1. Field of the Invention
This invention relates to apparatus for pumping fluids intravenously or intra arterially to patients, and more particularly, it relates to the control systems for such fluid infusion pumping apparatus which regulate the flow rates under which such pumping is accomplished.
2. Description of the Prior Art
In recent years there has been a considerably increased use of positive displacement fluid infusion pumping devices for delivering fluids intravenously or intra arterially to patients in hospitals or other patient care locations. These have to a large extent replaced the time honored gravity flow control systems primarily due to their much greater accuracy in determining delivery rates and dosages, their relative sophistication in permitting a flexible and controlled feed from multiple liquid sources, and particularly their ability to control with precision the amount of dangerous drugs delivered to a patient over a given period of time.
In a typical positive displacement fluid infusion pump system, stepping motors are used to gradually deliver the I.V. fluids by displacing an amount of liquid trapped within a chamber in very small increments. In this way, shock to the nervous system of a patient is minimized and a relatively uniform flow of fluid is obtained. Various prior United States patents disclosing typical fluid infusion pump control systems include U.S. Pat. Nos. 4,474,309 to Edward G. Solomon; 4,443,216 to Anthony G. Chappell; 4,396,385 to Timothy G. Kelly et al.; 4,392,849 to John H. Petre et al.; 4,308,866 to Roger W. Jelliffe et al.; 4,280,494 to Robert J. Cosgrove, Jr. et al.; 4,217,993 to Thurman S. Jess et al.; 4,126,132 to Peer M. Portner et al.; 4,037,598 to Heinz W. Georgi; and 3,985,133 to John Arthur Jenkins et al.
The United States patent to Solomon, U.S. Pat. No. 4,474,309, discloses a typical fluid infusion pump system wherein a small disposable cassette having a pumping chamber with a flexible diaphragm forming one exterior surface thereof is adapted to be placed in a driver or controller mechanism, and with the cassette including a programmable plunger adapted to interact with the diaphragm to pump fluid from the chamber. The plunger is driven by a stepping motor through a series of short, liquid-displacing steps and then retracted quickly once it has reached its maximum displacement in the chamber so as to assure as uniform a flow of liquid as possible. The inlet and outlet valves to the pumping chamber may be actuated from the driver (as is the case with the inlet valve in the Solomon patent disclosure) or they may be comprised of check valves controlled by the differential pressure provided by the pumping plunger (as in the case with the outlet valve in the Solomon disclosure patent). As is conventional with pumping systems of this type, the user sets a rate at which the fluid is to be delivered as well as the total dosage which is to be delivered by the apparatus. The system will then operate by providing the fluid at the predescribed rate in a generally uniform flow and will automatically shut down when the accumulated running total of liquid delivered equals the preset total dosage to be delivered.
While careful control of the pumping mechanism and limiting the delivery to short strokes spaced over time insures a more uniform flow rate, it has been found that physiological shocks may still occur in particular patients, especially when the delivery of I.V. solutions is started and also when it abruptly stops.