Understanding Guyton's venous return curves

Abstract

Based on observations that as cardiac output (as determined by an artificial pump) was experimentally increased the right atrial pressure decreased, Arthur Guyton and coworkers proposed an interpretation that right atrial pressure represents a back pressure restricting venous return (equal to cardiac output in steady state). The idea that right atrial pressure is a back pressure limiting cardiac output and the associated idea that "venous recoil" does work to produce flow have confused physiologists and clinicians for decades because Guyton's interpretation interchanges independent and dependent variables. Here Guyton's model and data are reanalyzed to clarify the role of arterial and right atrial pressures and cardiac output and to clearly delineate that cardiac output is the independent (causal) variable in the experiments. Guyton's original mathematical model is used with his data to show that a simultaneous increase in arterial pressure and decrease in right atrial pressure with increasing cardiac output is due to a blood volume shift into the systemic arterial circulation from the systemic venous circulation. This is because Guyton's model assumes a constant blood volume in the systemic circulation. The increase in right atrial pressure observed when cardiac output decreases in a closed circulation with constant resistance and capacitance is due to the redistribution of blood volume and not because right atrial pressure limits venous return. Because Guyton's venous return curves have generated much confusion and little clarity, we suggest that the concept and previous interpretations of venous return be removed from educational materials.

Fig. 1.

Guyton's venous return curve. Original data from Guyton et al. (12) are plotted showing the steady-state relation between flow (F = cardiac output = venous return) and right atrial pressure (P_RA) measured when flow was altered by limiting the inflow to an artificial pump with a collapsible tube. In the experiments of Guyton et al., a pump was used to bypass the right ventricle. Plotting the right atrial pressure on the abscissa incorrectly suggests that the right atrial pressure was the independent variable in the experiments. The line is a least-squares fit of Eq. A2 to the data, yielding R_VR = 5.08 mmHg·min·liter⁻¹ and P_MS = 5.97 mmHg. See Glossary for definitions of abbreviations.

Fig. 2.

Compliant circuit model. A: Fig. 5 from Guyton et al. (12), illustrating the model of the systemic circulation (used with permission). B: electrical circuit analog of model. See Glossary for definitions of abbreviations.

Fig. 3.

Behavior of Guyton's model with flow identified as the independent variable. A: original data from Fig. 1 are replotted with flow plotted on the abscissa. The intercept on the ordinate is the right atrial pressure (equal to the mean systemic pressure) when flow (cardiac output) is zero. Guyton et al. (12) reported the mean arterial pressure at the intercept of the abscissa as 112 mmHg. B: the calculated arterial pressure according to the model when flow is varied over the range defined in A. Note that as cardiac output is increased, arterial pressure increases and thus the arterial capacitor fills and the venous capacitor empties because the blood volume is constant. Circuit element parameters are set to values determined in the appendix: R_A = 95.2 mmHg·min·liter⁻¹, R_V = 0.069 mmHg·min·liter⁻¹, C_T/C_A = 19, and the mean systemic filling pressure is set to P_MS = 5.97 mmHg as determined in Fig. 1. See Glossary for definitions of abbreviations.

Fig. 4.

Influence of arterial resistance on circuit pressures. A: prediction from Guyton's model of the response of right atrial pressure when arterial resistance is varied with a constant cardiac output of 0.6 liter/min (an intermediate value from the data set in Figs. 1 and 3). B: the simultaneous change in arterial pressure as arterial resistance is varied. Note that as arterial resistance is increased arterial pressure increases, filling the arterial capacitor by shifting volume from the venous capacitor. Parameters are set to values indicated in Fig. 3. See Glossary for definitions of abbreviations.