CrossTalk opposing view: Guyton's venous return curves should not be taught

The thesis of this essay is that Guyton's model of the systemic circulation has been so misinterpreted by Guyton and others that it generates more confusion than insight. Because of the capacitance of arteries and veins, blood pressure will attain a finite value if blood flow is quickly stopped without a change in vascular tone. This static pressure is called the ‘mean systemic pressure’PMS. Guyton devised an experimental preparation in open chest dogs where an external artificial pump drew blood from the right atrium and delivered it to the pulmonary artery. The output flow rate of the pump was manually adjusted by raising and lowering a collapsible tube (Starling resistor) at the inlet (right atrial) side of the pump. In this way the steady state cardiac output flow of the left ventricle was set as the independent experimental variable. In other words, the artificial pump opened the circuit of the circulation and haemodynamically isolated the right atrium from the heart so that the systemic circulation could be studied in quasi-isolation. As will be explained, the confusion over Guyton's results and associated model can be traced to the mistaken interpretation of the measured right atrial pressure as an independent causal variable governing the behaviour of the system. Using the preparation with a Starling resistor, Guyton made steady state measurements of right atrial pressure at different pump output levels, including mean systemic pressure when pump output was zero (Guyton, 1955, 1959). An example of Guyton's original data is shown in Fig. 1A. Remember that these are steady state measurements where venous return equals cardiac output by definition. A, data from Guyton et al. (1959) are plotted showing the steady state relation between flow (F= cardiac output = venous return) and right atrial pressure (PRA) measured when flow was altered by limiting the inflow to an artificial pump with a collapsible tube. B, the data from A are replotted with flow plotted on the abscissa correctly indicating that cardiac output is the independent variable. The intercept on the ordinate is the right atrial pressure (equal to the mean systemic pressure) when flow (cardiac output) is zero. C, the calculated arterial pressure according to Guyton's model, with constant systemic vascular resistance, when flow is varied over the range defined in A and B. See Beard clearly it is the pump (cardiac) output. Guyton's second major misconception was the assertion that the driving pressure for steady state cardiac output (‘venous return’) is mean systemic pressure minus right atrial pressure (PMS–PRA). R VR is an equivalent resistance (Guyton's ‘resistance to venous return’) that is simply a consequence of this equation, and does not represent an actual physical structure or mechanism. It is misleading to think of PMS as a driving force for flow through a resistor. PMS is solely a function of the magnitude of blood volume and systemic vascular capacitance. In steady state, PMS is not a function of flow, and flow is not a function of PMS. However, right atrial pressure is a function of flow in the Guyton model as emphasized by Beard 591/23/5795 Disclaimer: Supplementary materials have been peer-reviewed but not copyedited. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.