Fundamentals of feedback control applied to microcomputer instrumentation design

Review article: closed-loop systems in anesthesia: is there a potential for closed-loop fluid management and hemodynamic optimization?

Closed-loop (automated) controllers are encountered in all aspects of modern life in applications ranging from air-conditioning to spaceflight. Although these systems are virtually ubiquitous, they are infrequently used in anesthesiology because of the complexity of physiologic systems and the difficulty in obtaining reliable and valid feedback data from the patient. Despite these challenges, closed-loop systems are being increasingly studied and improved for medical use. Two recent developments have made fluid administration a candidate for closed-loop control. First, the further description and development of dynamic predictors of fluid responsiveness provides a strong parameter for use as a control variable to guide fluid administration. Second, rapid advances in noninvasive monitoring of cardiac output and other hemodynamic variables make goal-directed therapy applicable for a wide range of patients in a variety of clinical care settings. In this article, we review the history of closed-loop controllers in clinical care, discuss the current understanding and limitations of the dynamic predictors of fluid responsiveness, and examine how these variables might be incorporated into a closed-loop fluid administration system.

Development and evaluation of a dual-pump microcomputer-based closed-loop arterial pressure control system

Hypertension after cardiac surgery is common and is associated with increased morbidity. It is usually managed by the infusion of short acting vasodilators. The use of a closed-loop computer system to control the infusion of a vasodilator has been shown to compare favourably with manual control. We have developed a closed-loop system for the ATARI 1040ST microcomputer to control arterial pressure by the simultaneous infusion of two vasodilators. The control program is based on a proportional-integral-derivative algorithm which has been adapted to allow control of two IMED 929 infusion pumps from one RS-232 port. All communication between the user and computer is carried out with a 'mouse', thus increasing acceptability of the system to ward staff. Cardiovascular data are collected on-line from the patient monitor via a custom-built analogue to digital convertor. This system was used to study glyceryl trinitrate and sodium nitroprusside in 24 patients requiring vasodilators after cardiopulmonary bypass. The study showed that in 14 of the patients hypertension was controlled by GTN alone and 10 required supplementary SNP. We have demonstrated that this dual-pump automatic arterial pressure control system is a satisfactory and safe method of administering two vasodilators simultaneously. It is suitable for both routine clinical and research uses.