Method of determining a system compressibility value of a medical membrane pump drive

Abstract

A method of determining a system compressibility value of a medical membrane pump drive is provided which includes moving to a first and second pressure level and detecting a first and second operating parameter value of the membrane pump drive. The system compressibility value is determined on the basis of the detected operating parameter values and the membrane of the membrane pump drive is supported at a rigid surface during the determination of the system compressibility value.

Claims

1. A method of determining a system compressibility value of a medical membrane pump drive of a medical device, the membrane pump drive including a membrane, a coupling surface for coupling a pump cassette to the medical device, a drive chamber arranged in the coupling surface, said drive chamber being closed by said membrane, a piston-in-cylinder unit including a piston movable in a cylinder, the cylinder connected to the drive chamber for adjusting a pressure in the drive chamber by moving the piston, and a pressure sensor for determining the pressure in the drive chamber, said membrane being deflected outwardly out of the drive chamber by excess pressure in the drive chamber and being deflected inwardly into the drive chamber by negative pressure in the drive chamber, the membrane being deformable from a flat configuration inwardly to a first maximum deflection where the membrane contacts a first rigid surface of the medical device and outwardly to a second maximum deflection where the membrane contacts a second rigid surface of the medical device or the pump cassette coupled to the medical device, the method comprising: moving the piston of the piston-in-cylinder unit to produce a first pressure level in the drive chamber, said first pressure level exceeding a maximum counter-pressure of the membrane such that the membrane is deflected inwardly to the first maximum deflection in contact with the first rigid surface or is deflected outwardly to the second maximum deflection in contact with the second rigid surface; detecting a first position of the piston of the piston-in-cylinder unit of the membrane pump drive associated with the first pressure level; moving the piston of the piston-in-cylinder unit to produce a second pressure level in the drive chamber, the second pressure level exceeding the maximum counter-pressure of the membrane such that the membrane is still deflected inwardly to the first maximum deflection in contact with the first rigid surface or is still deflected outwardly to the second maximum deflection in contact with the second rigid surface; detecting a second position of the piston of the piston-in-cylinder unit of the membrane pump drive associated with the second pressure level; and determining the system compressibility value on the basis of the first and second positions of said piston.

2. The method in accordance with claim 1, wherein the medical device includes a pump cassette receiver having a receiving surface supporting a rear side of the pump cassette when a front side of the pump cassette is coupled to the coupling surface, the method further comprising a step of coupling the pump cassette to the membrane pump drive, with the cassette being received in the pump cassette receiver in a coupled state; wherein the step of coupling the pump cassette to the membrane pump drive is performed after the step of determining the system compressibility value, and wherein the second rigid surface in contact with the membrane during the determination of the system compressibility value is the receiving surface of the pump cassette receiver.

3. The method in accordance with claim 1, wherein the pump cassette includes a pump chamber, the method further comprising a step of coupling the pump cassette to the membrane pump drive; wherein the step of determining the system compressibility value is performed after the step of coupling the pump cassette to the membrane pump drive and wherein the membrane is completely pressed into the pump chamber of the pump cassette and being supported at a rear wall of the pump chamber during the determining of the system compressibility value, such that the second rigid surface is formed by the rear wall of the pump chamber.

4. The method is accordance with claim 1, wherein the system compressibility value is determined at a negative pressure, with the membrane being completely drawn into the drive chamber of the membrane pump drive and contacting a rear wall of the drive chamber during the determining of the system compressibility value, such that the first rigid surface is formed by the rear wall of the drive chamber.

5. The method in accordance with claim 1, wherein the medical device includes an air cushion via which, in normal operation, the pump cassette is pressed toward the coupling surface of the membrane pump drive from its rear side, wherein the method further comprises the step of filling the air cushion to an operating pressure before determining the system compressibility value.

6. The method in accordance with claim 1, wherein a difference between the first and second pressure levels is greater than 5 mbar and less than 500 mbar.

7. The method in accordance with claim 1, wherein a first system compressibility value is determined at negative pressure values and a second system compressibility value is determined at excess pressure values.

8. The method in accordance with claim 1, wherein the system compressibility value is determined in dependence on a difference between the first position and the second position of the piston of the piston-in-cylinder unit.

