Method for compensating for pressure differences across valves in cassette type IV pump

• US 20030055375A1
• Filed: 10/29/2002
• Published: 03/20/2003
• Est. Priority Date: 12/17/1999
• Status: Active Grant

• Current Assignee: ICU Medical Incorporated
• Original Assignee: Hospira Incorporated (Pfizer Inc.)
• Inventors: Maske, Rudolph J., Holst, Peter A., Krajewski, David A.
• Granted Patent: US 6,942,636 B2
• Time in Patent Office: Days
• US Class Current: 604/67
• CPC Class Codes: A61M 2205/12 with interchangeable cassettes forming partially or totally the fluid circuit
• CPC Class Codes: A61M 2205/3355 Controlling downstream pump pressure
• CPC Class Codes: A61M 2205/50 with microprocessors or computers
• CPC Class Codes: A61M 5/14224 Diaphragm type
• CPC Class Codes: A61M 5/16809 by repeated filling and emptying of an intermediate volume pressure infusion using positive displacement pumps A61M5/142
• CPC Class Codes: A61M 5/16827 controlling delivery of multiple fluids, e.g. sequencing, mixing or via separate flow-paths infusion of multiple fluids without using a controller A61M5/1407
• CPC Class Codes: A61M 5/365 Air detectors A61M5/1684 takes precedence; in extracorporeal blood circuits A61M1/3626
• CPC Class Codes: G05D 16/2046 the plurality of throttling means being arranged for the control of a single pressure from a plurality of converging pressures G05D16/204 takes precedence

Abstract

A pump used to infuse a fluid into a patient is controlled in accordance with an algorithm that enables a microprocessor to monitor and adjust each pump cycle to compensate for a differential pressure between the pump'"'"'s inlet and outlet. The algorithm defines a fluid delivery protocol that is applied in controlling the operation of the pump to achieve a desired rate, volume, and timing of the fluid infusion. Fluid is delivered by the pump when a plunger compresses an elastomeric membrane overlying a fluid chamber. Due to the small volume of the chamber, an incremental change in the plunger position before the delivery stroke produces a significant change in the delivery pressure. At the beginning of a pump cycle, the microprocessor determines the differential pressure between the inlet and outlet of the pump, and adjusts the plunger position before the delivery stroke to compensate for the differential pressure. A retraction of the plunger from the home position decreases the delivery pressure of the fluid, and an advancement of the plunger increases it. After the position of the plunger is adjusted to compensate for the differential pressure, the pump cycle proceeds. Following the plunger stroke, the outlet pressure is used to determine the actual volume of fluid delivered. The duration of the plunger stroke in the next pump cycle is adjusted to compensate for any volume delivery error produced by the differential pressure compensation.

Claims

1. A pump for administering a fluid to a patient, comprising:

(a) a fluid displacement device that is adapted to couple with a drive unit and to couple in fluid communication with a source, said drive unit actuating the fluid displacement device to deliver a fluid drawn from the source to a patient by displacement of the fluid from a chamber disposed within the fluid displacement device into which the fluid flows from the source;

(b) a control unit that is coupled to the drive unit to control operation of the fluid displacement device; and

(c) at least one pressure sensor electrically coupled to the control unit and producing a signal indicative of a pressure of the fluid proximate at least an outlet of the fluid displacement device, said control unit being coupled to receive the signal from said at least one pressure sensor and using the signal to determine a differential pressure for the fluid during a pumping cycle, wherein said control unit determines a correction based upon the differential pressure and applies the correction in controlling the drive unit when displacing the fluid from the chamber, to compensate for the differential pressure, thus ensuring accurate delivery of the fluid to the patient.

2. The pump of claim 1, further comprising an inlet pressure sensor that produces a signal indicative of a pressure of a fluid proximate an inlet of the fluid displacement device, said control unit determining the differential pressure based upon a difference between the signal produced by the inlet pressure sensor and the pressure sensor disposed proximate the outlet of the fluid displacement device.

3. The pump of claim 1, wherein the fluid displacement device comprises a disposable cassette having an elastomeric membrane covering the chamber and wherein the drive unit includes a plunger that acts on the elastomeric membrane to displace the fluid from the chamber and into the patient, said correction comprising a change in a position of the plunger relative to the chamber before the plunger begins to displace the fluid from the chamber.

