Infusion device and method for administering a medical fluid to a patient

Abstract

An infusion device (1) for administering a medical fluid to a patient (P) comprises a pump mechanism (11) for administering a medical fluid to a patient (P) through a line set (3), a sensor device (14) for measuring a measurement value indicative of a pressure in the line set (3), and a processor device (15) configured to automatically set, in an initial phase, a threshold value (15) to a value determined based on at least a mechanical characteristic of the line set (3) and/or the pump mechanism (11) and to determine, in an operational phase, whether an occlusion condition in the line set (3) is present based on the measurement value and the threshold value (P.sub.thres) —Herein, the processor device (15) is configured to adjust the threshold value (P.sub.thres) if in the operational phase a pressure variation condition in the line set (3) different than an occlusion condition is detected by the processor device (15). In this way an infusion device is provided which may be able to reliably detect an occlusion condition even in case of multiple infusion devices acting on a line set in a multiline infusion.

Claims

1. An infusion device for administering a medical fluid to a patient, comprising: a pump mechanism for administering a medical fluid to a patient through a line set, a sensor for measuring a measurement value indicative of a pressure in the line set, and a processor configured to automatically set, in an initial phase, a threshold value to a value determined based on at least a mechanical characteristic of at least one of the line set and the pump mechanism and to determine, in an operational phase, whether an occlusion condition in the line set is present based on the measurement value and the threshold value, wherein the processor is configured to increase the threshold value if in the operational phase a pressure variation condition in the line set different than an occlusion condition is detected by the processor, wherein, for detecting the pressure variation condition, a slope value indicative of a slope of the pressure in the line set is determined based on measurement values obtained from the sensor, and wherein the processor is configured to conclude that the pressure variation condition different than the occlusion condition is present if the slope value deviates from an expected slope value by more than a predetermined margin.

2. The infusion device according to claim 1, wherein the expected slope value is determined by taking one or more of a compliance of a syringe connected to the line set, a compliance of the line set, a stiffness of the pump mechanism and a dimension of the syringe into account.

3. The infusion device according to claim 2, further comprising a memory configured to store values for one or more of the compliance of at least one syringe, the compliance of at least one line set, the stiffness of the pump mechanism and the dimension of at least one syringe.

4. The infusion device according to claim 1, wherein the processor is configured to conclude that the pressure variation condition different than the occlusion condition is present based on a condition that the slope value is larger than the expected slope value times a first predetermined constant.

5. The infusion device according to claim 1, wherein the processor is configured to set the threshold value to a value larger than the value of the threshold value set during the initial phase if the pressure variation condition different than the occlusion condition is detected.

6. The infusion device according to claim 1, wherein the processor is configured to reset the threshold value to the value of the threshold value set during the initial phase based on a condition that the slope value falls below the expected slope value times a second predetermined constant.

7. The infusion device according to claim 1, wherein the processor is configured, for determining the value of the threshold value during the initial phase, to determine a first threshold value candidate based on a desired time between a time of an occurrence of an occlusion in the line set and a time of the determining of the occlusion condition by the processor.

8. The infusion device according to claim 7, wherein the processor is configured to compute the first threshold value candidate using a stored compliance value, a set flow rate and the desired time.

9. The infusion device according to claim 8, wherein the processor is configured to compute the first threshold value candidate using the following equation: $P_{\mathrm{thresh},1}\left[\mathrm{bar}\right]=\frac{T_{\mathrm{desired}}\left[h\right].Math.f\left[\mathrm{ml}/h\right]}{C\left[\mathrm{ml}/\mathrm{bar}\right]}$ wherein P.sub.thres,1 represents the first threshold value candidate (in bar), T.sub.desired represents the desired time (in hours), C represents the stored compliance value (in ml/bar), and f represents the set flow rate (in ml/h).

10. The infusion device according to claim 8, further comprising a memory configured to store a multiplicity of compliance values associated with a multiplicity of at least one of different syringes and different line sets.

11. The infusion device according to claim 7, wherein the infusion device is a syringe pump comprising a receptacle for receiving a syringe having a tube containing a medical fluid and a piston movable with respect to the tube, and a pusher device for acting onto the piston for pumping the medical fluid from the tube towards a patient, wherein the processor is configured, for determining the value of the threshold value during the initial phase, to determine a second threshold value candidate computed based on a force error estimate of a possible deviation between an expected frictional force and a true frictional force occurring when moving the piston relative to the tube.