9. The method in accordance with claim 1, wherein the first position and the second position of the piston of the piston-in-cylinder unit is determined using a length sensor.

10. The method of claim 9, wherein the piston-in-cylinder unit and the drive chamber are filled with hydraulic fluid.

11. The method in accordance with claim 1, comprising the further steps of: coupling the membrane pump cassette to the membrane pump drive; pumping a fluid through a pump chamber of the membrane pump cassette by operating the membrane pump drive; moving the piston to produce a third pressure level and a fourth pressure level in the drive chamber; detecting a third position and a fourth position of the piston associated with the third and the fourth pressure levels, respectively; and determining at least one of an air proportion and an air quantity in the fluid pumped through the pump chamber of the membrane pump cassette on the basis of the third and fourth positions of the piston.

12. The method in accordance with claim 11, wherein the system compressibility value of the membrane pump drive is taken into account in the determination of the at least one of the air proportion and the air quantity; and wherein the third pressure level is equal to the first pressure level and the fourth pressure level is equal to the second pressure level.

13. The method of claim 1, wherein a transmission of pressure onto the membrane takes place hydraulically.

14. A membrane pump drive of a medical device comprising: a coupling surface for coupling a pump cassette to the membrane pump drive; a drive chamber arranged in the coupling surface, said drive chamber being closed by the membrane of said membrane pump drive, said membrane being deflected outwardly out of the drive chamber by excess pressure in the drive chamber and being deflected inwardly into the drive chamber by negative pressure in the drive chamber, the membrane being deformable from a flat configuration inwardly to a first maximum deflection where the membrane contacts a first rigid surface of the medical device and outwardly to a second maximum deflection where the membrane contacts a second rigid surface of the medical device or the pump cassette coupled to the medical device; a piston-in-cylinder unit including a piston movable in a cylinder, the cylinder connected to the drive chamber for adjusting a pressure in the drive chamber by moving the piston; a pressure sensor for determining the pressure in the drive chamber; and a controller, said controller being configured to carry out a method of determining a system compressibility value, the method including the steps of moving the piston of the piston-in-cylinder to produce a first pressure level in the drive chamber, wherein the first pressure level exceeds a maximum counter-pressure of the membrane such that the membrane is deflected inwardly to the first maximum deflection in contact with the first rigid surface or is deflected outwardly to the second maximum deflection in contact with the second rigid surface; detecting a first position of the piston of the piston-in-cylinder unit of the membrane pump drive associated with the first pressure level; moving the piston of the piston-in-cylinder unit to produce a second pressure level in the drive chamber, the second pressure level exceeding the maximum counter-pressure of the membrane such that the membrane is still deflected inwardly to the first maximum deflection in contact with the first rigid surface or is still deflected outwardly to the second maximum deflection in contact with the second rigid surface; detecting a second position of the piston of the piston-in-cylinder unit of the membrane pump drive associated with the second pressure level; and determining the system compressibility value on the basis of the first and second positions of the piston.

15. A blood treatment machine having a membrane pump drive in accordance with claim 14.

16. The blood treatment machine of claim 15, wherein the blood treatment machine is a dialysis machine.

17. The blood treatment machine of claim 15, further comprising a pump cassette receiver and an air cushion for pressing the pump cassette to the coupling surface of the membrane pump drive.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Preferred embodiments of the present invention will now be presented in more detail with reference to Figures and embodiment examples.

(2) There are shown:

(3) FIG. 1: a schematic representation of a membrane pump drive in accordance with the invention with a coupled pump chamber;

(4) FIG. 2: a section through the coupling region of a membrane pump drive in accordance with the invention with a coupled pump cassette; and

(5) FIG. 3: am embodiment of a pump cassette such as can be coupled to a membrane pump drive in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

(7) FIG. 1 shows an embodiment of a membrane pump drive 30 in accordance with the invention for a pumping of a medical fluid through the pump chamber 4 that is couplable to the membrane pump drive.

(8) The membrane pump drive has a drive chamber 1 at which a flexible membrane 2 is arranged. The flexible membrane 2 is arranged in a coupling surface 3 of the membrane pump drive such that a membrane, not recognizable in FIG. 1, of the pump chamber 4 can be coupled to the membrane 2 of the drive chamber such that it follows the movements of the membrane 2 of the drive chamber. The volume of the pump chamber 4 can therefore be varied by a movement of the membrane 2 out of or into the drive chamber 1. Fluid can be pumped by the pump chamber 4 by moving the membrane 2 by the corresponding switching of valves, not shown in any more detail in FIG. 1, that control the inflow or outflow to or from the pump chamber 4.

