US 6,497,680 B1 · Method for compensating for pressure differences across valves in cassette type IV pump

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.

Claims17

Dependent claims

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.

5 more elements

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.

9 more elements

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.

10 more elements

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, compromising 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 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;
(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;
(f) determining an actual volume of fluid delivered in a current pump cycle;
(g) 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;
(h) applying a change in the duration of the delivery stroke during the next pump cycle;
and
wherein the step of determining the change in the duration of a delivery stroke comprises the steps of:
(1) 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
(2) 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.