4. The pump of claim 1, further comprising an outlet valve disposed in the fluid displacement device distally of the chamber to control fluid flow from the chamber to the patient, wherein a first pressure reading is sampled by said control unit from the signal produced by said at least one pressure sensor at a point in a pump cycle when the outlet valve is closed, and a second pressure reading is sampled by said control unit at a point in the pump cycle while the outlet valve is opened to enable the fluid to flow from the chamber to the patient, said differential pressure comprising a difference between the first and the second pressure readings.

5. A pump employed to infuse a fluid flowing from a source into a patient, comprising:

(a) a disposable cassette having a housing that defines a fluid path from an inlet port that is adapted to couple in fluid communication with the source, to an outlet port that is adapted to couple in fluid communication with an infusion site on the patient, and including an inlet pressure sensor that produces a signal indicative of a pressure proximate the inlet port, and an outlet pressure sensor that produces a signal indicative of a pressure proximate the outlet port, said fluid path including;

(i) a chamber covered by an elastomeric membrane that when forced into the chamber, displaces the fluid from the chamber through the outlet port;

(ii) an inlet valve controlling flow of the fluid into the chamber from the inlet port, said inlet valve being open as fluid flows into the chamber from the inlet port and closed as the fluid is displaced through the outlet port; and

(iii) an outlet valve controlling flow of the fluid from the chamber toward the outlet port, said outlet valve being open when the fluid is displaced through the outlet port and closed as the fluid flows into the chamber from the inlet port;

(b) a drive unit adapted to couple with disposable cassette and including at least one prime mover and a plunger, said at least one prime mover moving the plunger relative to the chamber to deform the elastomeric membrane, thereby displacing fluid from the chamber toward the outlet port, said drive unit also actuating the inlet valve and outlet valve to open and close said valves; and

(c) a control unit, coupled to receive the signals from the inlet pressure sensor and the outlet pressure sensor, and coupled to the drive unit to control said at least one prime mover and thereby controlling movement of the plunger, said control unit determining a differential pressure from the signals produced by the inlet pressure sensor and the outlet pressure sensor and adjusting an initial position of the plunger relative to the chamber before the plunger displaces the fluid from the chamber, to compensate for the differential pressure, so that an accuracy of the pump in infusing the fluid into the patient is substantially improved.

6. The pump of claim 5, wherein the adjustment to the initial position of the plunger changes a pressure of the fluid in the chamber before the outlet valve is opened to enable the fluid to be displaced from the chamber.

7. The pump of claim 5, wherein as a function of the adjustment to the initial position of the plunger, the control unit changes a duration of time for the fluid to be displaced from the chamber by the plunger.

8. The pump of claim 5, wherein the plunger is always in contact with the elastomeric membrane during the pump cycle.

9. The pump of claim 5, wherein said differential pressure is less than 0.5 psi, after being compensated.

10. The pump of claim 5, wherein the control unit includes a processor coupled to a memory in which machine instructions comprising a control algorithm are stored, said machine instructions being executed by the processor and causing it to determine the differential pressure and to adjust the initial position of the plunger to compensate for the differential pressure.

11. The pump of claim 10, wherein the control algorithm uses a pressure sensed by the outlet pressure sensor, after the outlet valve has just opened and fluid is flowing through the outlet valve to the patient, to determine a calibration difference between the inlet pressure sensor and the outlet pressure sensor, and wherein said calibration difference is used by the algorithm when adjusting the initial position of the plunger to compensate for the differential pressure during a next pump cycle.

12. The pump of claim 10, wherein parameters of the control algorithm are empirically determined for a specific design of the disposable cassette and the drive unit.

13. The pump of claim 10, wherein when the control unit determines that the differential pressure between the inlet pressure sensor and the outlet pressure sensor is such that a greater pressure is indicated at the outlet pressure sensor than at the inlet pressure sensor, the control unit causes the plunger to advance toward the chamber to a position determined by the algorithm, and when the control unit determines that the differential pressure between the inlet pressure sensor and the outlet pressure sensor is such that a lower pressure is indicated at the outlet pressure sensor than at the inlet pressure sensor, the control unit causes the plunger to retract away from the chamber, to a position determined by the algorithm.

