Method of Adjustment for Hydrocephalus Valve

Abstract

A method of using an adjustable valve for the treatment of hydrocephalus in a patient, the adjustable valve including a fluid path having an inlet in fluid communication with a cranial cavity of the patient and an outlet in fluid communication with in another cavity of the patient, and an adjustment element to adjust a flow of a fluid in the fluid path, the method comprising the steps of modulating a setting of the adjustment element, measuring a pressure data of a gradient of a fluid pressure between the inlet and the outlet of the fluid path of the adjustable valve, and determining an optimal setting of the adjustment element of the adjustable valve, determining a failure of the adjustable valve, or characterizing cerebrospinal fluid (CSF) dynamics, based on the pressure data.

Claims

1. A method of using an adjustable valve for the treatment of hydrocephalus in a patient, the adjustable valve including a fluid path having an inlet in fluid communication with a cranial cavity of the patient and an outlet in fluid communication with in another cavity of the patient, and an adjustment element configured to adjust a flow of a fluid in the fluid path, the method comprising the steps of: modulating a setting of the adjustment element; measuring a pressure data of a gradient of a fluid pressure between the inlet and the outlet of the fluid path of the adjustable valve; and determining an optimal setting of the adjustment element of the adjustable valve, determining a failure of the adjustable valve, or characterizing cerebrospinal fluid (CSF) dynamics, based on the pressure data.

2. The method according to claim 1, wherein the step of measuring is performed over a determined period of time.

3. The method according to the claim 2, wherein during the step of measuring, no modulating of the setting is performed.

4. The method according to the claim 1 wherein the body of the patient is initially placed in a determined position which can be decubitus, upright or inclined.

5. The method according to the claim 1, wherein in the step of determining, the optimal setting of the adjustment element is further determined based on a determined position of a body of the patient.

6. The method according to the claim 1, wherein in the step of determining, the optimal setting of the adjustment element is further determined based on a first setting and a second setting of the adjustment element, the second setting being different from the first setting.

7. The method according to the claim 1, wherein in the step of determining, the failure of the adjustable valve is further determined based on a determined position of a body of the patient.

8. The method according to the claim 1, wherein in the step of determining, the failure of the adjustable valve is further determined based on a first setting and a second setting of the adjustment element, the second setting being different from the first setting.

9. The method according to the claim 1, wherein in the step of determining, the characterizing of the CSF dynamics is further determined based on a determined position of a body of the patient.

10. The method according to the claim 1, wherein in the step of determining, the characterizing of the CSF dynamics is further determined based on a first setting and a second setting of the adjustment element, the second setting being different from the first setting.

11. The method according to the claim 1, further comprising a step of: analyzing a profile of the pressure data of the gradient.

12. The method according to the claim 1, wherein the step of determining or the step of characterizing uses a mathematical model based on at least one of the following parameters: a previous setting, a variation of the fluid pressure, a determined period of time, a derivative dP/dt of the fluid pressure, pressure data measured with previous and current settings, a curve slope of the pressure data, a reference of pressure data, the amplitude or the frequency of the pressure curve or a position of a body of a patient.

13. The method according to the claim 1, further comprising the step of: changing a position of a patient for a determined period of time.

14. The method according to the claim 13 wherein the step of modulating the setting and the step of changing the position are performed successively and repeatedly.

15. The method according to the claim 14 wherein in the step of changing the position, the position of the patient is chosen to be decubitus, upright, or inclined.

16. The method according to the claim 1 further comprising the step of: determining a sign of a time derivative of the fluid pressure.

17. The method according to the claim 1, wherein the adjustable valve is in communication with a remote controller for adjusting the valve, and wherein the adjustable valve is configured to communicate the pressure data to the remote controller in a wireless manner.

18. The method according to the claim 1, wherein the pressure sensor is arranged in the fluid path.

19. The method according to the claim 1, wherein the step of characterizing further comprises the computing of E (brain elastance), Rout (residual drainage resistance), P0 (the pressure in the extradural venous system) and/or PP (peritoneum pressure).

20. A method of using an adjustable valve for the treatment of hydrocephalus of a patient, the adjustable valve including a fluid path having an inlet in fluid communication with a cranial cavity of the patient and an outlet in fluid communication with in another cavity of the patient, and an adjustment element configured to adjust the flow of a fluid in the fluid path, the method comprising the following steps: placing a body of the patient in a first determined position; measuring a first data of a pressure gradient between the inlet and the outlet of the fluid path; placing the body of the patient in a second determined position which is different from the first determined position; measuring a second data of a pressure gradient between the inlet and the outlet of the fluid path; and determining an optimal setting of the adjustment element, determining a failure of the adjustable valve, or characterizing cerebrospinal fluid (CSF) dynamics, based on the first data, the second data, and the first and the second position of the body.

21. The method according to the claim 20, wherein the steps of measuring is performed over a determined period of time.

22. The method according to the claim 20, further comprising the step of: modifying a setting of the adjustment element.

23. The method according to the claim 22, wherein in the step of determining, the optimal setting of the adjustment element is further determined based on a first setting and a second setting of the adjustment element.

24. The method according to the claim 22, wherein in the step of determining, the failure of the adjustable valve is further determined based on a first setting and a second setting of the adjustment element.

25. The method according to the claim 22, wherein in the step of determining, the characterizing of the CSF dynamics is further determined based on a first setting and a second setting of the adjustment element.

26. The method according to the claim 22, comprising a successive and repeated steps of modifying the setting and the patient position.

27. The method according to the claim 22, wherein the first and the second determined position can be decubitus, upright or inclined.

28. The method according to the claim 22, wherein the step of characterizing further comprises the computing of E (brain elastance), Rout (residual drainage resistance), P0 (the pressure in the extradural venous system) and/or PP (peritoneum pressure).

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0088] The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate the presently preferred embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain features of the invention.

[0089] FIG. 1 shows a passive flow regulator with drilled membrane and flexible substrate having a mesa;

[0090] FIG. 2 shows an adjustable passive flow regulator with drilled membrane with a rigid ball glued on the mesa, the force being applied by a cantilever spring (blade);

[0091] FIG. 3 shows the flow rate versus gap for multi-membrane flow regulator;

[0092] FIG. 4 shows the mean flow rate in the range 10 to 40 mbar versus the gap;

[0093] FIG. 5 shows inlet and outlet ports on the top;

[0094] FIG. 6 shows the pressure gradient through the device for a production rate of 15 ml/h as a function of the gap;

[0095] FIG. 7 shows the simulation of the effect of a partial occlusion on the device flow vs pressure characteristics of an auto-regulated shunt;

[0096] FIG. 8 shows the pressure drop in the fluidic restriction versus pressure drop in the flow regulation device;

[0097] FIG. 9 shows an illustration of the Dayson's law;

[0098] FIG. 10 shows the pulse amplitude (AMF) versus Intra-Cranial Pressure;

[0099] FIG. 11 shows the schematic flow rate through the device after change of the selector position from position i to position i+1 and i1;

[0100] FIG. 12 shows the equivalent electrical model for CSF dynamics analysis of a patient with a shunt;

[0101] FIG. 13A shows evolutions of the relative pressure sensor value and the ICP in case of a sudden increase of the production rate of 2 ml/h at t=0, for a flow regulating valve set at 20 ml/h and for an initial ICP of 10 mbar. Postural changes are described in Table 2;

[0102] FIG. 13B shows evolutions of the relative pressure sensor value and the ICP at a nominal production rate of 20 ml/h, for a flow regulating valve set at 20 ml/h and for an initial ICP of 10 mbar. Postural changes are described in Table 2;

[0103] FIG. 14 shows evolutions of the relative pressure sensor value and the ICP in case of a sudden decrease of the production rate of 2 ml/h at t=0, for a flow regulating valve set at 20 ml/h and for an initial ICP of 10 mbar. Postural changes are described in Table 2;

[0104] FIG. 15 shows the evolution of the relative pressure sensor value and the ICP according to the tuning procedure described in Table 3;

[0105] FIG. 16 shows the evolution of the relative pressure sensor value and the ICP according to the tuning procedure described in Table 4;

[0106] FIG. 17 shows the evolution of the relative pressure sensor value and the ICP according to the tuning procedure described in Table 5;

[0107] FIG. 18 shows the evolution of the ICP according to the tuning procedure described in Table 5, numerical constants being given in Table 6. The signal during the first 12 h of stabilization is not shown;

[0108] FIG. 19 shows a typical pressure profile in case of postural change (decubitus to upright position) in various conditions of occlusion, for a device exhibiting a large compliance;

[0109] FIG. 20 shows the evolution of the ICP according to the test protocol given in Table 7, using simulation data given in Table 6; and

[0110] FIG. 21 shows the evolution of the differential pressure sensor signal according to the test protocol given in Table 7, using simulation data given in Table 6.

[0111] Herein, identical reference numerals are used, where possible, to designate identical elements that are common to the figures. Also, the images are simplified for illustration purposes and may not be depicted to scale.

DETAILED DESCRIPTION OF THE SEVERAL EMBODIMENTS

[0112] In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration several embodiments of devices, systems and methods. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense. The invention is set forth and characterized in the independent claims, while the dependent claims describe other characteristics of the invention.

[0113] All scientific and technical terms used herein have meanings commonly used in the art unless otherwise specified. The definitions provided herein are to facilitate understanding of certain terms used frequently herein and are not meant to limit the scope of the present disclosure.

[0114] As used in this specification and the appended claims, the singular forms a, an, and the encompass embodiments having plural referents, unless the content clearly dictates otherwise.