12. The infusion device according to claim 11, wherein the second threshold value candidate is computed according to the following equation: $P_{\mathrm{thresh},2}\left[\mathrm{bar}\right]=\frac{F_{est}\left[gf\right]}{10.2.Math.S\left[{\mathrm{mm}}^2\right]}$ wherein P.sub.thres,2 represents the second threshold value candidate (in bar), F.sub.est represents the force error estimate (in gram force (gf)), and S represents the effective cross sectional surface (in mm.sup.2) of the tube, defined as S=π.Math.(D/2).sup.2, D being an inner diameter of the cylindrical tube.

13. The infusion device according to claim 11, wherein the threshold value during the initial phase is set to be a maximum of the first threshold value candidate and the second threshold value candidate.

14. A method for controlling an infusion device for administering a medical fluid to a patient, comprising: in an initial phase automatically setting, using a processor of the infusion device, a threshold value to a value determined based on at least a mechanical characteristic of at least one of a line set and a pump mechanism for administering a medical fluid to a patient through the line set, and in an operational phase determining, using the processor, whether an occlusion condition in the line set is present based on a measurement value indicative of a pressure in the line set and the threshold value, and increasing the threshold value if in the operational phase a pressure variation condition in the line set different than an occlusion condition is detected by the processor, wherein, for detecting the pressure variation condition, a slope value indicative of a slope of the pressure in the line set is determined based on measurement values obtained from a sensor, and wherein the processor is configured to conclude that the pressure variation condition different than the occlusion condition is present if the slope value deviates from an expected slope value by more than a predetermined margin.

15. An infusion device for administering a medical fluid to a patient, comprising: a pump mechanism for administering a medical fluid to a patient through a line set, a sensor for measuring a measurement value indicative of a pressure in the line set, and a processor configured to automatically set, in an initial phase, a threshold value to a value determined based on at least a mechanical characteristic of at least one of the line set and the pump mechanism and to determine, in an operational phase, whether an occlusion condition in the line set is present based on the measurement value and the threshold value, wherein the processor is configured to increase the threshold value if in the operational phase a pressure variation condition in the line set different than an occlusion condition is detected by the processor based on a comparison of the pressure variation condition and the occlusion condition, wherein the processor is configured, for determining the value of the threshold value during the initial phase, to determine a first threshold value candidate based on a desired time between a time of an occurrence of an occlusion in the line set and a time of the determining of the occlusion condition by the processor.

16. The infusion device according to claim 15, wherein the processor is configured to compute the first threshold value candidate using a stored compliance value, a set flow rate and the desired time.

17. The infusion device according to claim 16, wherein the processor is configured to compute the first threshold value candidate using the following equation: $P_{\mathrm{thresh},1}\left[\mathrm{bar}\right]=\frac{T_{\mathrm{desired}}\left[h\right].Math.f\left[\mathrm{ml}/h\right]}{C\left[\mathrm{ml}/\mathrm{bar}\right]}$ wherein P.sub.thres,1 represents the first threshold value candidate (in bar), T.sub.desired represents the desired time (in hours), C represents the stored compliance value (in ml/bar), and f represents the set flow rate (in ml/h).

18. The infusion device according to claim 16, further comprising a memory configured to store a multiplicity of compliance values associated with a multiplicity of at least one of different syringes and different line sets.

19. The infusion device according to claim 15, wherein the infusion device is a syringe pump comprising a receptacle for receiving a syringe having a tube containing a medical fluid and a piston movable with respect to the tube, and a pusher device for acting onto the piston for pumping the medical fluid from the tube towards a patient, wherein the processor is configured, for determining the value of the threshold value during the initial phase, to determine a second threshold value candidate computed based on a force error estimate of a possible deviation between an expected frictional force and a true frictional force occurring when moving the piston relative to the tube.

20. The infusion device according to claim 19, wherein the second threshold value candidate is computed according to the following equation: $P_{\mathrm{thresh},2}\left[\mathrm{bar}\right]=\frac{F_{est}\left[gf\right]}{10.2.Math.S\left[{\mathrm{mm}}^2\right]}$ wherein P.sub.thres,2 represents the second threshold value candidate (in bar), F.sub.est represents the force error estimate (in gram force (gf)), and S represents the effective cross sectional surface (in mm.sup.2) of the tube, defined as S=π.Math.(D/2).sup.2, D being an inner diameter of the cylindrical tube, and the threshold value, during the initial phase is set to be a maximum of the first threshold value candidate and the second threshold value candidate.