14. The pump of claim 13, wherein when the plunger is advanced from its initial position to a compensated position as determined by the algorithm, said advancement causes a greater deformation of the elastomeric membrane, and thus causes a volume of the chamber to decrease and a pressure within the chamber to increase;

and wherein when the plunger is retracted from its initial position to a compensated position as determined by the algorithm, said retraction causes a lesser deformation of the elastomeric membrane, and thus causes a volume of the chamber to increase and a pressure within the chamber to decrease.

15. The pump of claim 10, wherein the algorithm incorporates a first lookup table in which the plunger position is a function of a pressure associated with the inlet, and a second lookup table in which the plunger position is a function of a pressure associated with the outlet, and a compensated plunger position is determined by combining a result from both lookup tables.

16. The pump of claim 15, wherein the first and second lookup tables are empirically determined.

17. The pump of claim 10, wherein after the plunger has exerted a force on the elastomeric membrane and displaced the fluid from the chamber to the outlet port, the control unit uses the algorithm to determine the actual fluid volume displaced from the chamber, and to then determine a correction factor for modifying the next pump cycle to maintain a desired delivery rate of the fluid to the patient.

18. A system for maintaining a desired delivery rate of a fluid flow through an intravenous line, comprising:

(a) a pump chassis;

(b) a pump in fluid communication with the intravenous line and mountable within the pump chassis to receive a driving force from a driven member, said pump including an inlet port, an outlet port, an elastomeric membrane overlying a chamber comprising a portion of a fluid path extending between the inlet port and outlet port, said driven member exerting a force on the elastomeric membrane that displaces the fluid from the chamber and into the intravenous line;

(c) an inlet pressure sensor that produces an inlet pressure signal indicative of an inlet pressure upstream of the chamber;

(d) an outlet pressure sensor that produces an outlet pressure signal indicative of an outlet pressure downstream of the chamber; and

(e) a controller that is electrically coupled to the inlet pressure sensor and the outlet pressure sensor to receive the inlet and the outlet pressure signals, said controller employing the inlet pressure signal and the outlet pressure signal to determine a first correction factor to be applied to an initial position of the driven member, said first correction factor being expressed as a change in a position of the driven member relative to the chamber.

19. The system of claim 18, wherein when the first correction factor requires the driven member to be advanced toward the chamber, a pressure at which the fluid is released from the outlet port is increased;

and wherein when the first correction factor requires the driven member to be retracted away from the chamber, the pressure at which the fluid is released from the outlet port is decreased.

20. The system of claim 18, wherein the first correction factor is applied to change the position the driven member relative to the chamber at the beginning of each pump cycle.

21. The system of claim 18, wherein the controller monitors the outlet pressure signal to determine a pressure spike associated with the release of fluid from the outlet port and determines a second correction factor to be applied to the inlet pressure signal to correct for any calibration difference between the inlet port and the outlet port pressure sensors.

22. The system of claim 18, wherein the controller uses the inlet pressure signal and the outlet pressure signal to determine an actual volume of fluid delivered during each pump cycle, and determines a third correction factor to be applied to a duration of a delivery stroke in the next pump cycle, to compensate for a difference between the actual volume of the fluid delivered and a target volume.

23. The system of claim 18, wherein the controller further comprises a processor and a memory, and the memory stores a plurality machine instructions that cause the processor to perform a plurality of logical steps when the machine instructions are executed by the processor, said logical steps including:

(a) sampling the inlet pressure signal produced by the inlet pressure sensor, and the outlet pressure signal produced by the outlet pressure sensor during each pump cycle to determine said first correction factor, and applying the first correction factor during the current pump cycle;

(b) sampling the inlet pressure signal produced by the inlet pressure sensor and the outlet pressure signal produced by the outlet pressure sensor as the fluid is released from the outlet port, to determine a second correction factor. said second correction factor correcting for a calibration difference between said inlet and outlet pressure sensors, and applying the second correction factor to the inlet pressure signal during the next pump cycle, and

(c) sampling the inlet pressure signal produced by the inlet pressure sensor and the outlet pressure signal produced by the outlet pressure sensor after the fluid is released from the outlet port to determine a third correction factor, said third correction factor adjusting a duration of a pump delivery stroke, and applying the third correction factor during the next pump cycle.