[0115] As used in this specification and the appended claims, any direction referred to herein, such as top, bottom, left, right, upper, lower, and other directions or orientations are described herein for clarity in reference to the figures and are not intended to be limiting of an actual device or system. Devices and systems described herein may be used in a number of directions and orientations.

[0116] As used herein, have, having, include, including, comprise, comprising or the like are used in their open ended sense, and generally mean including, but not limited to.

[0117] As used in this specification and the appended claims, the term or is generally employed in its sense including and/or unless the content clearly dictates otherwise.

[0118] The term proximal as used herein, is a broad term and is used in its ordinary sense, including, without limitation, near to a point of reference such as an origin or a point of attachment.

[0119] The term distal as used herein, is a broad term and is used in its ordinary sense, including, without limitation, spaced relatively far from a point of reference, such as an origin or a point of attachment. The term substantially as used herein, is a broad term and is used in its ordinary sense, including, without limitation, being largely but not necessarily wholly that which is specified.

[0120] The wording position, human position or patient position refer to the physical configuration that the human body can take. For example, the human body can take the following basic position: standing, sitting, squatting, lying, . . . .

[0121] In this document, the decubitus position or horizontal position refers to a position taken by the patient which may be a lying position and more generally a position where the axis formed by the inlet of the implanted medical device (for example located in the cranial cavity of the patient) and the outlet of the implanted medical device (for example located in the peritoneal cavity) is substantially horizontal.

[0122] In this document the upright position or vertical position refers to a position taken by the patient which may be a standing position or a sitting position and more generally a position where the axis formed by the inlet of the implanted medical device (for example located in the cranial cavity of the patient) and the outlet of the implanted medical device (for example located in the peritoneal cavity or near the heart) is substantially vertical.

[0123] A change of position or postural change refers to a modification of the physical configurations taken by the human body. For example: from a horizontal position to a vertical position or from a vertical position to a horizontal position.

[0124] As employed herein, the term adjustment element refers to an element configured to adjust a parameter of the valve, for example but not limited to a parameter or setting to regulate the flow rate, the pressure threshold of the valve, open or close, open or closed position of the valve. In other terms, an adjustment element allows to regulate a variable of the fluid which flows from the cranial cavity to another cavity of the body patient, at least a part via the medical device.

[0125] As employed herein, the terms modulate and modulating refer to at least one change of a setting, for example the setting of the adjustment element of the medical device.

[0126] Examples of Medical Device Used by the Method

[0127] FIGS. 1 and 2 show two examples of the valve used by a flow regulator. Said flow regulator may comprise a substrate 2 and a membrane 1 which may be tightly fixed to a top surface of the substrate. Said regulator further comprises a cavity 10 which is arranged between the substrate 2 and the membrane 1. The distance between said substrate 2 and said membrane 1 corresponds to the depth of the cavity hereinafter called gap G. Said substrate 2 and/or said membrane 1 have through holes which are at least one inlet hole 3 and one outlet hole 4. Said holes permit the fluid to flow through said regulator. Inlet hole 3 and outlet hole 4 are in direct fluid communication with said cavity 10.

[0128] According to an example of the flow regulator, said membrane 1 comprises a lower face 11 which faces the cavity 10 and an upper face 12 on which the pressure of the fluid (also called P) is applied before said fluid enters in the cavity. Said membrane 1 comprises a flexible part in such a way that said membrane 1 comes into contact with said substrate 2 depending on the pressure of the fluid. Said contact may be total or partial, the main goal of this contact is to increase the fluidic resistance in such a way that the flow can be controlled even if the pressure of said fluid increases. When the pressure of the fluid increases, the depth of the cavity (gap G) decreases until the membrane 1 comes into contact, at least partially, to the substrate 2, thus the fluidic resistance increases for hindering said fluid flow.

[0129] The regulator may comprise several inlet holes 3. The holes positions and dimensions are arranged so that the fluid flow rate is passively regulated, depending on the fluid pressure, at least in a range of fluid pressure going from a first and at least a second predefined threshold values.

[0130] FIG. 1 shows a regulator comprising pillar. But the valve may have or not pillars. The pillar may be aligned to the inlet holes and may be located on the membrane and/or the rigid part. The position of the outlet may be located either on the top or the bottom or the side of the substrate or the membrane. Inlet and outlet may be on the same side. The position of the outlet could be arranged in the membrane or in the substrate.

[0131] Advantageously, the substrate 2 comprises a rigid part 6 and a flexible part 5. The rigid part may be surrounded by the flexible part. Due to the flexible part 5, a part of the substrate 6 is able to move downwardly and/or upwardly. In other words, a part of the substrate (to simplify the description, the rigid part refers to this part) can move to the membrane 1 or move away from the membrane 1 in such a way as to decrease or increase the gap G. Thus, the rigid part may come into contact with said membrane.

[0132] The regulator may comprise an adjustment means (also called adjustment element) designed to move and/or to maintain at least temporarily said rigid part at a given position. Said adjustment means allows applying a force F on the rigid part of substrate to adjust the gap G of the cavity.

[0133] Thanks to said adjustment means, the gap G is adjustable, the graph of FIG. 3 shows the effect of the gap on the flow rate. Thus, the changes of few microns around the nominal values of the gap induces an offset of the flow rate plateau while the pressure range of regulation is slightly shifted. FIG. 4 shows the means flow rate value, in the range of regulation 10 to 40 mbar, as a function of the gap. The linearity of this curve makes possible the use of a spring to adjust the flow rate. To achieve a displacement of only few microns, it is more relevant to control the force on the rigid part and not its displacement. Initially, the gap may be set at 25 m without deflection and pressure, the substrate being deflected only by compression force to simplify the assembly of the system.

[0134] The membrane may be much more flexible than the substrate. The pressure force transmitted by the membrane after contact with the substrate should be negligible compared to the restoring force of the substrate.

[0135] A force may be exerted by a spring, e.g. a flexible blade 14 in compression against a hard ball 13 glued on the mesa 6 as shown in FIG. 2. The ball may prevent substrate flexure by transmitting no couple. The force may be adjusted by the rotation of an elliptic came (not shown in the figures) in contact with the blade or by adjusting the height of the blade support. A rotor (which may comprise two axially opposed magnets) may be used to change the setting (the gap between the substrate and the membrane and/or the force applying on the mesa). Said rotor may comprise several predetermined setting which may be externally changed by a specific tool (e.g. a magnetic lock).

[0136] The medical device may further comprise a pressure sensor. For example, a sensor configured to measure a pressure gradient between the inlet of the medical device (for example the inlet catheter located in the cranial cavity, also called proximal catheter) and the outlet of the medical device (for example, the outlet catheter which may be located in the peritoneal cavity, also called distal catheter). This sensor may be a strain gages in a full Wheatstone configuration and may be implanted directly in or on the silicon membrane. The sensor may be connected to an electronic circuit comprising a coil to power externally the system, signal processing and wireless means to format and transfer the data to the end user (physician).

[0137] A push-pull system may be made by gluing a flexible blade onto the mesa and by attaching the spring onto this blade. The blocking of the substrate due to a flexure may be prevented by the flexibility of the blade attached to the mesa. The spring and the flexible blade may be a single preformed blade.

[0138] An adjustable valve, such shown by FIGS. 1, 2, and 5, may be arranged in a medical device comprising a housing 8, an inlet 15 adapted to be inserted in the cranial cavity and an outlet 16 adapted to be inserted in the other cavity. The medical device may comprise (upstream the adjustable valve) advantageously a check valve (e.g. a ball valve) and/or a reservoir to flush the medical device. The medical device may comprise a safety valve that opens at high pressure (e.g. ball valve).

[0139] Example of Numerical Simulations

[0140] An equivalent electrical model, based on the original study of Marmarou, has been used to simulate cerebrospinal fluid dynamics. A variable resistance has been added to simulate the shunt itself. FIG. 12 shows an example of an equivalent electrical model for CSF dynamics analysis of a patient with a shunt.

[0141] The CSF continuously produced by the patient is either stored in the ventricles or reabsorbed via the arachnoid villi or drained through the valve toward the peritoneum cavity. We do not consider here the fluctuations of ICP induced by the vasogenic system. For the sake of clarity, we assume first that the residual outflow resistance is infinite, the CSF being only diverted towards the peritoneum in steady state. The evolution of the pressure in the brain is derived by solving a differential equation, based on the diagram provided in FIG. 1, with the Euler's method.

[0142] The numerical constants used in the simulations are provided in Table 1 that shows numerical constants for the simulation of the active tuning of a hydrocephalus valve:

TABLE-US-00001 TABLE 1 Pss 10 (mbar) Po (mbar) 6.6 PP (mbar) 0 Rout (SI) Inf. z max (cm) 30 E (1/mL) 0.184 Pbo (mbar) 10.8 dt (s) 1

[0143] We consider a flow regulator set at 20 ml/h, the initial value of the ICP being equal to 10 mbar. In case of a sudden increase of the production rate, from 20 to 22 ml/h at t=0, the evolution of the ICP with time could be simulated as reported in the FIG. 13A. The scenario for the different postural change of the patient is summarized hereafter in Table 2, providing a description of the different postural changes of the patient with time:

TABLE-US-00002 TABLE 2 Time value constant (h) Postural change t0 0 decubitus to upright t1 +16 h upright to decubitus t2 +24 h decubitus to upright t3 +40 h upright to decubitus t4 +48 h decubitus to upright

[0144] The time t0 and t2 could correspond for instance to two successive patient's waking at 7 am while the time t1 and t3 could correspond to two successive patient's bedtime at 11 pm.

[0145] Unacceptable increase of ICP due to underdrainage is observed, in particular during the night (decubitus). This curve illustrates the interest in fine tuning the valve setting after implant.