Description

(1) The idea of the invention shall subsequently be described in more detail with reference to the embodiments shown in the figures. Herein:

(2) FIG. 1 shows a view of an infusion device constituted as a syringe pump;

(3) FIG. 2 shows a schematic diagram of a pressure rise over time in case of an occlusion; and

(4) FIG. 3 shows a schematic of a multiline infusion using multiple infusion devices.

(5) FIG. 1 shows an embodiment of an infusion device 1 in the shape of a syringe pump. The infusion device 1 comprises a housing 10 having a front face 100 and a display device 13 arranged thereon. The display device 13 may for example be a touch-sensitive display allowing a user to enter commands for operation of the infusion device 1 and displaying operational information regarding the process of an actual infusion operation.

(6) The infusion device 1 comprises a receptacle 12 in which a syringe 2 having a cylindrical tube 20 is arranged. A piston 21 is movable within the cylindrical tube 20 and is in engagement with a pusher device 11 of a pumping mechanism of the infusion device 1. At an end of the cylindrical tube 20 opposite the piston 21 a line set 3 extends from the cylindrical tube 20 towards a patient B, the line set 3 being connected to the cylindrical tube 20 at an end 30 and to the patient B at an end 31.

(7) The piston 21 comprises a head 210 facing away from the cylindrical tube 20 and being in abutment with the pusher device 11 of the infusion device 1. During operation of the infusion device 1, the pusher device 11 is electromotorically driven in an actuation direction A such that the piston 21 is moved into the cylindrical tube 20 and a medical fluid contained in the cylindrical tube 20 is delivered via the line set 3 towards the patient B.

(8) The infusion device 1 comprises a processor device 15 and a storage device 16. Via the processor device 15 the infusion operation of the infusion device 1 is controlled. In the storage device 16 operational parameters, such as mechanical characteristics of the syringe 2 used on the infusion device 1 as well as operational data, may be stored.

(9) During an infusion process a medical fluid, for example a medication or a nutritional fluid for the parenteral feeding of a patient or the like, is delivered from the cylindrical tube 20 via the line set 3 towards the patient B. For this, the piston 21 is continuously pushed into the cylindrical tube 20 in an actuation direction A such that a desired flow rate is obtained, which is programmed by a user prior to the start of the infusion operation.

(10) The line set 3 generally is made of a flexible tubing made for example from a PVC material. The line set 3 extends from the cylindrical tube 20 to the patient B and is, at its first end 30, in fluid connection with the cylindrical tube 20 and, at its second end 31, for example connected to a needle for providing an intravenous access to the patient B. During an infusion process an occlusion O in the line set 3 must be avoided and, if it nevertheless occurs, must be detected such that appropriate countermeasures to overcome the occlusion O can be taken. For this, a force sensor 14 is placed on the pusher device 11 facing the head 210 of the piston 21 for measuring a force exerted on the piston 21 during an infusion process. From a force measured by means of the force sensor 14 an estimate of the pressure within the syringe 2 can be obtained, such that the pressure within the syringe 2 and the line set 3 can be monitored. If it is found that the pressure within the syringe 2 and the line set 3 rises beyond a permissible threshold value, an alarm is triggered indicating that an occlusion O may be present in the system.

(11) FIG. 2 shows in a schematic diagram the pressure P over time t in case of an occlusion O. Generally, the pressure P is very small (almost 0) during normal infusion operation in case no occlusion O is present (see the pressure P prior to the time t0). If at the time t0 an occlusion O occurs, the pressure P will start to rise and will continue to rise (if the occlusion O does not disappear) until a threshold value P.sub.thres is exceeded, at which moment an alarm is triggered by the processor device 15 such that a user is warned of the occlusion O.

(12) The occlusion O, in the example of FIG. 2, occurs at time t0. Until the pressure threshold value P.sub.thres actually is exceeded by the pressure curve P at time t1, a substantial time duration T may pass, due to the continuous rise of the pressure P following the occlusion O at a finite slope, the slope of the pressure rise herein depending on a multiplicity of factors, for example the flow rate and the compliance of the system, in particular the compliance of the cylindrical tube 20 of the syringe 2 and of the line set 3 extending in between the cylindrical tube 20 and the patient B.

