SYSTEMIC FILLING PRESSURE IN INTACT CIRCULATION DETERMINED ON BASIS OF AORTIC VS CENTRAL VENOUS-PRESSURE RELATIONSHIPS

In the intact circulation, mean systemic filling pressure (P-sf) is determined by applying a series of inspiratory pause procedures (IPPs) and using Guyton's equation of venous return (Q(v)) and central venous pressure (P-cv): Q(v) = a - b x P-cv. During an IPP series, different tidal volumes are applied to set P-cv at different values. From the linear regression between Q(v) and P-cv, P-sf can be calculated as P-sf = a/b. Guyton's equation can also be written as Q(v) = (P-sf - P-cv/R(sd), where R(sd) is the flow resistance downstream of the places where blood pressure is equal to P-sf. During an IPP, a steady state is observed. Therefore, we can also formulate the following equation for flow: Q(s) = (P-ao - P-sf)/R(su), where Q(s) is systemic flow, R(su) is the systemic flow resistance upstream to P-sf, and P-ao is aortic pressure. Because both flows (Q(s) and Q(v)) are equal, it follows that P-ao = P-sf(1 + R(su)/R(sd)) - R(su)/R(sd) x P-cv. This equation implies a method to determine mean systemic filling pressure on the basis of P-ao measurements instead of flow determinations. Using 22 IPPs in 10 piglets, we determined the mean systemic filling pressure, and we compared the values obtained from the flow curves with those obtained from the aortic pressure curves. The mean difference between the two methods was 0.03 +/- 1.16 mmHg. With the use of P-ao measurements, the P-sf can be estimated as accurately as in using flow determinations. The advantage of the new method is that estimation of cardiac output is not required.