[0146] A reference curve corresponding to the typical evolution of the pressure (for example ICP) in case of postural change, following the scenario of Table 2, but without increase of the nominal flow rate which remains equal to 20 mL/h, is provided in the FIG. 13B. As expected, the ICP is unchanged.

[0147] The evolution of ICP, for a decrease of the production rate of 2 ml/h at t=0, leading to Qf=18 ml/h, are provided in the FIG. 14, using the same scenario than previously for the different postural changes. In that case overdrainage is observed during the main part of the day. Even if the large fluidic resistance of the flow regulator strongly limit the effect of this overdrainage on the ICP value, long term complications to such low mean ICP value could be expected.

[0148] The following simulation is based on the numerical simulation described above.

[0149] Method of Setting of the Regulator

[0150] We work on the principle that the pressure gradient monitored by the pressure sensor is equal to the difference of pressure between the top surface of the membrane (assumed to be equal to ICP) and the bottom surface of the membrane which is in pressure communication with the peritoneum. Thus, we assume the following rule:

P=ICP+HPPP(2)

[0151] Where ICP is the intra-cranial pressure, HP the hydrostatic pressure and PP the peritoneum pressure. Ideally, to set a hydrocephalus valve, the physician is mainly interested in the knowledge of the ICP and the flow rate (of CSF production rate) but the ICP cannot be measured by the sensor of our device (because our sensor is not an absolute pressure sensor). Other patient dependent parameters as the residual drainage resistance R.sub.out may be used and will be discussed hereafter.

[0152] As a general trend, but not limited to, the present invention is based on the combination of device setting change with patient postural change to make fast and reliable valve adjustment using the integrated pressure sensor. Furthermore, this new method is based on the use of a pressure gradient sensor, not to adjust the pressure at which the anti-siphon valve closes but to adjust the desired flow rate of the device. Thus, this new method consists in the combination of the change of the device setting (e.g. flow rate) with patient postural change. The monitoring and the analysis of the pressure gradient values during these changes allows the tuning of the device in accordance with the patient physiological characteristics.

[0153] For example, for a device initially set at 20 ml/h, and for a patient generating 25 ml/h of CSF, the differential pressure sensor will indicate a very large value (typically >20 mbar). For a patient generating 15 ml/h, the differential pressure measured will be small (typically <10 mbar).

[0154] According to one example, FIG. 6 shows the differential pressure monitored by the sensor as a function of the device setting (here referred as a gap in m, each setting position corresponding to a given gap). Here, the gap is the distance between a surface of the mesa and a surface of the membrane. For this example, the CSF production rate is 15 ml/h. According to one example, a change of setting can adjust the gap by step equivalent to change of about 5% of the nominal flow rate. We note I a setting position, I.sub.0 is the position that leads to a determined mean flow rate for example 20 ml/h. Thus, I.sub.0+1 is the setting that leads to 21 ml/h while the setting corresponds to 19 ml/h, and so on.

[0155] Optionally, the initial setting of the device correspond to a high flow rate setting (for example the greatest gap corresponding to the maximum flow rate of the device) so as to drain the excess of CSF in the patient's brain. Preferentially after the surgery, the patient stays in decubitus during several hours and the ICP stabilizes at around 10 mbar (for example).

[0156] A first method may comprise the following steps: [0157] Place the patient in a determined position (for example: upright position), [0158] Monitor the evolution of the pressure with time dP/dt [0159] Reduce the device setting I until the sign of dP/dt becomes positive.
We note I.sub.k the setting which allows changing the sign of time derivative dP/dt. Thus, the optimal setting is comprised between I.sub.k and I.sub.k+1.

[0160] A second method may comprise the following steps: [0161] Set the device at I.sub.0+j where I.sub.0+j may be the initial setting (for example after implant), [0162] Set the device at I.sub.0j, [0163] Place the patient in a determined position (for example: upright position), [0164] Monitor the evolution of the pressure with time dP/dt [0165] Increase the device setting until the sign of dP/dt becomes negative.
With this method, the optimal setting is comprised between I.sub.k and I.sub.k1.

[0166] A third method may consists in changing setting alternatively in positive and negative direction with respect the foreseen nominal value I.sub.0, in order to limit the change of ICP during the adjustment procedure. Indeed the ICP alternatively increases and decreases, limiting the overall change of the mean ICP value. The third method may comprise the following steps (j is the range of setting that will be used to adjust the device, k is the step of the setting search and n is the iterative number of adjustment steps): [0167] Set the device at I.sub.0+j where I.sub.0+j may be the initial setting with n=0 (for example after implant), [0168] Place the patient in a determined position (for example: upright position), [0169] Monitor the evolution of the pressure with time dP/dt. [0170] If dP/dt stays positive, use a j value larger in order to get the right sign of dP/dt, [0171] Set the device at I.sub.Nominal+(jkn)(1).sup.n using n=1; if dP/dt becomes negative set the device at I.sub.Nominal+(jkn)(1).sup.n using n=2 and so on.
The optimal setting is obtained when the change of setting of one k unit change the sign of dP/dt. To check out that the setting is correct, the pressure after a long stay in upright position shall be in a determined range for example 13+/6 mbar.

[0172] The method is still valid if a setting change does not correspond exactly to a fixed value of flow rate change. The mandatory requirement is just that an increase/decrease of the actual setting I will induce an increase/decrease of the mean flow rate.

[0173] As a general trend, the analysis of the sign of dP/dt is valid essentially when the pressure gradient is comprised in its regulation range, or in other words the functioning point of the device is located onto the plateau, where the flow is expected to be constant. The different methods are therefore more effective when the patient is in vertical or tilted position. The analysis of the evolution of pressure in case of partial or total occlusion is discussed hereafter.

[0174] Example of a Simulation Using the Method 1

[0175] We consider a valve having a nominal flow rate Q0 of 20 mL/h for the setting 0, the increase/decrease of 1 setting corresponding to an increase/decrease of 1 mL/h for the nominal flow rate through the valve (e.g. Q2=22 mL/h and Q4=16 mL/h). We assume now that the patient exhibits a production rate of 22 mL/h, while the other constants necessary to perform the simulations are given in Table 1.

[0176] After implant the patient is in decubitus with the valve wide open in order to reduce the ICP to a value slightly lower than its basal pressure P.sub.b. The patient is then placed in upright position while the setting is decreased until stabilization and finally an increase of the pressure, the best setting being here the penultimate one.

[0177] After the stabilization period at the best setting the patient comes back in decubitus to check out that the ICP is not affected by postural change and that its value is within the initial range estimated during an Infusion Test. A typical protocol is given in the Table 3 showing an adjustment protocol for the determination of the best setting of the valve using the method.

TABLE-US-00003 TABLE 3 Time Valve constant value setting Patient's position t0 0 h +5 (initial decubitus setting) t1 +12 h +4 upright t2 +12 h 45 +3 upright t3 +13 h 30 +2 upright t4 +14 h 15 +1 upright t5 +15 h +2 decubitus

[0178] The ideal setting of the device is the setting +2, leading to a flow rate through the valve equal to the production rate. The evolutions of the ICP and the sensor signal during the valve tuning are provided in FIG. 15. The change of the slope is an indication that the new setting is too low. The final postural change of the patient is used to check the effectivity of the anti-siphoning effect of the device in case of correct valve tuning.

[0179] Example of a Simulation Using the Method 2

[0180] We still assume a production rate of 22 mL/h. This production rate is estimated during an Infusion Test. Therefore we are able to limit the range of setting during the tuning at +/3 around this estimated value. The tuning procedure is summarized in Table 4 showing an adjustment protocol for the determination of the best setting of the valve using the method 2 (best Setting=+2). The best tuning is +2. To increase the change of pressure during the change of setting, the patient is ideally placed in upright (resp. decubitus) position during an increase (resp. decrease) of the setting.

TABLE-US-00004 TABLE 4 Time Valve Patient's constant value setting position t0 0 h +5 (initial decubitus setting) t1 +12 h 1 decubitus t2 +12 h 45 +1 upright t3 +13 h 30 +3 upright t4 +14 h 15 +2 upright t5 +15 h +2 decubitus

[0181] Example of a Simulation Using the Method 3

[0182] We still assume a production rate of 22 mL/h. This production rate is estimated during an Infusion Test. Therefore we are able to limit the range of setting during the tuning at +/3 around this estimated value. The tuning procedure is summarized in Table 5 showing an adjustment protocol for the determination of the best setting of the valve using the method 2 (best Setting=+2). The best tuning is +2. To increase the change of pressure during the change of setting, the patient is ideally placed in upright (resp. decubitus) position during an increase (resp. decrease) of the setting.

TABLE-US-00005 TABLE 5 Time Valve Patient's constant value setting position t0 0 h +5 (initial decubitus setting) t1 +12 h 1 decubitus t2 +12 h 45 +3 upright t3 +13 h 30 +1 decubitus t4 +14 h 15 +2 upright t5 +15 h +2 decubitus

[0183] Modelling the Arterial Pressure Oscillations

[0184] In order to get a more realistic picture of the pressure trend during the device setting, the change of the cerebral artery volume due to heart action shall be considered. It is assumed that this volume change is balanced by the same CSF volume fluctuations.