(13) Generally, the pressure in the line set 3 will rise according to the following equation:

(14) $P=\frac{f.Math.t}{C}.$

(15) Herein, P denotes the pressure, f denotes the flow rate, t denotes the time, and C denotes the compliance.

(16) Making use of the above equation, a threshold value P.sub.thres may be determined—in an initial phase prior to the actual operation for administering a medical solution to a patient B—in order to obtain a fast occlusion detection. Namely, if the compliance C and the flow rate f is known, by assuming a desired time which shall lapse between the actual occurrence of an occlusion and the moment at which the pressure exceeds the pressure threshold value P.sub.thres an optimum pressure threshold P.sub.thres can be determined, which is low such that a fast occlusion detection may be obtained.

(17) If however the pressure threshold value P.sub.thres is chosen too small, false alarms may be triggered, which may hinder the usability of the infusion device 1.

(18) Hence, in one embodiment, within an infusion device 1 the pressure threshold value P.sub.thres may be chosen according to two criteria, the first criteria taking into account the desired time and the second criteria serving to limit the likelihood for false alarms.

(19) According to the first criteria, a first pressure threshold value candidate can for example be determined according to the following equation:

(20) $P_{\mathrm{thres},1}\left[\mathrm{bar}\right]=\frac{T_{\mathrm{desired}}\left[h\right].Math.f\left[\mathrm{ml}/h\right]}{C\left[\mathrm{ml}/\mathrm{bar}\right]}.$

(21) Herein, P.sub.thres,1 represents the first threshold value candidate in bar, T.sub.desired represents the desired time (in hours) between the time t0 of an occurrence of an occlusion O in the line set 3 and the time t1 at which the pressure value exceeds the threshold value P.sub.thres, C represents the compliance value (in ml/bar), and f represents the flow rate (in ml/h).

(22) The compliance value C associated with the syringe 2 and the line set 3 is stored in the storage device 16 of the infusion device 1. The storage device 16 in this regard may store a multiplicity of compliance values C for different syringes 2 and different line sets 3 such that, by inputting for example the type of syringe 2 to the infusion device 1, the processor device 15 may refer to the compliance value C associated with the particular syringe 2 used on the system for computing the estimate of the duration T.

(23) The storage device 16 may for example store a compliance value for a syringe 2 of a particular manufacturer and a particular volume. The storage device 16 may in addition store a default value for a syringe 2 of a particular volume, which may be used in case a particular syringe 2 of a particular manufacturer is not explicitly defined in the infusion device 1. A line set 3 may be identified by its length, its inner and/or outer diameter or the like and by its manufacturer, and associated with a particular type of line set 3 a particular compliance value may be stored in the storage device 16.

(24) The storage device 16 may store a constant value for the compliance. Just as well it is conceivable that the storage device 6 stores a nonlinear relation for the compliance depending for example on the pressure in the system.

(25) If this first pressure threshold value candidate would be chosen as the pressure threshold value P.sub.thres, a fast occlusion detection would be obtained. This however may potentially come at the expense of a possibly increased likelihood of a false alarm if the pressure threshold value P.sub.thres is chosen too small in this way.

(26) Therefore, a second criteria may—optionally—be applied in addition to determine the threshold value P.sub.thres.

(27) The second criteria may be derived based on the following background:

(28) To observe the pressure in the line set 3, the force applied to the piston head 210 of the piston 21 by means of the pusher device 11 is measured by the sensor 14 placed in between the pusher device 11 and the piston head 210. The force measured in this way allows for an indirect measurement of the pressure within the cylindrical tube 20, which generally equals the pressure in the line set 3. In particular, the pressure in the cylindrical tube 20 depends on the measured force according to the following relation:

(29) $P=\frac{F-F_0}{S}.$

(30) Herein, P denotes the pressure, F denotes the measured force, F.sub.0 denotes a frictional force component and S denotes the effective surface by which the piston 21 acts onto the liquid contained in the cylindrical tube 20. The effective surface S is substantially determined by the inner diameter of the cylindrical tube 20.

(31) By determining the pressure P in this way and by comparing the determined pressure P to a predefined threshold P.sub.thres it can then be concluded whether an occlusion O is present in the line set 3 or not. In particular, if it is found that the pressure P rises above the threshold P.sub.thres, it is concluded that an occlusion O is present.