(9) The pump chamber 4 is in this respect typically part of a pump cassette not shown in any more detail in FIG. 1 that preferably represents a disposable. In this respect, the pump chamber is typically formed by a corresponding shaping of a hard part of the pump cassette that is covered by a flexible film forming the membrane of the pump chamber.

(10) The present invention would, however, also be usable in the same way for membrane pumps in which the drive chamber and the pump chamber are fixedly connected to one another or are integrated in a common pumping apparatus.

(11) In the embodiment shown in FIG. 1 it is in this respect a piston membrane pump that has a piston-in-cylinder unit 7 that is in hydraulic communication with the drive chamber 6 via the hydraulic line 12. The piston-in-cylinder unit 7 is in this respect driven by a drive 10 that acts on the piston 8 of the piston-in-cylinder unit 7 and moves it in the cylinder 9. The distance the piston 8 is traveled in the cylinder 9 is detected or measured by a length sensor 11 associated with the piston-in-cylinder unit 7.

(12) The pressure side 25 of the piston-in-cylinder unit 7 is in this respect in fluid communication with the drive chamber 1 via the fluid line 12, with the pressure side 25, the fluid line 12 and the drive chamber 1 being filled with hydraulic fluid. The adjustment movement of the piston 8 is hereby transmitted to the membrane 2 of the drive chamber 1. The membrane 2 of the drive chamber 1 is therefore arched convexly outwardly or is pulled concavely into the inner space of the drive chamber on a corresponding change of the hydraulic volume of the piston-in-cylinder unit 7 by moving the piston 8.

(13) The volume change of the drive chamber 1 required for the fluid conveying in the pump chamber 4 is accordingly brought about by actuating the piston-in-cylinder unit 7. The hydraulic fluid is pressed into or sucked out of the drive chamber 1 by actuating the piston 8. The membrane 2 is hereby actuated whose movement is transmitted onto the pump chamber 5 and varies its volume.

(14) The membrane pump drive furthermore has a pressure sensor 13 via which the pressure of the hydraulic fluid in the hydraulic system and thus the pressure in the drive chamber 1 can be measured. The pressure prevailing in the drive chamber 1 in this respect correspondswith the exception of a possible counter-pressure of the membrane 2to the counter-pressure prevailing in the pump chamber 4 such that the pressure in the pump chamber 4 can also simultaneously be determined via the pressure sensor 13.

(15) The membrane pump drive furthermore has a control, not shown, that is connected to the length sensor 11 and to the pressure sensor 13 and evaluates the measured signals. The control furthermore controls the drive 10 of the membrane pump drive and the valves for controlling the fluid flow into and out of the pump chamber 4.

(16) Such a piston membrane pump has the advantage that it conveys fluid with a very exact quantity, with the totally conveyed quantity being able to be precisely balanced since the pump volume corresponds to the stroke volume of the piston-in-cylinder unit 7 and can be exactly measured by the length sensor 11.

(17) The control of the membrane pump drive of the present invention in this respect first has a function in accordance with the second aspect of the present invention by which an air proportion and/or an air quantity in the fluid conveyed by the membrane pump can be determined. It can be prevented by this function that air bubbles that are present in the pump chamber 4 falsify the balancing of the fluid conveyed through the air chamber 4.

(18) A measurement phase that can be interposed between the pumping process with every stroke is provided for determining the air proportion or the air quantity. First, in this respect, fluid is sucked into the pump chamber 4 by moving the membrane 2 in accordance with the usual pumping process. The cut-off valves of the pump chamber 4 are thereupon closed such that a closed fluid volume results and a first, predefined pressure level p.sub.a is moved to and the associated position of the piston 8 is determined by actuating the drive 10. A second pressure level p.sub.e is thereupon in turn traveled to and the associated position of the piston 8 is likewise determined by actuating the drive 10. If the fluid enclosed in the pump chamber 4 has a certain gas proportion, it is compressed by the pressure increase, which corresponds to a corresponding change of the volume of the pump chamber 4. This volume difference can be determined by the positions of the piston 9 present at the starting pressure and at the end pressure.