[0185] The production rate of CSF includes therefore an additional term in cos (t):

Q.sub.f=Q.sub.f0+C.sub.AP.sub.A cos(t)(3)

Where is the heart rate angular frequency, C.sub.A the cerebral artery compliance and P.sub.A the amplitude of the sinusoidal perturbation:

P.sub.A(t)=P.sub.AO+P.sub.A sin(t)(4)

The numerical constants used in the simulations are provided in Table 6 showing simulations constants for modelling the effect of cerebral artery compliance on ICP, while the adjustment protocol the given in Table 5:

TABLE-US-00006 TABLE 6 Pss (mbar) 10 Po (mbar) 9 PP (mbar) 0 R.sub.out 13 (mmHg/mL/min) z max (cm) 30 E (1/mL) 0.2 Pbo (mbar) 14 (S.sup.1) 6.28 E C.sub.A P.sub.A 0.05 dt (s) 0.05

[0186] The evolution of the ICP after the 12 h of ICP stabilization is provided in FIG. 18. The adjustment methods are not affected by the sinusoidal trend of the ICP. Low-pass filter may be used for ICP profile analysis.

[0187] Failure Detection Methods, in Case of Total Occlusion:

[0188] Two methods may be used: the DC analysis method (stationary) and/or the AC analysis method.

[0189] 1. DC Analysis Method

[0190] If the total occlusion is located in the proximal (i.e. before the valve) and/or distal (i.e. after the valve) catheter, the pressure between the top and bottom surfaces of the membrane will equilibrate, leading to a pressure gradient equal to zero in both positions (horizontal or vertical position). After stabilization, we have: P.sub.hP.sub.v=0.

[0191] For an occlusion near the device (i.e. proximal or just after the device outlet), we do not expect a change of pressure by postural changes. But, an occlusion at the end of the distal catheter induces a sudden increase of pressure and then quickly relax to zero. In that latter case, a total liquid volume corresponding to the change of volume of the fluidic line upward the regulator will pass through the device, inducing a movement of the membrane which can be detected by the sensor.

[0192] Because of the elasticity of the fluidic line it could be possible to discriminate between proximal or distal occlusion if this latter occlusion takes place just after the device (i.e. the valve) and not at the end of the distal catheter. But, if the elasticity of the proximal cavity (in communication with the top surface of the flexible membrane of the valve) is small, the pressure will remain equal to zero during the postural change of the patient and the possibility to distinguish between proximal and distal occlusions is lost.

[0193] Thus, a total occlusion diagnosis can be detected or confirmed by these observations. The method for detecting an occlusion may comprise: [0194] Provide a medical device adapted to regulate a fluid which flows from the cranial cavity to another cavity of the body patient, the medical device comprising: [0195] An inlet located in the cranial cavity, in which the fluid enters in the medical device [0196] An outlet located in the other cavity, by which the fluid flows out the medical device [0197] A pressure sensor which senses the differential pressure between the cranial cavity and the other cavity [0198] Record a first differential pressure measurement when the patient is in decubitus or upright position, [0199] Change the position of the patient [0200] Record a second differential pressure measurement, [0201] Compare the first and the second differential pressure data, [0202] Determine an occlusion based on this comparison.

[0203] For example, an occlusion may be detected when the difference between said first and second pressure measurements is equal or close to zero, or lower in absolute value to a reference value. The location of the occlusion (distal or proximal) may be based on the presence of a transient peak of pressure during postural change.

[0204] A purge may be necessary to release this occlusion. The purge is made possible by a filling port that is externally accessible via a syringe.

[0205] 2. AC Analysis Method

[0206] The total occlusion may be detected by analyzing the AC signal of the ICP pressure (for example via a mathematical model). The AC component of ICP due to change of blood volume in the brain are no longer transmitted to the device for proximal occlusion. This method is the best way to distinguish proximal and distal occlusion.

[0207] The method for detecting an occlusion may comprise: [0208] Provide a medical device adapted to regulate a fluid which flows from the cranial cavity to another cavity of the body patient, the medical device comprising: [0209] An inlet located in the cranial cavity, in which the fluid enters in the medical device [0210] An outlet located in the other cavity, by which the fluid flows out the medical device [0211] A pressure sensor which senses the differential pressure between the cranial cavity and the other cavity [0212] Record a first set of pressure measurements, [0213] Process the first set of measurements, [0214] Determine an occlusion based on the first set of measurements, for example by comparing the amplitudes of the different components of the differential pressure oscillations with a reference.
The step of processing may comprise the characterization (amplitude and/or frequency) of the different components of the differential pressure oscillations, using for instance a Fourier Transform.

[0215] In Case of Partial Occlusions:

[0216] 1. If the Partial Occlusion is Located Inside the Valve:

[0217] Because the device shows the smallest restriction in the fluidic pathway between the brain and the peritoneum, this kind of occlusion have the highest probability to occur. These restrictions are the holes in the membrane and the gap between the membrane and the pillars. Any occlusion in the device (e.g. located in the holes) leads to an increase of the measured value of the pressure gradient through the device.

[0218] For a given device correctly adjusted to the patient CSF production rate, this kind of occlusion will lead to an increase of the pressure at least equivalent to a change of position because of the translation of the Q(P) curve towards the low flow rate. This increase of pressure can be observed in decubitus, and a change of position will increase again the measured value.

[0219] The method for detecting an occlusion may comprise: [0220] Provide a medical device adapted to regulate a fluid which flows from the cranial cavity to another cavity of the body patient, the medical device comprising: [0221] An inlet located in the cranial cavity, in which the fluid enters in the medical device [0222] An outlet located in the other cavity, by which the fluid flows out the medical device [0223] A pressure sensor which senses the differential pressure between the cranial cavity and the other cavity [0224] Record a first differential pressure measurement when the patient is in decubitus or upright position, [0225] Change the position of the patient [0226] Record a second differential pressure measurement, [0227] Compare the first and the second differential pressure data [0228] Determine an occlusion based on this comparison, for example if the difference between said first and second pressures is equal or close to zero, or lower in absolute value to a reference value.

[0229] A purge may be necessary to flush the device from any protein residual. A new valve adjustment may be required after detection of partial occlusion.

[0230] 2. If the Partial Occlusion is Located in Proximal and/or Distal Catheter:

[0231] We consider now a partial occlusion equivalent to a fluidic resistance of 3.8E11 Pa.Math.s/m3 (for example). The change of the device characteristics is shown in FIG. 7. The nominal regulated flow rate is equal to 20 ml/h in both cases, there is just a shift induced by the additional fluidic resistance. We consider a patient having a normal ICP of 10 mbar. In case of partial occlusion equivalent to said fluidic resistance of 3.8E11 Pa.Math.s/m3, the ICP rises up to 30 mbar but the sensor only measures 10 mbar which corresponds to the pressure drop through the device. By design, the device is blind to partial occlusion if the device is originally well adjusted to the effective CSF production rate of the patient. The pressure drop in this fluidic restriction versus the pressure drop in the device itself is shown in FIG. 8. When the patient moves from decubitus to upright position, the anti-gravity properties of the device are still active with a partial occlusion and the sensor signal is not affected by partial occlusion: the pressure drop in this restriction is constant during the movement while the pressure drop through the device has increased of 30 mbar which corresponds to the hydrostatic pressure HP.

[0232] In normal situation the sensor shall experience a negative pressure while in case of partial occlusion, considering the former case with an ICP of 30 mbar induced by said occlusion, the pressure will equilibrate and the sensor will not measure this negative pressure.

[0233] A first approach consists in using a tilting chair and placing the patient not at 90 but at an angle comprised between 10 to 60 and to analyze the decrease of the measured pressure as a function of this angle, after a stabilization period. Other methods to confirm the hypothesis of a partial occlusion may be used, for example a change of setting or a AC method.

[0234] Change of Setting Methods

[0235] The typical patient characteristics (E, p.sub.0 and R.sub.out) have been determined before or after the shunt implantation to characterize the CSF dynamics of the patient. Where E is the cerebral elasticity or elastance coefficient, p.sub.0 is the pressure in the extradural venous system and R.sub.out is the residual drainage resistance. The initial p(t) curve could be therefore used as a reference.

[0236] In order to check the eventual presence of an occlusion it is possible to use the method described previously consisting in changing the setting of the device and monitoring the evolution of the pressure.

[0237] In case of occlusion the pressure measured by the sensor is no longer equal to the ICP even when the patient is in decubitus and when we neglect the peritoneum pressure.

[0238] In case of partial occlusion that leads to an increase of ICP, we expect a change of the CSF dynamics because of the decrease of the brain compliance at high ICP.

[0239] The fitting of the curve p(t) during the test of setting change will give the best coefficient of determination R.sup.2 for a reduced value of E.

[0240] Moreover, in case of occlusion, the slope of the curve Q(P) before and after the plateau will decrease. This decrease will result in a change of the typical curve p(t) during a test according to the previous method.

[0241] The decrease of this slope induces an increase of the measured variation of pressure during these tests by a factor proportional to the ratio of the ICP.

[0242] It is therefore possible to extract the real value of the ICP by doing a simple rule of three: we use a +1 method consisting in changing the position setting of the device of +1 (to increase of 1 unit the nominal setting of the device).

[0243] The pressure change after stabilization is proportional to p.sub.b(i):

[00001]$\begin{array}{cc}p\left(t=\right)=p_b\left(i+1\right)=\left(\frac{Q_{\mathrm{valve}}\left(p_b\left(i\right),i\right)}{Q_{\mathrm{valve}}\left(p_b\left(i\right),i\right)+.Math..Math.Q}\right).Math.p_b\left(i\right)=\frac{p_b\left(i\right)}{1+\frac{.Math..Math.Q}{Q_{\mathrm{valve}}\left(p_b\left(i\right),i\right)}}& \left(5\right)\end{array}$

If the partial occlusion leads to an increase of the ICP of a factor 3, the relative change of pressure during the change of setting +1 will be also increased by a factor 3 according to the reference value recorded just after implantation.