(32) Whereas F is measured and S is known from the geometrical dimensions of the cylindrical tube 20 of the syringe 2, the frictional force component F.sub.0 may vary in dependence on the specific syringe 2 used on the system, wherein the frictional force component F.sub.0 generally is dependent on the position of the piston 21 within the cylindrical tube 20 and on the velocity by which the piston 21 is moved relative to the cylindrical tube 20 during an infusion process.

(33) The frictional force component F.sub.0 may for example depend on the following parameters: the syringe brand, model and batch, the pushing velocity, the position of the piston on its full travel range, the temperature, the waiting time between syringe preparation and infusion start, the liquid inside the syringe, and the pressure. (The catheter size, the extension line diameter and length and the drug viscosity generally can be considered to have no influence on the frictional force. But these parameters may of course have an influence on the pressure.)

(34) In addition, the structural characteristics in particular of the cylindrical tube 20 in which the piston 21 is moved may vary along the travel range of the piston 21 relative to the cylindrical tube 20. In particular, the cylindrical tube 20 may not exhibit a constant diameter, but the diameter may (slightly) change over position, i.e. it may decrease or increase. From such structural variations, a variation of the frictional force over the position may arise.

(35) Hence, for a particular syringe of a particular model, a particular batch, a particular volume and a particular brand a very specific dependence of the frictional force on the position may arise. Generally, the frictional force may be obtained from a statistical analysis by measuring different syringes of different types, models and volumes with respect to their friction as a function of position of the piston 21 relative to the tube 20 and, possibly, also as a function of the velocity by which the piston 21 is moved relative to the tube 20. From such statistical analysis for example a mean frictional force associated with a particular syringe (of a particular type, model, brand and volume) may be stored in the storage device 16. In addition, also the standard deviation of this mean frictional force may be stored for the particular syringe, the standard deviation indicating, as known from statistics, a possible deviation of a true frictional force from the stored mean frictional force.

(36) Generally, if a particular syringe 2 is used and if the true frictional force occurring between the piston 21 and the tube 20 is equal or at least close to the mean frictional force, the pressure value derived from the equation

(37) $P=\frac{F-F_0}{S}.$
will be (close to) 0 if no occlusion O is present, because it can be assumed that the measured force F is equal (or at least close) to the mean frictional force used as frictional force component F0 in the above equation. If however the true frictional force significantly deviates from the stored mean frictional force and if the stored mean frictional force is used in the above equation, the derived pressure value P deviates from the actual pressure P in the line set 3, which possibly may give rise to false alarms.

(38) The likelihood for false alarms hence depends on the possible deviation of the true frictional force from the frictional force component used in the above equation. If in the above equation the stored mean frictional force (obtained for example from a statistical analysis of a multiplicity of syringes of a particular type, model, volume and brand) is used, the standard deviation expresses a possible deviation of a used syringe from the stored mean frictional force.

(39) Hence, the stored standard deviation for a particular syringe (of a particular type, model, volume and brand) can be used to derive a second criteria, namely a second pressure threshold value candidate. For example, by multiplying the standard deviation by a constant factor, for example a factor of 2, a force error estimate may be derived, which then can be used to calculate the second threshold value candidate according to the following equation:

(40) $P_{\mathrm{thres},2}\left[\mathrm{bar}\right]=\frac{F_{\mathrm{est}}\left[\mathrm{gf}\right]}{10.2.Math.S\left[{\mathrm{mm}}^2\right]}$

(41) Herein, P.sub.thres,2 represents the second threshold value candidate (in bar), F.sub.est represents the force error estimate (in gram force (gf)) computed from for example the standard deviation (by multiplying the standard deviation by a constant factor), and S represents the effective cross sectional surface (in mm.sup.2) of the tube (20), defined as S=π.Math.(D/2).sup.2, D being the inner diameter of the cylindrical tube 20.

(42) There are other possibilities to compute the force error estimate F.sub.est. For example, the force error estimate may be computed by multiplying the mean frictional force stored in the storage device 16 by a constant factor, for example a factor of 1.5. Or the force error estimate may be stored directly as a parameter for different syringes of different types, models, volumes and brands within the storage device 16.

(43) The actual pressure threshold P.sub.thres then is chosen in the infusion device 1 as the maximum of the first threshold value candidate and the second threshold value candidate. In addition, limits may be defined, such that the pressure threshold value may not be lower than a lower limit (for example 50 mmHg) and not be higher than an upper limit (for example 900 mmHg).