(19) The control calculates the air quantity contained in the pump chamber from the values thus acquired, i.e. the air volume V.sub.at contained there at atmospheric pressure. For this purpose, the control assumes Boyle's law that reads for an isothermal state change, i.e. while neglecting a temperature change:
pV=constant.

(20) Starting from this, different states of the measurement phase can be equated to:
V.sub.atp.sub.at=V.sub.ap.sub.e=V.sub.eP.sub.e.

(21) While observing the relationship that the difference volume V.sub.diff is determined by the difference of the starting volume and of the end volume, that is V.sub.diff=V.sub.aV.sub.e, the actual gas volume at atmospheric pressure V.sub.at can be acquired therefrom:

(22) $V_{\mathrm{at}}=\frac{V_{\mathrm{diff}}}{\left(\frac{p_{\mathrm{at}}}{p_a}-\frac{p_{\mathrm{at}}}{p_a+p_{\mathrm{diff}}}\right)}$

(23) Depending on the specifically used pump method, it must be taken into account with this formula that the pressure measured on the hydraulic side of the membrane pump via the pressure sensor 13 may not exactly correspond to the pressure in the pump chamber 4, but differs by a specific value from this pressure due to the tension of the membrane 2. In a first variant of the method, the determination of the air proportion can, however, take place with a non-deflected membrane 2 so that the influence of the membrane can be neglected. In a second variant, the starting pressure p.sub.a can in contrast be corrected by a difference pressure p.sub.mem due to the membrane between the hydraulic side and the pump side. It can be stored in the control, for example. It is hereby possible to carry out the determination of the air proportion while the membrane 2 has been drawn very far into or completely into the drive chamber 1 such that the complete pump volume is utilized. The differential pressure p.sub.mem between the hydraulic side and the pump side due to the membrane can in this respect be determined in the activation phase. Depending on the ratio between the pressures on the hydraulic side and the differential pressure p.sub.mem due to the membrane and the required precision, the differential pressure p.sub.mem can optionally also be neglected, however.

(24) The volume difference entering into the above formula is determined by the distance of the piston S.sub.diff covered on the compression from the pressure level p.sub.a to the pressure level p.sub.e, and its area A.sub.K.

(25) However, it must be taken into account in this respect that the movement of the piston 8 on the pressure change from p.sub.a to p.sub.e is not exclusively due to the air volume in the pump chamber 4. For the membrane pump drive itself also has a certain yielding or system compressibility under pressure changes. Factors are in this respect in particular the air that can collect in the hydraulic system and a certain flexibility of the hydraulic line 12. The piston 8 would therefore move by a certain distance S.sub.0 solely due to this system compressibility on a pressure change from p.sub.a to p.sub.e even if no air at all were contained in the pump chamber 4 and the latter were thus non-compressible.

(26) The actual volume V.sub.at of the air contained in the pump chamber 4 thus results while taking account of the system compressibility value S.sub.0 characterizing the system compressibility.

(27) $V_{\mathrm{at}}=\frac{\left(S_{\mathrm{diff}}-S_0\right).Math.A_k}{\left(\frac{p_{\mathrm{at}}}{p_a}-\frac{p_{\mathrm{at}}}{p_a+p_{\mathrm{diff}}}\right)}$

(28) Since, in accordance with the second aspect of the present invention, two previously fixed pressure levels p.sub.a and p.sub.e are actively traveled to on the determination of the air volume in the pump chamber 4, the system compressibility value S.sub.0 characterizing the system compressibility can be determined exactly for this pressure change. Inaccuracies that resulted in accordance with the prior art due to the use of the pressure level obtained by gravity filling as the starting pressure level p.sub.a are hereby avoided.

(29) The control of the membrane pump drive in accordance with the invention in this respect preferably has a second function via which the system compressibility value S.sub.0 can be determined. The first and second pressure levels p.sub.a and p.sub.e are also moved to, for example in the activation phase, for this purpose and the corresponding positions of the piston 8 are detected. In order in this respect only to take effects into consideration that are due to the system compressibility of the membrane pump drive and not for instance to the compressibility of the components coupled to the membrane pump drive, the determination takes place in a state of the membrane pump drive in which the membrane 2 is supported at a rigid surface. This can be achieved, for example, in that the determination of the system compressibility value takes place in a pressure range in which the membrane 2 has been deflected to a maximum outwardly or inwardly.