[0244] In brief: a distal or proximal occlusion will be therefore detected by analyzing the curve p(t) and by comparing this curve with a reference curve obtained just after implantation.

[0245] The method for detecting an occlusion may comprise: [0246] Provide a medical device adapted to regulate a fluid variable which flows from the cranial cavity to another cavity of the body patient, the medical device comprising: [0247] An inlet located in the cranial cavity, in which the fluid enters in the medical device [0248] An outlet located in the other cavity, by which the fluid flows out the medical device [0249] An adjustable valve in fluid communication with the inlet and the outlet of the medical device, said adjustable valve being adapted to be set by a caregiver in such a way as to adjust the fluid variable, [0250] A pressure sensor which senses the differential pressure between the cranial cavity and the other cavity [0251] Change the fluid variable according to a new setting, [0252] Monitor over time the differential pressure, [0253] Analyze the curve of the monitored differential pressure (for example: compare the curve of the monitored differential pressure to reference data) [0254] Determine an occlusion based on this comparison, for example the change of the differential pressure after a change of setting being increased by the presence of a partial occlusion.

[0255] AC Method

[0256] According to Lemaire J. J. et al., Slow Pressure Waves in the Cranial Enclosure, Acta neurochirurgica Vol. 144.3 (2002), pp. 243-254, this reference herewith incorporated by reference in its entirety, the analysis of the slow pressure waves in the cranial cavity could be also used to for assessment of CSF disorders.

[0257] Just after implantation, typical AC components of the ICP signal will be recorded as reference for standard ICP, when the device is correctly adjusted to the patient production rate of ICP.

[0258] A decrease of the measured AC amplitude of the pressure could be attributed to an increase of the ICP. This change of the AC amplitude could be correlated to the former test to confirm the presence of a partial occlusion.

[0259] The method for detecting an occlusion may comprise: [0260] Provide a medical device adapted to regulate a fluid which flows from the cranial cavity to another cavity of the body patient, the medical device comprising: [0261] An inlet located in the cranial cavity, in which the fluid enters in the medical device [0262] An outlet located in the other cavity, by which the fluid flows out the medical device [0263] A pressure sensor which senses the differential pressure between the cranial cavity and the other cavity [0264] Record a first set of measurements, [0265] Process the first set of measurements, [0266] Determine an occlusion based on the first set of measurements, for example by comparing the amplitudes of the different components of the differential pressure oscillations with a reference.

[0267] The step of processing may comprise the characterization (amplitude and/or frequency) of the different components of the differential pressure oscillations, using for instance a Fourier Transform.

[0268] In Case of Leakage

[0269] According to the device design, we should consider two kinds of leakage: [0270] a leakage due to a membrane crack or [0271] a leakage due to particle between the pillars and the membrane leading to a permanent gap (valve leakage).

[0272] 1. In Case of a Leakage Due to a Membrane Crack:

[0273] By design the silicon membrane of the device is expected to sustain pressure of several bars. In case of a shock on the patient head, the pressure sensor could be used to detect a mechanical failure of the membrane. The pressure sensor is preferably made of a strain gauge implanted directly in the silicon membrane. The detector circuit at the surface of the membrane could be therefore considered as a crack-guard: any crack in the membrane will lead to an open detector circuit.

[0274] 2. In Case of Valve Leakage:

[0275] The device may be not sensitive to small particles (lower than few um in diameter (for example 2 um)). In case of contamination with larger particles or residuals (proteins . . . ), we expect an increase of the derivation flow through the valve for a given ICP. In a first approximation we assume that this permanent gap is equivalent to a change of the device setting that leads to an increase of the nominal flow rate.

[0276] If typical symptoms of overdrainage are observed, the first analysis method consists in doing an adjustment of the device with a monitoring of the p(t) curve during this process.

[0277] An abnormal increase of the nominal flow rate should be rather attributed to a valve leakage rather than an increase of CSF production if this phenomenon is sudden and permanent.

[0278] The method for detecting an valve leakage may comprise: [0279] Provide a medical device adapted to regulate a fluid variable which flows from the cranial cavity to another cavity of the body patient, the medical device comprising: [0280] An inlet located in the cranial cavity, in which the fluid enters in the medical device [0281] An outlet located in the other cavity, by which the fluid flows out the medical device [0282] An adjustable valve in fluid communication with the inlet and the outlet of the medical device, said adjustable valve being adapted to be set by a caregiver in such a way as to adjust the fluid variable, [0283] A pressure sensor which senses the differential pressure between the cranial cavity and the other cavity [0284] Change the fluid variable according to a new setting, [0285] Monitor over time the differential pressure, [0286] Analyze the curve of the monitored differential pressure (for example: compare the curve of the monitored differential pressure to reference data) [0287] Determine a valve leakage based on this comparison.

[0288] The presence of a valve leakage may be equivalent to a positive change of setting (larger valve opening), the device is intended to be well adjusted after reduction of the valve opening (negative change of setting).

[0289] Simulation of Failure Analysis

[0290] We assume first that the volume displaced by the flexible membrane per unit of pressure is large and almost constant over the range of pressure 10 to 40 mbar. In that case, we show in FIG. 19 the evolution of the pressure sensor signal in various cases, comprising notably either proximal or distal occlusion. We assume occlusion in the proximal catheter or at the entry of the distal catheter (i.e. just after the valve itself). The water column due to the distal catheter in vertical position has an effect on the sensor signal depending on the localization of the occlusion.

[0291] For a proximal occlusion, we expect a first peak of pressure when the patient moves from decubitus to upright position. This effect is due to the compliance of the proximal cavity that will be contracted by the negative pressure generated by the water column. A linear relaxation is then observed as shown in FIG. 19. This linear behavior is related to the fact that the flow rate induced by the movement of the flexible membrane is constant.

[0292] We consider the presence of a distal occlusion in a device that exhibits a small compliance: the flexible membrane will remain all the time at equilibrium and there is no possibility to localize the occlusion using this method. The analysis of the short-term oscillations of the sensor signal is then required: in case of proximal occlusion a strong reduction of pressure oscillations due to the vasogenic system is expected.

[0293] Typical example considering a residual drainage function via arachnoid villi and variation around this value to simulate partial occlusion and leakage: the simulation data are summarized in Table 7 showing simulation data for failure analysis purpose. The initial ICP value is assumed to be equal to Pb0 (i.e. 10.8 mbar).

TABLE-US-00007 TABLE 7 P.sub.ss (mbar) 10 P.sub.o (mbar) 6.6 PP (mbar) 0 R.sub.out nominal (SI) 3.6.sup.E11 R.sub.out partial occlusion 3.6.sup.E12 (SI) R.sub.out leakage (SI) 3.6.sup.E10 z max (cm) 30 E (1/mL) 0.184 Pbo (mbar) 10.8 dt (s) 1

[0294] In Table 8, a typical test protocol for failure analysis is shown.

TABLE-US-00008 TABLE 8 Valve Patient's Time constant value setting position t0 0 h 5 decubitus t2 +2 h 00 0 upright
The evolution of the intracranial pressure is provided in FIG. 20 while the evolution of the pressure monitored by the differential sensor is given in FIG. 21.

[0295] For sake of clarity, the values of R.sub.out corresponding to Nominal, Leakage and Semi-occlusion curves are here significantly different (one order of magnitude).

[0296] The analysis of the pressure trends is straightforward. In case of leakage the partial closing of the valve (setting 5) is not able to generate underdrainage while in case of semi-occlusion a large increase of the pressure is observed.

[0297] Method for a Non-Invasive Characterization of the CSF Dynamics

[0298] In a general manner, any information about CSF flow or pressure is fundamental to diagnose and characterize cerebral disease. In case of abnormal function of the cerebrospinal fluid outflow system, several methods are described in the prior art part of this document.

[0299] Standard Model for CSF Dynamics

[0300] We do not consider fluctuation due to periodic changes of the cerebral arterial blood volume and changes of venous outflow generated by respiration. We assume a constant production rate of CSF. Therefore the production of CSF shall be balanced between storage and absorption. This absorption Q.sub.abs is proportional to the difference of pressure between the intracranial pressure p=ICP and the pressure in the sagittal sinuses p.sub.ss (from Dawson's law, see FIG. 9):

[00002]$\begin{array}{cc}{Q}_{\mathrm{abs}}=\frac{p-p_{\mathrm{ss}}}{p_{\mathrm{out}}}& \left(6\right)\end{array}$

where R.sub.out is the resistance to CSF absorption. The storage of CSF is proportional to the cerebrospinal compliance C(p) and the rate of change of CSF pressure

[00003]$\frac{\mathrm{dp}}{\mathrm{dt}}.Math.\text{:}$

[00004]$\begin{array}{cc}\mathrm{Storage}=C\left(p\right).Math.\frac{\mathrm{dp}}{\mathrm{dt}}& \left(7\right)\end{array}$

[0301] The compliance of the cerebrospinal space C(p) is constant up to given pressure p.sub.opt and increases when the CSF pressure is larger than p.sub.opt:

[00005]$\begin{array}{cc}C\left(p\right)=\frac{1}{E\left(p-p_0\right)}.Math..Math..Math.\mathrm{for}.Math..Math..Math.p>p_{\mathrm{opt}}& \left(8\right)\end{array}$

where E is the cerebral elasticity or elastance coefficient which is patient specific. The compliance of the brain decreases for high ICP. By writing the conservation of matter we finally derive:

[00006]$\begin{array}{cc}\frac{1}{E\left(p\left(t\right)-p_0\right)}.Math.\frac{\mathrm{dp}\left(t\right)}{\mathrm{dt}}+\frac{p\left(t\right)-p_b}{R_{\mathrm{out}}}=Q_{\mathrm{ext}}\left(t\right)& \left(9\right)\end{array}$

Where Q.sub.ext(t) is the rate of external volume addition; the term p.sub.b, which is the baseline CSF pressure (resting pressure), is used in the latter formula instead of p.sub.ss which is difficult to measure.