(44) By applying the above outlined scheme, the pressure threshold value P.sub.thres may be determined by the infusion device 1 in the initial phase prior to the actual operation in an automatic manner. The threshold value P.sub.thres hence does not need to be set by a user, but is determined automatically by the infusion device 1 upon initial configuration of the system.

(45) The automatic computing of the threshold value P.sub.thres is reliable and works well for the occlusion detection during actual operation in particular in situations in which the infusion device 1 acts on a line set 3 having a single infusion line, hence with no other, external pressure sources (such as other infusion devices) present on the line set 3.

(46) However, in multiline infusions such as illustrated in FIG. 3, in which multiple infusion devices 1A, 1B, 1C act on a common line set 3 via infusion lines 300A, 300B, 300C connected to each other at a junction point 32, the pressure within the line set 3 is influenced by all infusion devices 1A, 1B, 1C, such that a pressure variation in the line set 3 does not originate only from a single pressure source, but from the multiple infusion devices 1A, 1B, 1C. This may lead to a situation in which for example a bolus is given by one of the infusion devices 1A, 1B, 1C causing a steep rise of pressure in the line set 3, potentially leading to the observation of a pressure rise at another infusion device 1A, 1B, 1C beyond the threshold value P.sub.thres as automatically determined during the initial phase at this other infusion device 1A, 1B, 1C. Such pressure rise hence may lead to the triggering of a false alarm at the other infusion device 1A, 1B, 1C.

(47) In order to differentiate, at an infusion device 1, an occlusion condition from a pressure variation condition in line set 3 other than an occlusion condition, it herein is proposed to monitor the slope of the pressure rise in the line set 3 versus an expected slope, i.e., a slope that can be expected during an occlusion condition. If it is found at an infusion device 1A, 1B, 1C that the slope of the pressure rise in the system substantially deviates from the expected slope, it can be concluded that the pressure rise is not due to an occlusion in the line set 3, but due to another one of the infusion devices 1A, 1B, 1C, for example due to a bolus given by another infusion device 1A, 1B, 1C.

(48) An occlusion will generally cause a rise of the measured force according to a rather well-defined slope, which can be determined when mechanical characteristics of the system such as the compliance of the line set 3, the compliance of the syringe 2 and the stiffness of the mechanical system of the pusher device 11 are known. The expected slope is the theoretical slope that the pressure should follow in case the line is occluded at the catheter level. It depends for example on the flowrate, the syringe mechanical properties (especially the syringe stopper stiffness), the syringe pump mechanical properties (especially the pusher stiffness), the line set mechanical properties (the tube compliance), and the fluid properties (which can be neglected if it is assumed that the fluid to be pumped is an incompressible liquid).

(49) The pressure slope can either be expressed referring to time or referring to volume. Expressing the expected slope with reference to volume, the expected slope at a position i during movement of the piston 21 of the comes out to be:

(50) $\mathrm{Expected\_slope}\left(i\right)\left[\mathrm{bar}/\mathrm{ml}\right]=\frac{dP\left(i\right)\left[\mathrm{bar}\right]}{\mathrm{dVolume}\left(i\right)\left[\mathrm{ml}\right]}$

(51) The expected slope is equivalent to a volumetric stiffness, which is the inverse of the system compliance. One can therefore write

(52) 0$\frac{1}{\mathrm{Volumetric\_Stiffness}\left[\mathrm{bar}/\mathrm{ml}\right]}=\underset{k=1}{\overset{3}{.Math.}}\frac{1}{\mathrm{Volumetric\_Stiffness}\left(k\right)\left[\mathrm{bar}/\mathrm{ml}\right]}$$\mathrm{Where}\{\begin{array}{c}\mathrm{Volumetric\_Stiffness}\left(1\right)\left[\mathrm{bar}/\mathrm{ml}\right]=\frac{1}{\mathrm{Syringe\_Compliance}\left[\mathrm{ml}/\mathrm{bar}\right]}\\ \mathrm{Volumetric\_Stiffness}\left(2\right)\left[\mathrm{bar}/\mathrm{ml}\right]=\frac{1}{\mathrm{Line\_Compliance}\left[\mathrm{ml}/\mathrm{bar}\right]}\\ \mathrm{Volumetric\_Stiffness}\left(3\right)\left[\mathrm{bar}/\mathrm{ml}\right]=\frac{100.Math.\mathrm{Pusher\_Stiffness}\left[\mathrm{gf}/\mathrm{mm}\right]}{{\mathrm{Syringe\_Surface}\left[{\mathrm{mm}}^2\right]}^2}\end{array}$
and the expected slope comes out to be:

(53) $\mathrm{Expected\_slope}\left[\mathrm{bar}/\mathrm{ml}\right]=\frac{1}{\begin{array}{c}\mathrm{Syringe\_Compliance}\left[\mathrm{ml}/\mathrm{bar}\right]+\\ \mathrm{Line\_Compliance}\left[\mathrm{ml}/\mathrm{bar}\right]+\\ \frac{{\mathrm{Syringe\_Surface}\left[{\mathrm{mm}}^2\right]}^2}{100.Math.\mathrm{Pusher\_Stiffness}\left[\mathrm{gf}/\mathrm{mm}\right]}\end{array}}$

(54) This can be converted to a slope by millimeter, assuming that for a different syringe 1 mm is equivalent to (syringe_Surface S [mm.sup.2]/1000) ml:

(55) $\mathrm{Expected\_slope}\left[\mathrm{bar}/\mathrm{mm}\right]=\frac{\mathrm{Syringe\_Surface}\left[{\mathrm{mm}}^2\right]}{\begin{array}{c}1000.Math.\begin{array}{c}(\mathrm{Syringe\_Compliance}\left[\mathrm{ml}/\mathrm{bar}\right]+\\ \mathrm{Line\_Compliance}\left[\mathrm{ml}/\mathrm{bar}\right])\end{array}+\\ 10.Math.\frac{{\mathrm{Syringe\_Surface}\left[{\mathrm{mm}}^2\right]}^2}{\mathrm{Pusher\_Stiffness}\left[\mathrm{gf}/\mathrm{mm}\right]}\end{array}}$

(56) This also can be converted to gf/mm. Assuming that for a given syringe F[gf]=10.2*P[bar]*S[mm.sup.2], the slope in bar/mm can be converted into a slope in gf/mm:

(57) $\mathrm{Expected\_slope}\left[\mathrm{gf}/\mathrm{mm}\right]=\frac{0.0102.Math.{\mathrm{Syringe\_Surface}\left[{\mathrm{mm}}^2\right]}^2}{\begin{array}{c}\begin{array}{c}(\mathrm{Syringe\_Compliance}\left[\mathrm{ml}/\mathrm{bar}\right]+\\ \mathrm{Line\_Compliance}\left[\mathrm{ml}/\mathrm{bar}\right])\end{array}+\\ \frac{{\mathrm{Syringe\_Surface}\left[{\mathrm{mm}}^2\right]}^2}{\mathrm{Pusher\_Stiffness}\left[\mathrm{gf}/\mathrm{mm}\right]}\end{array}}$

(58) Example parameter values for a 5 cc syringe of a particular brand and a particular infusion device are summarized in Table 3:

(59) TABLE-US-00001 TABLE 3 Parameter Value syringe_Compliance 0.0566 ml/bar Line_Compliance 0.145 ml/bar Pusher_Stiffness 9279 gf/mm syringe inner diameter 11.87 mm syringe surface S 110.66 mm.sup.2

(60) Using these parameters, the following values for the expected slope are obtained:
Expected_Slope[bar/ml]=4.65[bar/ml]
Expected_Slope[bar/mm]=0.514[bar/mm]
Expected_Slope[gf/mm]=568.8[gf/mm]

(61) This expected slope is independent of the flow rate.

(62) Thus, it can be assumed that, in the above case, the expected slope in case of an occlusion will be close to 0.5 bar/mm for the particular syringe and the particular infusion device for which the parameters are valid.

(63) The expected slope may for example be computed for the particular parameters of the line set 3, the syringe 2 and the device 1 in use upon installation of a particular syringe 2 in connection with a line set 3 on the infusion device 1.

(64) In order to allow the computation of the expected slope, the storage device 16 of the infusion device 1 stores relevant parameters of different syringes 2 and line sets 3 which potentially may be used on the infusion device 1. In the storage device 16 in particular a syringe compliance for one or multiple syringes 2, a line compliance for one or multiple line sets 3, a pusher stiffness of the pusher device 11 (including its driving mechanism), and geometric data of one or multiple syringes 2, in particular an inner diameter of the cylindrical tube 20, may be stored. Hence, upon identification of a particular syringe 2 and a particular line set 3 by a user, for example by inputting suitable control data into the infusion device 1 upon installation of the syringe 2 and the line set 3 on the infusion device 1, the expected slope can be computed and can be used as it shall be described subsequently.

