Mean systemic filling pressure as a characteristic pressure for venous return

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Abstract
Guyton's theory on venous return, implying a linear relationship between blood flow and central venous pressure, was tested in an intact circulation after thoracotomy and airtight chest closure. In eleven Yorkshire pigs (approx. 10 kg) we measured flow in the pulmonary artery and aorta and pressure in the central veins and aorta during pentobarbital anesthesia and mechanical ventilation. To change central venous pressure different lung volumes were randomly applied at intervals of 5 min in a series of inspiratory hold procedures of 7.2 s. During these short periods hemodynamic steady state circumstances were met without involvement of cardiovascular control mechanisms. We confirmed the linear relationship between venous return and central venous pressure and derived mean systemic filling pressure from the regression equation. Mean systemic filling pressure was on average 10.5±2.3 (SD) mm Hg. The time dependent changes during the inspiratory hold procedure showed that the increase in central venous pressure was the primarily dependent variable, followed by a decrease in venous return and right ventricular output. After a delay of 2–4 heart beats also a decrease in left ventricular output and aortic pressure occurred. Subsequently, the lower venous return during inspiratory hold was mainly sustained by the lower aortic pressure, but nevertheless fulfilled the linear relationship mentioned above. For analysis of flow and pressure changes in the systemic circulation during changes of central venous pressure a tube of constant flow resistance was used as a conceptual model. Consequently, the point where mean systemic filling pressure exists during normal flow conditions was predicted at a characteristic location in the peripheral venous system. Downstream from this point blood pressure will rise and vessel capacity will be filled up during increases in central emptying vessel capacity partially.