Computer control of the infusion of vasoactive drugs

Abstract

In the Cardiac Surgical Intensive Care Unit (CICU) of the University of Alabama Hospital in Birmingham, Alabama, the mean arterial pressure (MAP) is regulated by computer (H-P 21 MX) controlled infusion of vasodilating agents and has been performed in 1100 hypertensive patients following open heart surgery.

The MAP controller was developed based on investigation of the dynamics of the physiological responses of hypertensive patients to trimethaphan camsylate (Arfonad) and sodium nitroprusside (Nipride). Representative responses were selected for investigation by digital simulation of a computer programmed version of a proportional-integral-derivative (PID) controller algorithm. The PID controller parameters were tuned to elicit a response with acceptable settling time and minimal overshoot during the time course of the simulated MAP.

Eight IMED 929 (IMED Corporation, San Diego, CA) digitally controlled infusion pumps interfaced with our computer based CICU system are used to regulate the blood pressure of patients following cardiac surgery. A microprocessor based blood pressure controller employing this algorithm has been developed and is being evaluated.

For system design, analysis, and evaluation, the essential aspects of the arterial blood pressure (MAP) and its response to nitroprusside has been modeled. The arterial pressure is considered the sum of nonstationary stochastic background activity and a change due to the nitroprusside (characterized by a transfer function). The model was applied to the simulation of a clinically used controller and has provided insight into responses observed in the treatment of patients. The model is presently being used for computer-aided design and evaluation of an adaptive controller.