(65) Knowing the expected slope, generally an occlusion condition can be differentiated from other pressure variation conditions, in particular in a multiline infusion scenario. For example, during the operation of an infusion device 1A, 1B, 1C, which may not know that other infusion devices 1A, 1B, 1C are present and are operated to administer medical fluids to a patient B via the same line set 3, the slope of a pressure variation in the line set 3 may be monitored by the infusion device 1A, 1B, 1C and may be compared to the expected slope in order to test whether the threshold value P.sub.thresh determined automatically in the initial phase may be assumed reliable and adequate or should be adjusted. If during operation of the infusion device 1A, 1B, 1C it is found that a slope value determined based on measurement values of the sensor device 14 indicative of a pressure variation in the line set 3 deviates by more than a predefined margin from the expected slope, it may be concluded that a pressure variation condition other than an occlusion condition is present in the line set 3, such that it may be determined that the threshold value P.sub.thresh may have to be adjusted.

(66) In one embodiment, the processor device 15 of the infusion device 1A, 1B, 1C may be configured to adjust the threshold value P.sub.thresh if it is found that the slope value is larger than a predefined first constant α1 times the expected slope value:
slope value>expected slope.Math.α1

(67) The constant α1 herein may have a value in the range between 1 and 10, for example between 2 to 4, in particular 3. If it hence is found that the slope value of the pressure rise as determined from current measurement values of the sensor device 14 is substantially larger than the expected slope, it is concluded that the pressure rise is not due to an occlusion in the line set 3, but to another pressure source external to the infusion device 1A, 1B, 1C. In this case it is concluded that the threshold value P.sub.thresh determined automatically in the initial phase is no longer adequate, and consequently the threshold value P thrash is adapted by the processor device 15 of the infusion device 1A, 1B, 1C.

(68) For example, the threshold value P.sub.thresh in this case may be set to a maximum default value of the system. Alternatively, the threshold value P.sub.thresh may be set to the value of the currently measured pressure plus a predefined margin Δ.sub.pressure, equal to for example 50 mmHg.

(69) For the subsequent operation, then, the adjusted threshold value P.sub.thresh is used, the adjusted threshold value P.sub.thresh being substantially larger than the initially set threshold value P.sub.thresh. In this way, hence, false alarms may be avoided during abnormal pressure variation conditions.

(70) If during the subsequent operation it is found that the slope of the pressure rise decreases, and if it furthermore is found that the actually measured pressure also decreases, the threshold value P.sub.thresh may be reset to the initially set threshold value P.sub.thresh. For example, if it is found that the slope value falls below the expected slope value times a second predefined constant α2 and furthermore the currently measured pressure in the line set 3 falls below an initially measured pressure (prior to the adjustment of the threshold value P.sub.thresh), the threshold value P.sub.thresh may be reset to the initially set threshold value P.sub.thresh.

(71) The second predefined constant α2 may have a value in the range between 0.5 and 10. The second predefined constant α2 may have the same value as the first predefined constant α1, or may differ from the first predefined constant al.

(72) The infusion device 1A, 1B, 1C hence assumes regular operation using the threshold value P.sub.thresh which has automatically been determined and set during the initial phase.

(73) Because dependent on the operation and a characteristic of a pressure rise observed during the actual operation the initially set threshold value P.sub.thresh is variably adjusted, reliability of an occlusion detection may be improved, in particular by avoiding false alarms in multiline infusion situations. An infusion device 1, 1A, 1B, 1C hence may provide for a reliable, fast occlusion detection as well as a limited amount of false alarms.

(74) The invention is not limited to the embodiments described above, but can be carried out in an entirely different fashion. In particular, the invention generally is applicable to different kinds of infusion devices used for different purposes for administering a medical fluid into a patient.

(75) The expected slope may be determined in a different way, for example in a calibration measurement on a particular syringe and a particular line set. The described procedure hence only serves as an example.

LIST OF REFERENCE NUMERALS

(76) 1 Infusion device 10 Housing 100 Front face 11 Pusher device 12 Receptacle 13 Display device 14 Force sensor 15 Processor device 16 Storage device 2 Pumping device (syringe) 20 Cylindrical tube 21 Piston 210 Piston head 3 Line set 30, 31 End 300A, 300B, 300C Line section 32 Junction point A Actuation direction B Patient O Occlusion P Pressure P.sub.thres Pressure threshold t time T Duration