24. The system of claim 18, wherein a plurality of samples are averaged to minimize an effect of variations in pressure, individual samples being taken during one of a predefined interval of time and a duration of a step of a prime mover that is providing the driving force.

25. A method for minimizing an error in a disposable cassette pump used for infusing a fluid into a patient, comprising the steps of:

(a) sensing an inlet pressure of the cassette pump during a pump cycle;

(b) sensing an outlet pressure of the cassette pump during the pump cycle;

(c) determining a differential pressure between the inlet pressure and the outlet pressure;

(d) using the differential pressure, determining a compensation pressure that should be applied to the fluid in the cassette pump before infusing the fluid into the patient; and

(e) applying the compensation pressure to the fluid in the cassette pump before infusing the fluid into the patient.

26. The method of claim 25, wherein the fluid is displaced from the cassette pump by a driven member, the step of applying the pressure compensation comprising the step of adjusting a position of the driven member relative to the cassette pump.

27. The method of claim 25, further comprising the steps of:

(a) sensing a pressure associated with the outlet port of the cassette pump when the fluid is being delivered to the patient;

(b) determining a correction factor for a calibration difference between an inlet pressure sensor and an outlet pressure sensor used to sense the inlet pressure and the outlet pressure; and

(c) applying the correction factor for the calibration difference to one of the inlet and outlet pressures sensed in a next pump cycle.

28. The method of claim 25, further comprising the steps of:

(a) determining an actual volume of fluid delivered in a current pump cycle;

(b) determining a change in a duration of a delivery interval of the cassette pump that is required to compensate for any difference between the actual volume of fluid delivered and a desired volume of fluid to be delivered; and

(c) applying the change in the duration of the delivery interval in the next pump cycle.

29. A method for adjusting at least one parameter of an infusion pump for infusing a fluid into a patient, to maintain an accurate delivery of the fluid, comprising the steps of:

(a) providing a cassette having a housing that defines a fluid path between an inlet port adapted to couple in fluid communication with the source and an outlet port adapted to couple in fluid communication with an infusion site on the patient, said fluid path including a chamber from which fluid is displaced through the outlet port;

(b) monitoring at least one of an inlet pressure proximate the inlet port and an outlet pressure proximate the outlet port;

(c) providing a drive unit adapted to couple with the cassette and including a plunger that displaces the fluid from the chamber through the outlet port;

(d) determining an adjustment to an initial position of the plunger relative to the chamber before the plunger displaces the fluid from the chamber, to compensate for a differential pressure between the inlet pressure and the outlet pressure; and

(e) changing a position of the plunger relative to the chamber before the plunger begins to displace the fluid from the chamber, in accord with the adjustment to the initial position determined in the preceding step.

30. The method of claim 29, wherein the step of determining the adjustment comprises the steps of:

(a) determining a first corrected position of the plunger using a lookup table that defines a position of the plunger relative to the chamber as a function of the inlet pressure;

(b) determining a second corrected position of the plunger using a lookup table that defines a position of the plunger relative to the chamber as a function of the outlet pressure to determine a second corrected position of the driven member; and

(c) moving the plunger to a final corrected position as a function of the first and the second corrected positions.

31. The method of claim 29, further comprising the steps of:

(a) sensing the outlet pressure just as the fluid is being infused into the patient;

(b) determining a correction factor for a calibration difference between an inlet pressure sensor and an outlet pressure sensor used to determine the inlet pressure and the outlet pressure; and

(c) applying said correction factor to the inlet pressure sensed in a next pump cycle.

32. The method of claim 29, further comprising the steps of:

(a) determining an actual volume of fluid delivered in a current pump cycle;

(b) determining a change in a duration of a delivery stroke of the plunger required to compensate for any difference between the actual volume of fluid delivered and a desired volume of fluid to be delivered; and

(c) applying a change in the duration of the delivery stroke during the next pump cycle.

