The present invention relates to a fluid infusion system which provides program delivery of intravenous fluids at a continuous flow rate or in a substantially linearized rate of increase or decrease.
Some medical conditions disrupt the normal processes by which the human body extracts nutrition from food, requiring intravenous administration of large quantities of hypertonic dextrose. This procedure is known as total parenteral nutrition or TPN.
The infusion of fluids into the human body is usually accomplished by means of administration set in conjunction with a metering apparatus which controls the flow rate of fluid through the set. Peristaltic-type pumps, which function by repetitively compressing an expanding a section of tubing, have proven particularly attractive for use in metering apparatus since they do not introduce the possibility of leaks or contamination into the system, while providing positive flow of fluid through the system.
In applications where medication is being administered, it is necessary to operate the peristaltic pump to infuse only a predetermined volume of fluid at a predetermined rate. Patients need a control system for peristaltic-type infusion pumps which enable them to preset a quantity of fluid and the rate of infusion. The system must continuously monitor the performance of the pump to insure compliance with the preset parameters and alert the operator in the event of a malfunction.
Because of the physiological effects associated with the sudden infusions of fluid into the human body, it is often desirable to gradually increase and decrease the rate at which the infusion system delivers the fluid. Rather than initiating the pump at the full rate of delivery, it is preferable that the system ramps or tapers up to the full delivery rate. Likewise, it is often desireable that the system gradually tapers down the delivery rate at the end of the cycle, rather than abruptly stopping fluid delivery.
This is particularly critical when the fluid being infused contains high concentrations of glucose. A sudden infusion of glucose directly into the blood stream can create substantial adverse effects on the patient, including disorientation and unconsciousness. Similar effects can also occur if the delivery rate is suddenly decreased.
A major problem with existing peristaltic-type pumps is that the resulting delivery of fluid occurs in a series of pulses or surges, the frequency of which is equal to the frequency of the passage of successive rollers in contact with the delivery tube. The effect is that fluid is delivered at widely varying rates during a pump cycle, which is unacceptable where uniformity of delivery rate is required.
The pulses or surges of fluid usually produced by peristaltic-type pumps are difficult to accurately convert into linear rates of increase or decrease. Use of a programmable infusion pump with taper-on and taper-off capabilities is discussed in U.S. Pat. No. 4,785,799, issued to Schoon et al. Schoon discloses a taper function with the number of steps dependent on an arbitrarily chosen increment of 5 ml/hr. The number of steps depends on the difference between the starting rate and the steady-state rate.
Programming of ambulatory infusion systems can be a complicated process. The average patient finds it difficult to calculate the appropriate infusion rate and period. This calculation is made extremely more complicated when tapers are involved. With most medical infusion pumps currently available, if the user attempts to program an infusion cycle that exceeds the minimum or maximum flow rate of the pump, an error message is given and the user is forced to recalculate the infusion parameters, rather than having the system suggest a valid parameter value.
Patient operated infusion systems also require a back-up capability which retains system parameters, even if power is lost. However, non-volatile memory devices have a limited useful life, which degrades in proportion to the number of write-sequences executed. This problem is further complicated by the fact that the amount of information necessary to back-up during a taper cycle is significantly greater then during continuous or plateau fluid delivery.
Finally, it is often desireable to retain previously programmed delivery cycles. However, pre-set infusion cycles should not interfere with the patients ability to "customize" the cycle. Specifically, a new delivery cycle should begin if any of the infusion parameters are changed and the patient needs the option to initiate the taper up or down sequence at any time during "plateau" or continuous fluid delivery.