3 Applying inspiratory holds of increasing airway pressure levels Pcv increases and CO decreases to a steady-state level ( Figure 1). Recently we demonstrated that it was possible to determine Pmsf at the bedside in mechanically ventilated postcardiac surgery patients with an intact circulation. The difference between Pmsf and right atrial pressure or central venous pressure (Pcv) during steady-state flow represents the pressure gradient for venous return, and if CO is known, one can calculate the resistance to venous return as the ratio of driving pressure to flow. 2 Pmsf reflects the mean weighted up-stream pressure for venous return to the heart. When the heart is stopped and a large arteriovenous fistula opened, arterial and venous pressures rapidly equilibrate to one pressure, which is called mean systemic filling pressure (Pmsf). 1 For short time periods venous return and CO can differ, but averaged over time venous return must be equal to CO. In the classical view cardiac output (CO) is determined by cardiac function (contractility, heart rate), preload and afterload, despite Guyton’s studies on venous return.
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