Three dimensional closed loop control of the human circulation.

In the control of the circulation it is suggested there is merit in describing states which are the object of each therapy. Considering the determinants of venous return rather than those of cardiac output, we suggest that the mean systemic filling pressure is a parsimonious description of the volume state. A method is described of deriving a mean systemic filling pressure analogue based on stopping the flow in a notional regional systemic circulation. The mean systemic filling pressure (PM) forms a quantitative link between volume therapy and circulatory dynamics. The systemic vascular resistance is a conventional measure of the state of arteriolar resistance. We suggest that the ratio (PM - PRA)/PM (where PRA is the right atrial pressure) is a global measure of the effect of cardioactive drugs. Studies are reported in which the mean blood pressure, right atrial pressure and cardiac output are simultaneously controlled within desired tolerances using state based control.