33. The method of claim 32, wherein the step of determining the change in the duration of a delivery stroke comprises the steps of:

(a) using lookup tables to determine a plunger extend position as a function of an outlet differential pressure, and an error volume at a plunger home position as a function of the inlet pressure; and

(b) determining the duration of the delivery stroke in the next pump cycle required to maintain a desired delivery rate as a function of a time when a previous extend stroke was completed, a time a current extend stroke began, a nominal stroke volume, a user specified delivery rate, a nominal extend stroke position, a plunger corrected position prior to the extend stroke, the plunger extend position, and the error volume.

34. The method of claim 29, wherein the step of monitoring one of the inlet pressure and the outlet pressure comprises the steps of:

(a) sampling said at least one of the inlet pressure and the outlet pressure a plurality of time during a defined time interval, producing a plurality of pressure samples;

(b) determining an average inlet pressure from the plurality of pressure samples of the inlet pressure, if monitored, and an average outlet pressure from the plurality of pressure samples of the outlet pressure, if monitored; and

(c) employing the average inlet pressure for the inlet pressure, if monitored, and employing the average outlet pressure for the outlet pressure, if monitored, to minimize an effect of varying pressure samples during the defined time interval.

35. A method for determining a difference in a calibration between an inlet pressure sensor and an outlet pressure sensor in an infusion pump for infusing a fluid into a patient, comprising the steps of:

(a) providing a cassette having a housing that defines a fluid path between an inlet port adapted to couple in fluid communication with the source and an outlet port adapted to couple in fluid communication with an infusion site on the patient, said cassette including an inlet pressure sensor that produces a signal indicative of a pressure proximate the inlet port, and an outlet pressure sensor that produces a signal indicative of a pressure proximate the outlet port, said fluid path including;

(i) a chamber covered by an elastomeric membrane that when forced into the chamber, displaces the fluid from the chamber through the outlet port;

(ii) an inlet valve controlling a flow of the fluid into the chamber from the inlet port, said inlet valve being open as the fluid flows into the chamber from the inlet port and closed as the fluid is displaced through the outlet port; and

(iii) an outlet valve controlling a flow of the fluid from the chamber toward the outlet port, said outlet valve being open when the fluid is displaced through the outlet port and closed as the fluid flows into the chamber from the inlet port;

(b) opening the inlet valve while the outlet valve is closed, thus filling the chamber with fluid, and closing the inlet valve when the chamber is filled with a desired volume of fluid;

(c) determining a pressure proximate the inlet port and a pressure proximate the outlet port using the inlet and outlet pressure sensors;

(d) adjusting a position of the elastomeric membrane such that a pressure of the fluid within the chamber is equivalent to the pressure proximate the outlet port; and

(e) opening the outlet valve, and using the outlet pressure sensor, determining if a pressure spike accompanies the opening of the outlet valve, said pressure spike being indicative of a calibration difference between the inlet pressure sensor and the outlet pressure sensor.

36. A method for determining a difference between a targeted intake fluid volume, and an actual intake fluid volume in an infusion pump for infusing a fluid into a patient, comprising the steps of:

(a) providing a cassette having a housing that defines a fluid path between an inlet port adapted to couple in fluid communication with a source and an outlet port adapted to couple in fluid communication with an infusion site on the patient, including an inlet pressure sensor that produces a signal indicative of a pressure proximate the inlet port, said fluid path including;

(i) a chamber covered by an elastomeric membrane that when forced into the chamber by a driven member displaces the fluid from the chamber through the outlet port;

(ii) an inlet valve controlling a flow of the fluid into the chamber from the inlet port, said inlet valve being open as the fluid flows into the chamber from the inlet port and closed as the fluid is displaced through the outlet port; and

(iii) an outlet valve controlling a flow of the fluid from the chamber toward the outlet port, said outlet valve being open when the fluid is displaced through the outlet port and closed as the fluid flows into the chamber from the inlet port;

(b) opening the inlet valve while the outlet valve is closed, moving the driven member to a first position such that the elastomeric membrane adjusts a volume of the chamber to correspond to the targeted intake volume, thus filling the chamber with fluid;

(c) determining a first pressure proximate the inlet port using the inlet pressure sensor;

(d) moving the driven member to a second position such that the elastomeric membrane is forced into the chamber, thus decreasing the volume of the chamber, the inlet pressure sensor determining a second pressure proximate the inlet port that exceeds the first pressure proximate the inlet port by a predetermined amount;

(e) determining the actual intake fluid volume as a function of the first pressure proximate the inlet port, the second pressure proximate the inlet port, the first position of the driven member, and the second position of the driven member; and

(f) determining a difference between the targeted intake fluid volume and the actual intake fluid volume.

