The use of Guyton’s approach to the control of cardiac output for clinical fluid management

Figures

• 

  Fig. 1. Limitations to venous return and maximum VR (Vmax) with normal pleural pressure (A) and Positive pleural pressure (B). In A, when VR intersects the 0 line (atmospheric pressure) venous collapse occurs. A increase in the cardiac curve (dotted line) in this state does not increase cardiac output (closed red circle). In B, a rise in pleural pressure results in venous collapse and flow limitation at a pressure above atmospheric pressure. A shift of the venous return curve to the right is then needed to increase VRmax. This can occur by giving volume or by conversion of unstressed into stressed volume through reflex mechanisms or drugs (see text). RAP is right atrial pressure and Q represents cardiac output.

• 

  Fig. 2. Control of cardiac output by the interaction of the venous return function (left upper) and cardiac function (right upper). When RAP = MSFP there is no flow. Lowering RAP (by the pumping heart), allows for venous return to occur. The intersection of the cardiac function and return function (centre) gives the “working” right atrial pressure (RAP), “working” cardiac output and “working” venous return. “Q” is flow. See text for further details. Blue lines indicate VR and red line cardiac function. A decrease in cardiac output (Q) with a fall in RAP (left lower)indicates a primarily “return” problem. Fluids are likely the best choice. A decrease in Q with an increase in RAP (right lower) indicates a primarily “Pump” problem. An inotrope is likely the best clinical choice. See text for further details.