Fig. 1

Relation between changes in cardiac output (CO) by end-inspiratory hold maneuvers and both right atrial pressure (Pra) defining the venous return curve and mean arterial pressure (Pa) defining the arterial pressure-flow relations in a ventilated anesthetized patient. Arterial critical closing pressure (Pcc) is the zero-flow extrapolation of the ventricular output curve, just as mean circulatory filling pressure (Pmsf) is the zero-flow extrapolation of the venous return curve. The pressure difference between Pcc and Pmsf is the vascular waterfall. The pressure difference between arterial pressure and Pcc is the tissue perfusion pressure. In this example from a post-operative cardiac surgery patient for a mean arterial pressure of 75 mmHg and a Pcc is 35 mmHg, the tissue perfusion pressure is (75 mmHg-35 mmHg), or 40 mmHg.