[0302] This equation could be solved easily in case of constant infusion rate test with Q.sub.ext(t)=Q.sub.const:

[00007]$\begin{array}{cc}p\left(t\right)=p_0+\frac{\left(\frac{p_b-p_0}{R_{\mathrm{out}}}+Q_{\mathrm{const}}\right).Math.\left(p_b-p_0\right)}{\frac{p_b-p_0}{R_{\mathrm{out}}}+Q_{\mathrm{const}}\left(e^{-E\left[\frac{p_b-p_0}{R_{\mathrm{out}}}+Q_{\mathrm{const}}\right].Math.t}\right)}& \left(10\right)\end{array}$

For a bolus injection of V, the evolution of the pressure becomes:

[00008]$\begin{array}{cc}{p}_b=p_0+\frac{\left(p_b-p_0\right).Math.e^{E\left[.Math..Math.V+\left(\frac{p_b-p_0}{R_{\mathrm{out}}}\right).Math.t\right]}}{1+e^{E.Math..Math..Math..Math.V}\left(e^{E\left[\frac{p_b-p_0}{R_{\mathrm{out}}}\right].Math.t}-1\right)}& \left(11\right)\end{array}$

This bolus injection is usually used to determine the pressure volume index PVI:

[00009]$\begin{array}{cc}\mathrm{PVI}=\frac{.Math..Math.V}{\log \left(\frac{p_p-p_0}{p_b-p_0}\right)}& \left(12\right)\end{array}$

Where p.sub.p is the peak pressure just after the bolus injection.

[0303] According to Marmarou et al. Compartmental analysis of compliance and outflow resistance of the cerebrospinal fluid system, J. Neurosurg. Vol. 43, 523534 (1975) and Avezaat C. J. J. et al. Clinical observations on the relationship between cerebrospinal fluid pulse pressure and intracranial pressure, Acta Neurochirurgica, Vol. 79, pp. 13-29 (1986), these publications herewith incorporated by references in their entirety, the evolution of the intracranial pressure for increasing volume has an exponential form:

p=(p.sub.bp.sub.0)e.sup.EV+p.sub.0(13)

[0304] Because the craniospinal space is enclosed in the rigid skull, the compliance is small, typically from 0.1 to 0.3 ml.sup.1, according to Cieslicki, K., Mathematical modelling of the infusion test, Pol. J. Med. Phys. Eng Vol. 13, pp. 33-54 (2007), this publication herewith incorporated by reference in its entirety. The former relationship is useful to determine the pulse amplitude (AMF) of CSF due to the change of the blood volume in the brain after heart contraction, since this abrupt change of volume is similar to a bolus injection (see FIG. 10):

AMP+p.sub.pp.sub.b=(p.sub.bp.sub.0)(e.sup.EV1)(14)

[0305] The analysis of the slow pressure waves in the cranial cavity could be also used to for assessment of CSF disorders, according to Lemaire J. J. et al., Slow Pressure Waves in the Cranial Enclosure, Acta Neurochirurgica, Vol. 144: pp. 243-254 (2002), this publication being herewith incorporated by reference in its entirety.

[0306] Eklund A. et al., Assessment of cerebral fluid outflow resistance, Med. Bio. Eng. Comput. 45, 719-735 (2007) reviewed the different methods of investigation of the CSF dynamics by active infusion of artificial CSF and pressure recording, this publication being herewith incorporated by reference in its entirety. After implantation of a shunt, these methods are still used to verify the good functioning of the device, according to Weerakkody et al., Clinical assessment of cerebrospinal fluid dynamics in hydrocephalus; Guide to interpretation based on observational study, Acta Neurol. Scand. Vol. 124, pp. 85-98 (2011), this publication being herewith incorporated by reference in its entirety.

[0307] Because these methods usually require the placement of two needles in the lumbar canal, an in vivo (but non-invasive, i.e. without needle) determination of the shunt functionality is desirable. Methods for the in vivo adjustment of the shunt to the patient production rate of CSF are also required.

[0308] New Method for a Non-Invasive Characterization of the CSF Dynamics

[0309] The new method is related to the analysis of the pressure sensor signal to determine physiological parameters of the CSF dynamics of the patient. By adding an auto-regulated shunt in the CSF fluidic pathway, we should introduce a new term in the equation of the conservation of the matter:

Q.sub.f+Q.sub.ext=Q.sub.abs+Q.sub.storage+Q.sub.valve(15)

where Q.sub.f is the formation rate of CSF, Q.sub.ext is the externally injected flow of liquid, Q.sub.abs is the absorbed rate of CSF through the arachnoid villi, Q.sub.storage is the rate of liquid storage due to pressure change and Q.sub.valve the flow of CSF through the valve.

[0310] We note the following designations:

[0311] p=intracranial CSF pressure

[0312] p.sub.ss=sagittal sinuses pressure

[0313] p.sub.0=pressure in the extradural venous system

[0314] p.sub.b(i)=baseline ICP pressure, at rest in decubitus, for the setting i

[0315] Q.sub.valve(p.sub.b(i), i)=flow through the valve at equilibrium, for the setting i

[0316] R.sub.out=fluidic resistance to CSF outflow towards physiological routes

[0317] P.sub.reg=pressure range wherein the regulated flow is constant

[0318] PP=peritoneum pressure

[0319] We assume first that the compliance of the shunt is very small compared to the brain itself. To simplify the formalism of the evolution of the pressure with time, we assume that the peritoneum pressure PP is equal to zero. The real gradient of pressure through the valve is reduced of PP (typically 2 mbar). An example of a full model including PP is given in annex.

[0320] A simplified view of the CSF flow through the regulator is shown in FIG. 11. The device after adjustment is assumed to be in position i with a flow rate at rest of Q.sub.valve(p.sub.b(i), i). The valve fluidic characteristics after change of the device selector position of +/1 step are also shown. We note Q the absolute change of the nominal flow rate of the device in these two new positions.

[0321] At rest, the former equation reduces to:

[00010]$\begin{array}{cc}{Q}_f=\frac{p_b\left(i\right)-p_{\mathrm{ss}}}{R_{\mathrm{out}}}+Q_{\mathrm{valve}\left(p_b\left(i\right),i\right)}& \left(16\right)\end{array}$

[0322] Method: setting +1 and patient in decubitus position.

[0323] We can rewrite the full equation after the change of the setting of +1:

[00011]$\begin{array}{cc}\frac{p_b\left(i\right)-p_{\mathrm{ss}}}{R_{\mathrm{out}}}+Q_{\mathrm{valve}}\left(p_b\left(i\right),i\right)+Q_{\mathrm{ext}}=\frac{p-p_{\mathrm{ss}}}{R_{\mathrm{out}}}+\frac{1}{E\left(p-p_0\right)}.Math.\frac{\mathrm{dp}}{\mathrm{dt}}+Q_{\mathrm{valve}}\left(p,i+1\right)& \left(17\right)\\ .Math.\mathrm{Therefore}& \\ \frac{\mathrm{dp}}{\mathrm{dt}}=E\left(p-p_0\right)\left[\frac{p_b\left(i\right)-p}{R_{\mathrm{out}}}+Q_{\mathrm{ext}}+Q_{\mathrm{valve}}\left(p_b\left(i\right),i\right)-Q_{\mathrm{valve}}\left(p,i+1\right)\right]& \left(18\right)\end{array}$

[0324] The functioning point of de device jumps of +Q from the curve i to i+1 after change of the selector setting of +1, the pressure being equal to p.sub.b(i) at t=0 according to FIG. 11. There is then a relaxation up to an ICP equal to p.sub.b(i+1), the flow rate being approximated by:

[00012]$\begin{array}{cc}{Q}_{\mathrm{valve}}\left(p,i+1\right)=Q_{\mathrm{valve}}\left(p_b\left(i\right),i+1\right).Math.\frac{p}{p_b}=\left[Q_{\mathrm{valve}}\left(p_b\left(i\right),i\right)+.Math..Math.Q.Math.\frac{p}{p_b}\right]& \left(19\right)\end{array}$

Finally, the equation of the CSF dynamics state of the system satisfies to:

[00013]$\begin{array}{cc}\frac{\mathrm{dp}}{\mathrm{dt}}=E\left(p-p_0\right)\left[\frac{p_b\left(i\right)-p}{R_{\mathrm{out}}}+Q_{\mathrm{ext}}+Q_{\mathrm{valve}}\left(p_b\left(i\right),i\right).Math.\left(1-\frac{p}{p_b}\right)-.Math..Math.Q.Math.\frac{p}{p_b}\right]& \left(20\right)\end{array}$

[0325] We assume now that Q.sub.ext=0. This equation can be written in the form:

[00014]$\begin{array}{cc}\mathrm{dt}=\frac{\mathrm{dp}}{+.Math..Math.p+.Math..Math.p^2}& \left(21\right)\\ \mathrm{with}& \\ =-{\mathrm{Ep}}_0\left(Q_{\mathrm{valve}}\left(p_b\left(i\right),i\right)+\frac{p_b}{R_{\mathrm{out}}}\right)& \left(22\right)\\ =E\left(\frac{p_b+p_0}{R_{\mathrm{out}}}+Q_{\mathrm{valve}}\left(p_b\left(i\right),i\right)\left[1+\frac{p_0}{p_b}\right]+.Math..Math.Q.Math.\frac{p_0}{p_b}\right)& \left(23\right)\\ =-E\left[\frac{1}{R_{\mathrm{out}}}+\frac{1}{p_b}.Math.\left(Q_{\mathrm{valve}}\left(p_b\left(i\right),i\right)+.Math..Math.Q\right)\right]& \left(24\right)\end{array}$