(30) The determination of the system compressibility value can in this respect take place both with a pump cassette coupled to the coupling surface 3 of the membrane pump drive and without a coupled pump cassette.

(31) The mechanical design of an embodiment of a membrane pump drive in accordance with the invention to which a pump cassette can be coupled is in this respect shown in more detail in FIG. 2. The membrane pump drive has a machine block 20 at which the coupling surface 3 is arranged for coupling the pump cassette 14. The drive chamber 1 provided with the flexible membrane 2 is in this respect let into the coupling surface 3 and is in hydraulic communication with the piston-in-cylinder unit 7, not shown in any more detail here, via the hydraulic line 12 with fluid 19.

(32) The pump cassette 14 is in this respect inserted into a pump cassette receiver 15 for coupling to the coupling surface 3 such that the rear side of the pump cassette is supported at a receiving surface of the pump cassette receiver 15. The receiving surface in this respect has a corresponding spherically shaped cut-out for this purpose in the region of the pump chamber 4 that is designed as a bulge of the rear side of the pump cassette.

(33) After the insertion of the cassette 14, the cassette receiver 15 is pressed toward the coupling surface 3 via an air cushion 18 that is arranged at the rear side and that is in turn supported at a device wall 17. For this purpose, the air cushion is acted on by a corresponding operating pressure that can be, for example, between 1,500 and 2,500 mbar.

(34) In the embodiment, the pump cassette receiver 15 is designed as a drawer that can be moved in and out in the direction 21 to insert a cassette. The machine block 20 can furthermore be placed onto the pump cassette 14 in the direction of movement 22. After the pushing in of the drawer 15 and the placing on of the machine block 20, the air cushion 18 is then pressurized to achieve a secure coupling of the pump cassette 14 to the coupling surface 3.

(35) Alternatively to the constructive design shown in FIG. 2, the pump cassette receiver 15 could, however, also be designed as a door, for example, that is opened for inserting the pump cassette 14 and is closed for placing the pump cassette 14 at the coupling surface. The air cushion 18 would be integrated into the door in this case.

(36) In this respect, an embodiment of a pump cassette 14 is shown in FIG. 3 that has two pump chambers 4 and 4. The pump cassette in this respect comprises a hard part into which the fluid-conducting channels and the pump chambers are let and is covered by a flexible film with respect to the coupling surface. The pump cassette in this respect inter alia has the valves 23 and 24 via which the fluid flow into and out of the pump chambers 4 and 4 can be controlled. The valves are in this respect likewise actuated via actuators arranged in the machine block 20.

(37) The determination in accordance with the invention of the system compressibility value in this respect preferably takes place in the activation phase of the membrane pump, but can in this respect be carried out both with an inserted pump cassette and without an inserted pump cassette.

(38) If the determination is carried out as long as no pump cassette 14 was inserted, the membrane is supported on the receiving surface 16 of the pump cassette receiver 15 during the carrying out of the measurements. If in contrast the determination is carried out with an inserted pump cassette, the membrane 2 is supported on the rear wall 5 of the pump chamber 4 and thus on the hard part of the pump cassette. With an inserted pump cassette, at least one of the valves that control the fluid flows into and out of the respective pump chamber has to be open for this purpose. The determination of the system compressibility value in this respect advantageously takes place before the filling of the pump cassette with fluid or while the pump chamber is in communication, for example, with the dialysis bag or the drainage back via the fluid connections.

(39) The fact that the membrane contacts the receiving surface 16 of the pump cassette receiver or the rear wall of the pump chamber during the measurement phase is achieved by correspondingly high pressure levels p.sub.a and p.sub.e that provide a complete deflection of the membrane during the measurement process. The tension of the membrane 2 is in this respect already completely overcome by the reaching of the first pressure level p.sub.a. On the pressure increase to the second pressure level p.sub.e, the membrane is then supported on a rigid counter-surface such that the membrane or components coupled thereto has/have no influence on the determination of the system compressibility value.

