Guyton's theory on venous return, implying a linear relationship between blood flow and central venous pressure, was tested in 12 mechanically ventilated ICU patients during standard care.

The central venous pressure was changed by applying four different constant inspiratory plateau pressures over 12 seconds. Mean values of central venous pressure and cardiac output were measured with pulse contour analysis over the last 3 seconds of this plateau period and were plotted against each other to construct a venous return curve. During the inspiratory plateau periods, hemodynamic steady-state circumstances were met without an observable change in cardiovascular control mechanisms. Two different volemic states were created: normovolemia in the supine position (SUP) and hypervolemia by volume loading with 0.5 l intravenously (SUP-V).

Guyton's linear venous return pressure–flow relationship was confirmed. The average slope of the relation during SUP was not significantly different from the slope during SUP-V. The mean systemic filling pressures derived from these venous return curves during SUP and SUP-V were 18.8 ± 4.5 mmHg and 29.1 ± 5.2 mmHg, respectively (P < 0.001). During SUP the calculated total circulatory mean compliance was 0.98 ml/mmHg/kg and the mean stressed volume was 1,677 ml.

The mean systemic filling pressure, systemic compliance and stressed volume can be determined in mechanically ventilated patients with intact circulation using inspiratory pause procedures. These results may imply a physiological tool to assess the volume state of the circulation as well as fluid responsiveness of mechanically ventilated patients in the ICU.