[0326] General solutions of this equation are:

[00015]$\begin{array}{cc}\frac{2.Math.{\tan }^{-1}\left(\frac{+2.Math..Math..Math.p}{\sqrt{4.Math.-{}^2}}\right)}{\sqrt{4.Math.-{}^2}}& \left(25\right)\end{array}$

boundary conditions are:

[00016]$\begin{array}{cc}.Math.p\left(t=0\right)=p_b\left(i\right)& \left(26\right)\\ .Math.\mathrm{and}& \\ p\left(t=\right)=p_b\left(i+1\right)=\left(\frac{Q_{\mathrm{valve}}\left(p_b\left(i\right),i\right)}{Q_{\mathrm{valve}}(p_b\left(i\right),i+.Math..Math.Q}\right).Math.p_b\left(i\right)=\frac{p_b\left(i\right)}{1+\frac{.Math..Math.Q}{Q_{\mathrm{valve}}\left(p_b\left(i\right),i\right)}}& \left(27\right)\end{array}$

The unknown parameters E, R.sub.out and p.sub.0 can be determined by fitting the curve p(t) with the function tan.sup.1.

[0327] Method: Setting +1 and Patient in Vertical Position

[0328] The same test can be performed in vertical position, the analysis being simplified in the range of pressure p.sub.b(i)+P.sub.reg to p.sub.b(i) wherein the flow rate through the valve is constant:

[0329] For p.sub.b(i)<p<p.sub.b(i)+P.sub.reg:

Q.sub.valve(p+HP,i+1)=Q.sub.valve(p.sub.b(i)+HP,i)+Q(28)

Where the hydrostatic pressure is noted HP. We obtain

[00017]$\begin{array}{cc}\frac{\mathrm{dp}}{\mathrm{dt}}=.Math.E\left(p-p_0\right).Math..Math.\left[\frac{p_b\left(i\right)-p}{R_{\mathrm{out}}}+Q_{\mathrm{ext}}+Q_{\mathrm{valve}}\left(p_b\left(i\right)+\mathrm{HP},i\right)-Q_{\mathrm{valve}}\left(p+\mathrm{HP},i+1\right)\right]& \left(29\right)\end{array}$

Within this plateau of pressure, the equation for the CSF dynamics becomes, if we assume that Q.sub.ext=0:

[00018]$\begin{array}{cc}\frac{\mathrm{dp}}{\mathrm{dt}}=E\left(p-p_0\right)\left[\frac{p_b\left(i\right)-p}{R_{\mathrm{out}}}-.Math..Math.Q\right]& \left(30\right)\end{array}$

we get:

[00019]$\begin{array}{cc}\mathrm{dt}=\frac{\mathrm{dp}}{{}^{}+{.Math.}^{}.Math.p+{.Math.}^{}.Math.p^2}& \left(31\right)\end{array}$

[0330] Leading to similar general solutions with the parameters:

[00020]$\begin{array}{cc}{}^{}=-{\mathrm{Ep}}_0\left(\frac{p_b}{R_{\mathrm{out}}}-.Math..Math.Q\right)& \left(32\right)\\ {}^{}=E\left(\frac{p_b+p_0}{R_{\mathrm{out}}}-.Math..Math.Q\right)& \left(33\right)\\ {}^{}=-\frac{E}{R_{\mathrm{out}}}& \left(34\right)\end{array}$

with the boundary condition

p(t=0)=p.sub.b(i)(35)

[0331] Method: setting 1 and patient in decubitus position.

[0332] By changing the setting of the device of 1, we get:

[00021]$\begin{array}{cc}\frac{\mathrm{dp}}{\mathrm{dt}}=E\left(p-p_0\right)\left[\frac{p_b\left(i\right)-p}{R_{\mathrm{out}}}+Q_{\mathrm{ext}}+Q_{\mathrm{valve}}\left(p_b\left(i\right),i\right)-Q_{\mathrm{valve}}\left(p,i-1\right)\right]& \left(36\right)\end{array}$

We assume again that Q.sub.ext=0 and we restrict first the analysis on the plateau of the Q(p) curve, wherein the flow rate through the valve is constant. We assume also in a first approximation that P.sub.reg does not depends on i (see FIG. 11). The functioning point of de device jumps of Q from the curve i to i1 after change of the selector setting of 1, the pressure being equal to p.sub.b(i) at t=0 according to FIG. 11.

[0333] For p.sub.b(i)<p<p.sub.b(i)+P.sub.reg the flow rate is constant:

Q.sub.valve(p,i1)=Q.sub.valve(p.sub.b(i),i)Q(37)

For p.sub.b(i)<p<p.sub.b(i)+P.sub.reg the equation of the CSF dynamics state of the system satisfies to:

[00022]$\begin{array}{cc}\frac{\mathrm{dp}}{\mathrm{dt}}=E\left(p-p_0\right)\left[\frac{p_b\left(i\right)-p}{R_{\mathrm{out}}}+.Math..Math.Q\right]& \left(38\right)\end{array}$

we get:

[00023]$\begin{array}{cc}\mathrm{dt}=\frac{\mathrm{dp}}{{}^{}+{}^{}.Math.p+{}^{}.Math..Math.p^2}& \left(39\right)\end{array}$

Leading to similar general solutions with the parameters:

[00024]$\begin{array}{cc}{}^{}=-{\mathrm{Ep}}_0\left(.Math..Math.Q+\frac{p_b}{R_{\mathrm{out}}}\right)& \left(40\right)\\ {}^{}=E\left(\frac{p_b+p_0}{R_{\mathrm{out}}}+.Math..Math.Q\right)& \left(41\right)\\ {}^{}=-\frac{E}{R_{\mathrm{out}}}& \left(42\right)\end{array}$

with the boundary condition

p(t=0)=p.sub.b(i)(43)

[0334] For p>pb(i)+Preg the flow rate through the valve is:

[00025]$\begin{array}{cc}{Q}_{\mathrm{valve}}\left(p,i-1\right)=p\left(\frac{Q_{\mathrm{valve}}\left(p_b\left(i\right),i\right)-.Math..Math.Q}{p_b\left(i\right)+.Math..Math.p_{\mathrm{reg}}}\right)& \left(44\right)\end{array}$

This part of the curve could be analyzed using the same kind of general solutions, the boundary conditions including continuity conditions between the two ranges of pressure.

[0335] Method: Setting 1 and Patient in Vertical Position

[0336] The same test can be performed the patient being in vertical position, the same formula applies by changing p.sub.reg by HP, the hydrostatic pressure, those pressures being assumed to be equal in a first approximation. In the latter case, we get:

[00026]$\begin{array}{cc}\frac{\mathrm{dp}}{\mathrm{dt}}=E\left(p-p_0\right)\left[\frac{p_b\left(i\right)-p}{R_{\mathrm{out}}}+Q_{\mathrm{ext}}+Q_{\mathrm{valve}}\left(p_b\left(i\right)+\mathrm{HP},i\right)-Q_{\mathrm{valve}}\left(p+\mathrm{HP},i-1\right)\right]& \left(45\right)\end{array}$

[0337] We assume that Q.sub.ext=0. We can write, according to FIG. 11, with HP=p.sub.reg:

[00027]$\begin{array}{cc}{Q}_{\mathrm{valve}}\left(p+\mathrm{HP},i-1\right)=\left(\frac{Q_{\mathrm{valve}}\left(p_b\left(i\right)+\mathrm{HP},i\right)-.Math..Math.Q}{p_b\left(i\right)+\mathrm{HP}}\right).Math.\left(p+\mathrm{HP}\right)& \left(46\right)\end{array}$

Therefore the differential equation for the CSF pressure is:

[00028]$\begin{array}{cc}\frac{\mathrm{dp}}{\mathrm{dt}}=E.Math..Math.\left(p-p_0\right)[.Math..Math.\frac{p_b\left(i\right)-p}{R_{\mathrm{out}}}+.Math..Math.Q_{\mathrm{valve}}\left(p_b\left(i\right)+\mathrm{HP},i\right).Math.\left(1-\frac{p+\mathrm{HP}}{p_b\left(i\right)+\mathrm{HP}}\right)-.Math..Math.Q\left(\frac{p+\mathrm{HP}}{p_b\left(i\right)+\mathrm{HP}}\right)]& \left(47\right)\end{array}$

We finally solve this equation using general formulae

[00029]$\begin{array}{cc}\mathrm{dt}=\frac{\mathrm{dp}}{{{{}^{}}^{}}^{}+{{{}^{}}^{}}^{}.Math.p+{{{}^{}}^{}}^{}.Math.p^2}& \left(48\right)\\ \mathrm{where}& \\ {{{}^{}}^{}}^{}=-E_{p_0}\left(Q_{\mathrm{valve}}\left(p_b\left(i\right)+\mathrm{HP},i\right)+\frac{p_b}{R_{\mathrm{out}}}-\left(\frac{Q_{\mathrm{valve}}\left(p_b\left(i\right)+\mathrm{HP},i\right)+.Math..Math.Q}{1+\frac{p_b}{\mathrm{HP}}}\right)\right)& \left(49\right)\\ {{{}^{}}^{}}^{}=E\left(\frac{p_b+p_0}{R_{\mathrm{out}}}+Q_{\mathrm{valve}}\left(p_b\left(i\right)+\mathrm{HP},i\right)\left[1+\frac{p_0-\mathrm{HP}}{p_b+\mathrm{HP}}\right]+.Math..Math.Q\left(\frac{p_0-\mathrm{HP}}{p_b+\mathrm{HP}}\right)\right)& \left(50\right)\\ \mathrm{and}& \\ {{{}^{}}^{}}^{}=-E\left[\frac{1}{R_{\mathrm{out}}}+\frac{Q_{\mathrm{valve}}\left(p_b\left(i\right)+\mathrm{HP},i\right)+.Math..Math.Q}{p_b+\mathrm{HP}}\right]& \left(51\right)\end{array}$

[0338] In practice, a preliminary infusion test is performed to determine if a shunt shall be placed or not. During this test it is possible to determine these parameters E, R.sub.out, p.sub.0 and even PP if a complete model is used.