(40) Since the tension of the membrane 2 is overcome at a pressure level of approximately 50 mbar, a suitable first pressure level lies at approximately 200 mbar; a suitable second pressure level at approximately 300 mbar.

(41) In this respect, the same pressure levels are preferably used for determining the system compressibility value that are also used for determining the air volume in the fluid conveyed by the pump. The distance the piston 8 covers on the pressure increase from p.sub.a to p.sub.e can hereby simply be used as the system compressibility value S.sub.0. The system compressibility value S.sub.0 thus results as the difference from the position values of the membrane pump drive determined from the pressure levels p.sub.a and p.sub.e.

(42) As already presented above, the air content of the hydraulic fluid and the stiffness of the hydraulic hoses represent the main influence factors on the system compressibility. However, mechanical tolerances and a deformation of the mechanical components can also result in a certain yielding of the system and thus in an increase in the system compressibility value.

(43) The determination of the system compressibility value therefore preferably takes place after the air cushion 18 has been filled to the operating pressure such that the pump cassette receiver 15 is pressed toward the coupling surface 3. The influence the clearance of the pump cassette receiver and a possible mechanical deformation of the involved mechanical components could have on the determination of the system compressibility value is hereby reduced. In addition, the determination of the system compressibility value hereby takes place in the same situation that is then also present in the determination of the air proportion during the pump operation. The pump cassette receiver is in this respect pressed toward the coupling surface via the air cushion 18 independently of whether the determination of the system compressibility value is carried with or without an inserted pump cassette.

(44) To the extent that the system compressibility value is in this respect influenced by the operating pressure of the air cushion, the regulation tolerance of the internal air cushion pressure can be restricted for a further increase in the accuracy.

(45) In accordance with the above-described variants, the determination of the system compressibility value takes place at excess pressure such that the membrane 2 arches outwardly out of the drive chamber 1 and is supported on an outwardly arranged counter-surface.

(46) The determination in accordance with the invention of the system compressibility value can, however, also be reached at vacuum levels. In this case, the vacuum levels are selected such that the membrane 2 is supported on the rear wall 6 of the drive chamber 1. A suitable first pressure level is in this respect approximately 200 mbar; a suitable second pressure level approximately 300 mbar. The suitable vacuum levels thus correspond by amount to the suitable excess pressure levels.

(47) In the event that the determination of the system compressibility value takes place at vacuum levels p.sub.a and p.sub.e, work is preferably also carried out in accordance with the above-shown second aspect with vacuum levels p.sub.a and p.sub.e to determine the air volume in the fluid pumped through the pump chamber.

(48) If work is carried out in this respect with vacuum levels on the determination of the system compressibility value, the mechanical properties of the design of the air cushion, the pump cassette receiver and the machine block do not affect the measurement.

(49) Provision can furthermore be made in accordance with the invention respectively to determine a system compressibility value with excess pressure levels and with vacuum levels. The mechanical properties of the design of the air cushion, the pump cassette mount and the machine block can be determined by the use of both methods and they can be separated from the properties due to the hydraulic system.

(50) The determination of the system compressibility value at vacuum can likewise take place with or without an inserted pump cassette. If it takes place with an inserted pump cassette, the valves with which the pump chamber communicates with further components should be open.

(51) The system compressibility value determined in accordance with the invention can in this respect, on the one hand, enter into the determination of the air volume of the conveyed medical fluid as shown above. It in this respect allows a more exact balancing of the fluids moved through the membrane pump since the air volume in the pumped fluids can be determined more precisely.

(52) The determination of the system compressibility value can furthermore be used to verify the quality of the degassing of the hydraulic system. In this respect, for example, as soon as the system compressibility value exceeds a certain threshold, a degassing of the hydraulic system can be carried out or its necessity can be displayed.

(53) The membrane pump drive in accordance with the invention is preferably used in a blood treatment device for pumping medical fluids, in particular for pumping blood or dialyzate. The membrane drive pump in accordance with the invention is in this respect particularly preferably used in a dialysis machine, with the membrane pump being used for pumping the dialyzate into the abdomen of the patient or for removing the dialyzate from the abdomen of the patient.

(54) The invention being thus described, it will be apparent that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be recognized by one skilled in the art are intended to be included within the scope of the following claims.