Figure 4.

Ventilatory effects on the relationship between cardiac function and venous return at different potential stages in sepsis. Cardiac function is indicated by curved lines, and venous return by straight lines. (a) With spontaneous breaths the pressure around the heart falls relative to atmospheric pressure, which is represented by a leftward shift of the cardiac function curve. Under basal conditions the cardiac function curve intersects the venous return curve near the plateau of the venous return curve, so that the leftward shift produces only a small or no increase in right heart filling with inspiration. (b) If the right atrial pressure starts from a higher value and the fall in pleural pressure is increased because of changing pulmonary mechanics, then the inspiratory increase in right heart stroke volume is increased. (c) If the heart is functioning on the plateau of the cardiac function curve, there is no increase in right heart filling with inspiration. However, there is an increase in the transmural pressure (pressure across the wall) of the heart. (d) When a patient is ventilated with positive pressure, inspiration moves the cardiac function curve to the right. There is thus an inspiratory fall in right ventricular stroke volume and a rise in venous pressure, which can affect upstream organs such as liver and kidney. (e) The reduction in cardiac output with continuous positive pressure can be reversed by giving volume, which shifts the venous return curve to the right (dark circle). However, when positive pressure is removed this will result increased cardiac filling, and if the left ventricle does not handle the increased flow (open circle) pulmonary oedema may result. Pra, right atrial pressure; TM, transmural pressure.