[0339] The best strategy is therefore to use these parameters as constant and to check after implantation the good functioning of the device. In functioning, it is also possible to determine the change of E and R.sub.out with time in a non-invasive way by simply changing the setting of the device and by monitoring the pressure signal with time.

[0340] In sum, below some of the features of the method are highlighted. [0341] Patient in decubitus [0342] Change the setting of the device of one unit [0343] Monitor the evolution of the pressure [0344] Analyse the pressure curve to determine the best fitting parameters E (brain elastance), R.sub.out, P.sub.0 and/or PP. [0345] Optionally, compare these values with preliminary data obtained before the shunt placement during an infusion test or just after the shunt implantation using this method [0346] Optionally, check if the potential change of pressure curve is due to change of the CSF outflow characteristics or a shunt failure

[0347] Thus the method for a non-invasive characterization of the CSF dynamics may comprise the following steps: [0348] Provide a medical device adapted to regulate a variable (such as a flow rate or a pressure) of the fluid which flows from the cranial cavity to another cavity of the body patient, the medical device comprising: [0349] An inlet located in the cranial cavity, in which the fluid enters in the medical device [0350] An outlet located in the other cavity, by which the fluid flows out the medical device [0351] An adjustable valve in fluid communication with the inlet and the outlet of the medical device, said adjustable valve being adapted to be set by a caregiver in such a way as to adjust the fluid variable, [0352] A pressure sensor which senses the differential pressure between the cranial cavity and the other cavity [0353] Change the fluid variable according to a new setting [0354] Monitor over time the differential pressure, [0355] Analyse the curve of the monitored differential pressure, for example using a mathematical model derived from the cerebrospinal fluid conservation [0356] Determine E (brain elastance), Rout, P0 and/or PP (for example by using a mathematical model)

[0357] An alternative method could be: [0358] The parameters E, R.sub.out, P.sub.0 and PP are determined after implantation by an injection test via the filling of the reservoir septum with a needle and the monitoring of the pressure with time [0359] The change of setting test as described before is then performed to check the good functioning of the device by fitting the curve p(t) using the parameters obtained during the injection test

[0360] A General Mathematical Approach is Given Below:

[0361] We consider the full model that includes the peritoneum pressure PP. We only derive the equation for the method +1 in decubitus, other formula can be derived easily by applying the same formalism.

[0362] At rest, the equation of mass conservation writes:

[00030]$\begin{array}{cc}{Q}_f=\frac{p_b\left(i\right)-p_{\mathrm{ss}}}{R_{\mathrm{out}}}+Q_{\mathrm{valve}}\left(p_b\left(i\right)-\mathrm{PP},i\right)& \left(52\right)\end{array}$

We can rewrite the full equation after the change of the setting of +1:

[00031]$\begin{array}{cc}\frac{p_b\left(i\right)-p_{\mathrm{ss}}}{R_{\mathrm{out}}}+Q_{\mathrm{valve}}\left(p_b\left(i\right)-\mathrm{PP},i\right)+Q_{\mathrm{ext}}=\frac{p-p_{\mathrm{ss}}}{R_{\mathrm{out}}}+\frac{1}{E\left(p-p_0\right)}.Math.\frac{\mathrm{dp}}{\mathrm{dt}}+Q_{\mathrm{valve}}\left(p-\mathrm{PP},i+1\right)& \left(53\right)\\ \mathrm{Therefore}& \\ \frac{\mathrm{dp}}{\mathrm{dt}}=E\left(p-p_0\right)\left[\frac{p_b\left(i\right)-p}{R_{\mathrm{out}}}+Q_{\mathrm{ext}}+Q_{\mathrm{valve}}\left(p_b\left(i\right)-\mathrm{PP},i\right)-Q_{\mathrm{valve}}\left(p-\mathrm{PP},i+1\right)\right]& \left(54\right)\end{array}$

The functioning point of de device jumps of +Q from the curve i to i+1 after change of the selector setting of +1, the pressure drop in the device being equal to p.sub.b(i)PP at t=0. There is then a relaxation up to an ICP equal to p.sub.b(i+1), the flow rate being approximated by:

[00032]$\begin{array}{cc}{Q}_{\mathrm{valve}}\left(p-\mathrm{PP},i+1\right)=Q_{\mathrm{valve}}\left(p_b\left(i\right)-\mathrm{PP},i+1\right).Math.\left(\frac{p-\mathrm{PP}}{p_b-\mathrm{PP}}\right)=\left[Q_{\mathrm{valve}}\left(p_b\left(i\right)-\mathrm{PP},i\right)+.Math..Math.Q\right].Math.\left(\frac{p-\mathrm{PP}}{p_b-\mathrm{PP}}\right)& \left(55\right)\end{array}$

Finally the equation of the CSF dynamics state of the system satisfies to:

[00033]$\begin{array}{cc}\frac{\mathrm{dp}}{\mathrm{dt}}=E\left(p-p_0\right)\left[\frac{p_b\left(i\right)-p}{R_{\mathrm{out}}}+Q_{\mathrm{ext}}+Q_{\mathrm{valve}}\left(p_b\left(i\right)-\mathrm{PP},i\right).Math.\left(1-\frac{p-\mathrm{PP}}{p_b-\mathrm{PP}}\right)-.Math..Math.Q\left(\frac{p-\mathrm{PP}}{p_b-\mathrm{PP}}\right)\right]& \left(56\right)\end{array}$

[0363] We assume now that Q.sub.ext=0. This equation can be written in the form:

[00034]$\begin{array}{cc}\mathrm{dt}=\frac{\mathrm{dp}}{a+\mathrm{bp}+{\mathrm{cp}}^2}& \left(57\right)\\ \mathrm{with}& \\ a=-{\mathrm{Ep}}_0(.Math.Q_{\mathrm{valve}}\left(p_b-\mathrm{PP},i\right).Math.\left(1+\frac{1}{\frac{p_b}{\mathrm{PP}}-1}\right)+\frac{p_b}{R_{\mathrm{out}}}+.Math..Math.Q\left(\frac{1}{\frac{p_b}{\mathrm{PP}}-1}\right))& \left(58\right)\\ b=E\left(\frac{p_b+p_0}{R_{\mathrm{out}}}+Q_{\mathrm{valve}}\left(p_b\left(i\right)-\mathrm{PP},i\right)\left[1+\frac{p_0+\mathrm{PP}}{p_b-\mathrm{PP}}\right]+.Math..Math.Q\left(\frac{p_0+\mathrm{PP}}{p_b-\mathrm{PP}}\right)\right)& \left(59\right)\\ c=-E\left[\frac{1}{R_{\mathrm{out}}}+\frac{\left(Q_{\mathrm{valve}}\left(p_b\left(i\right)-\mathrm{PP},i\right)+.Math..Math.Q\right)}{p_b-\mathrm{PP}}\right]& \left(60\right)\end{array}$

General solutions of this equation are:

[00035]$\begin{array}{cc}\frac{2.Math..Math.{\tan }^{-1}\left(\frac{b+2.Math..Math.\mathrm{cp}}{\sqrt{4.Math..Math.\mathrm{ac}-b^2}}\right)}{\sqrt{4.Math.\mathrm{ac}-b^2}}& \left(61\right)\end{array}$

Boundary conditions are:

[00036]$\begin{array}{cc}p\left(t=0\right)=p_b\left(i\right)& \left(62\right)\\ \mathrm{and}& \\ p\left(t=\right)=p_b\left(i+1\right)=\frac{p_b\left(i\right)-\mathrm{PP}}{1+\frac{.Math..Math.Q}{Q_{\mathrm{valve}}\left(p_b\left(i\right)-\mathrm{PP},i\right)}}& \left(63\right)\end{array}$

The unknown parameters E, R.sub.out, PP and p.sub.0 can be determined by fitting the curve p(t) with the function tan.sup.1.

LIST OF ELEMENTS

[0364] The following reference numerals have been used to designate some of the elements. [0365] 1 Membrane [0366] 2 Substrate [0367] 3 Inlet hole [0368] 4 Outlet hole [0369] 5 Flexible part [0370] 6 Mesa or rigid part [0371] 7 O-ring [0372] 8 Housing [0373] 9 Adjustment element [0374] 10 Cavity [0375] 11 Lower face [0376] 12 Upper face [0377] 13 Ball [0378] 14 Blade [0379] 15 Inlet of the medical device [0380] 16 Outlet of the medical device [0381] 17 Adjusting means [0382] 18 Outside the body [0383] 19 Inside the body

[0384] While the invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments, and equivalents thereof, are possible without departing from the sphere and scope of the invention. Accordingly, it is intended that the invention not be limited to the described embodiments, and be given the broadest reasonable interpretation in accordance with the language of the appended claims.