37. The method of claim 36, wherein the predetermined amount is about 1 psi.

38. The method of claim 36, wherein the difference between the targeted intake fluid volume and the actual intake fluid volume is used to increase the accuracy of the fluid infusion, further comprising the step of adding the difference between the targeted intake fluid volume and the actual intake fluid volume to a targeted intake fluid volume of a subsequent pump cycle.

39. The method of claim 36, wherein functional relationships between the intake fluid volume, the proximate pressure, and the position of the driven member are empirically determined.

40. A pump for administering a fluid to a patient, comprising:

(a) a fluid displacement device that is adapted to couple with a drive unit and to couple in fluid communication with a source, said drive unit actuating the fluid displacement device to deliver a fluid drawn from the source to a patient by displacement of the fluid from a chamber disposed within the fluid displacement device into which the fluid flows from the source;

(b) a control unit that is coupled to the drive unit to control operation of the fluid displacement device; and

(c) at least one pressure sensor electrically coupled to the control unit and producing a signal indicative of a pressure of the fluid within said chamber, said control unit being coupled to receive the signal from said at least one pressure sensor and using the signal to determine a differential pressure for the fluid during a pumping cycle, wherein said control unit determines a correction based upon the differential pressure and applies the correction in controlling the drive unit when displacing the fluid from the chamber, to compensate for the differential pressure, thus ensuring accurate delivery of the fluid to the patient.

41. A pump employed to infuse a fluid flowing from a source into a patient, comprising:

(a) a disposable cassette having a housing that defines a fluid path from an inlet port that is adapted to couple in fluid communication with the source, to an outlet port that is adapted to couple in fluid communication with an infusion site on the patient, said fluid path including;

(i) a chamber covered by an elastomeric membrane that when forced into the chamber, displaces the fluid from the chamber through the outlet port, said chamber including a pressure sensor that produces a signal indicative of a pressure in the chamber

(ii) an inlet valve controlling flow of the fluid into the chamber from the inlet port, said inlet valve being open as fluid flows into the chamber from the inlet port and closed as the fluid is displaced through the outlet port; and

(iii) an outlet valve controlling flow of the fluid from the chamber toward the outlet port, said outlet valve being open when the fluid is displaced through the outlet port and closed as the fluid flows into the chamber from the inlet port;

(b) a drive unit adapted to couple with disposable cassette and including at least one prime mover and a plunger, said at least one prime mover moving the plunger relative to the chamber to deform the elastomeric membrane, thereby displacing fluid from the chamber toward the outlet port, said drive unit also actuating the inlet valve and outlet valve to open and close said valves; and

(c) a control unit, coupled to receive the signals from the pressure sensor, and coupled to the drive unit to control said at least one prime mover and thereby controlling movement of the plunger, said control unit determining a differential pressure from the signals produced by the pressure sensor and adjusting an initial position of the plunger relative to the chamber before the plunger displaces the fluid from the chamber, to compensate for the differential pressure, so that an accuracy of the pump in infusing the fluid into the patient is substantially improved.

42. A system for maintaining a desired delivery rate of a fluid flow through an intravenous line, comprising:

(a) a pump chassis;

(b) a pump in fluid communication with the intravenous line and mountable within the pump chassis to receive a driving force from a driven member, said pump including an inlet port, an outlet port, an elastomeric membrane overlying a chamber comprising a portion of a fluid path extending between the inlet port and outlet port, said driven member exerting a force on the elastomeric membrane that displaces the fluid from the chamber and into the intravenous line;

(c) at least one pressure sensor that produces a pressure signal indicative of a pressure in the chamber;

(d) a controller that is electrically coupled to the at least one pressure sensor to receive pressure signals at various parts of a pump cycle, said controller employing the pressure signals to determine a first correction factor to be applied to an initial position of the driven member, said first correction factor being expressed as a change in a position of the driven member relative to the chamber.

