Feedback control of mean aortic pressure in a dynamic model of the cardiovascular system

Orbital measurements of the cardiac function of Space Shuttle crew members have shown an initial increase in cardiac stroke volume upon entry into wei ghtlessness, followed by a gradual reduction in stroke volume to a level ap proximately 15% less than preflight values. In an effort to explain this re sponse, it was hypothesized that gravity plays a role in cardiac filling. A mock circulatory system was designed to investigate this effect. Prelimina ry studies carried out with this system on the NASA KC-135 aircraft, which provides brief periods of weightlessness, showed a strong correlation betwe en cardiac filling, stroke volume, and the presence or absence of gravity. The need for extended periods of high quality zero gravity was identified t o verify this observation. To accomplish this, the aircraft version of the experiment was reduced in size and fully automated for eventual integration into a Get Away Special canister to conduct an orbital version of the expe riment. This article describes the automated system, as well as the develop ment and implementation of a control algorithm for the servoregulation of t he mean aortic pressure in the orbital experiment. Three nonlinearities tha t influence the ability of the apparatus to regulate to a mean aortic press ure of 95 mm Hg were identified and minimized. In preparation for a Space S huttle flight, the successful function of the servoregulatory scheme was de monstrated during ground tests and additional test flights aboard the KC-13 5. The control algorithm was successful in carrying out the experimental pr otocol, including regulation of mean aortic pressure. The algorithm could a lso be used for the automated operation of long-term tests of circulatory s upport systems, which may require a scheduled cycling of the pumping condit ions on a daily basis.