43. A method for minimizing an error in a disposable cassette pump used for infusing a fluid into a patient, comprising the steps of:

(a) sensing a first pressure within a pumping chamber of the cassette pump during a first part of a pump cycle;

(b) sensing a second pressure within a pumping chamber of the cassette pump during a second part of a pump cycle;

(c) determining a differential pressure between the first pressure and the second pressure;

(d) using the differential pressure, determining a compensation pressure that should be applied to the fluid in the chamber before infusing the fluid into the patient; and

(e) applying the compensation pressure to the fluid in the chamber before infusing the fluid into the patient.

44. A method for determining a difference between a targeted intake fluid volume and an actual intake fluid volume in an infusion pump employed for infusing a fluid into a patient, comprising the steps of:

(a) providing a cassette having a housing that defines a fluid path between an inlet port adapted to couple in fluid communication with a source and an outlet port adapted to couple in fluid communication with an infusion site on the patient, said fluid path including;

(i) a chamber covered by an elastomeric membrane that when forced into the chamber by a driven member displaces the fluid from the chamber through the outlet port;

(ii) an inlet valve controlling a flow of the fluid into the chamber from the inlet port, said inlet valve being open as the fluid flows into the chamber from the inlet port and closed as the fluid is displaced through the outlet port;

(iii) a pressure sensor that produces a signal indicative of a pressure in the chamber; and

(iv) an outlet valve controlling a flow of the fluid from the chamber toward the outlet port, said outlet valve being open when the fluid is displaced through the outlet port and closed as the fluid flows into the chamber from the inlet port;

(b) opening the inlet valve while the outlet valve is closed, and while the inlet valve is open, moving the driven member to a first position such that the elastomeric membrane adjusts a volume of the chamber to correspond to the targeted intake volume, thus filling the chamber with fluid;

(c) determining a first pressure in the pumping chamber using the pressure sensor;

(d) moving the driven member to a second position such that the elastomeric membrane is forced into the chamber, thus decreasing the volume of the chamber, the pressure sensor determining a second pressure in the pumping chamber that exceeds the first pressure by a predetermined amount;

(e) determining the actual intake fluid volume as a function of the first pressure, the second pressure, the first position of the driven member, and the second position of the driven member; and

(f) determining a difference between the targeted intake fluid volume and the actual intake fluid volume.

45. A method for determining a difference between a targeted delivery fluid volume and an actual delivery fluid volume in an infusion pump employed for infusing a fluid into a patient, comprising the steps of:

(a) providing a cassette having a housing that defines a fluid path between an inlet port adapted to couple in fluid communication with a source and an outlet port adapted to couple in fluid communication with an infusion site on the patient, including an outlet pressure sensor that produces a signal indicative of a pressure proximate the outlet port, said fluid path including;

(i) a chamber covered by an elastomeric membrane that when forced into the chamber by a driven member displaces the fluid from the chamber through the outlet port;

(ii) an inlet valve controlling a flow of the fluid into the chamber from the inlet port, said inlet valve being open as the fluid flows into the chamber from the inlet port and closed as the fluid is displaced through the outlet port; and

(iii) an outlet valve controlling a flow of the fluid from the chamber toward the outlet port, said outlet valve being open when the fluid is displaced through the outlet port and closed as the fluid flows into the chamber from the inlet port;

(b) opening the outlet valve while the inlet valve is closed, and while the outlet valve is open, determining a first pressure proximate the outlet port using the outlet pressure sensor;

(c) moving the driven member to a first position such that the elastomeric membrane reduces a volume of the chamber to correspond to the targeted delivery volume, thus emptying the chamber of fluid;

(d) determining a second pressure proximate the outlet port using the outlet pressure sensor;

(e) closing the outlet valve while the inlet valve is still closed, and then opening the inlet valve while the outlet valve is closed;

(f) while the inlet valve is open, moving the driven member to a second position such that the elastomeric membrane expands a volume of the chamber to correspond to the targeted delivery volume, thus filling the chamber with fluid;

(g) determining the actual delivery fluid volume as a function of the first pressure proximate the outlet port, the second pressure proximate the outlet port, the first position of the driven member, and the second position of the driven member; and

(h) determining a difference between the targeted delivery fluid volume and the actual delivery fluid volume.