Question 13

Discuss the regulation of cardiac output. Illustrate your answer by using a graph to describe the important physiological relationships.

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College Answer

Most candidates approached this question by defining cardiac output as stroke volume × heart 
rate and then discussing the determinants of cardiac output - preload, contractility, afterload and 
heart rate rather than focusing on the regulation of cardiac output. Under preload a brief 
description of the Frank Starling mechanism was required. Important was the concept that at rest 
cardiac output is controlled almost entirely by peripheral factors that determine venous return. 
These concepts were best illustrated by graphing vascular function (venous return vs right atrial 
pressure) and cardiac function (cardiac output vs right atrial pressure) curves. Then 
demonstrating on these curves the factors that affected preload, contractility and afterload such 
as changes in blood volume, sympathetic and parasympathetic stimulation and exercise as 
examples. Also important to demonstrate on these curves was the fact that venous return and 
cardiac output are equal at steady state. Most candidates tried to illustrate these cardiovascular 
concepts with a series of left ventricular pressure volume loops rather than use the vascular and 
cardiac function curves. They then went on to demonstrate via these pressure volume loops the 
effects of changes in preload, contractility and afterload on stroke volume. Candidates who took 
this approach were not penalised, if there were clear, correct diagrams with explanations 
indicating comprehension of these concepts. On the whole graphs were poorly drawn and were 
not well integrated into the answer. Some candidates also wasted time by unnecessarily 
describing excitation-contraction coupling and sympathetic nerve reflex pathways

Recommended sources: Guyton Textbook of Medical Physiology 11th edition pgs 241-243

Discussion

With "regulation" the operative term in the stem, one might even be tempted to interpret this question as being about the cardiac reflexes and other systems which adjust cardiac output in response to changing conditions. However, from the extremely specific CICM answer,  one can clearly see that they wanted a Guytonian explanation of the determinants of cardiac output, centered around the vascular and cardiac function curves. 

  • Cardiac output, at a steady state, is determined by venous return. 
  • This relationship is described by the cardiac and vascular function curves, which intersect at the point which describes the resting steady state, where cardiac output and venous return are equal.

    cardiac and vascular function curves superimposed
     
  • The cardiac function curve is cardiac output as a function of right atrial pressure.
    • This curve describes the Frank-Starling relationship
      As contractility increases, the curve shifts up
    • A plateau is seen with higher RA pressures
  • The vascular function curve is venous return as a function of right atrial pressure.
    • Crosses the x-axis at the mean systemic filling pressure
    • A plateau is seen with right atrial pressure below 0 mmHg.
  • Changes to the operating conditions of the cardiovascular system can change the position of this equilibrium point in a predictable manner:
    • An increase in preload (volume, MSFP) increases cardiac output, up to a maximum (plateau) permitted by contractility and heart rate, and is associated with an increase in the right atrial pressure.
      Guyton diagram with increasing MSFP
    • An increase in contractility increases the cardiac output at any given volume/MSFP, and is associated with a decrease in right atrial pressure
      effect of changes in contractility on Guyton's diagram
    • An increase in peripheral vascular resistance sequesters blood in the arterial circulation, decreases the venous return and decreases cardiac output.
      effect of increasing peripheral vascular resistance on Guyton's diagram

References

Beard, Daniel A., and Eric O. Feigl. "Understanding Guyton's venous return curves." American Journal of Physiology-Heart and Circulatory Physiology 301.3 (2011): H629-H633.

Guyton, Arthur C. "Regulation of cardiac output." Anesthesiology: The Journal of the American Society of Anesthesiologists 29.2 (1968): 314-326.

Guyton, A. "Circulatory Physiological: Cardiac Output and Its Regulation." Philadelphia and London: WB Saunders Company (1963).

Guyton, Arthur C. "Determination of cardiac output by equating venous return curves with cardiac response curves." Physiological reviews 35.1 (1955): 123-129.

Levy, MATTHEW N. "The cardiac and vascular factors that determine systemic blood flow." Circulation research 44.6 (1979): 739-747.

Guyton, Arthur C., Arthur W. Lindsey, and Berwind N. Kaufmann. "Effect of mean circulatory filling pressure and other peripheral circulatory factors on cardiac output." American Journal of Physiology-Legacy Content 180.3 (1955): 463-468.

Guyton, Arthur C., James H. Satterfield, and Jeff W. Harris. "Dynamics of central venous resistance with observations on static blood pressure." American Journal of Physiology-Legacy Content 169.3 (1952): 691-699.

Guyton, Arthur C., Dimitri Polizo, and George G. Armstrong. "Mean circulatory filling pressure measured immediately after cessation of heart pumping." American Journal of Physiology-Legacy Content 179.2 (1954): 261-267.

Guyton, Arthur C., et al. "Venous return at various right atrial pressures and the normal venous return curve." American Journal of Physiology-Legacy Content 189.3 (1957): 609-615.

Persichini, Romain, et al. "Effects of norepinephrine on mean systemic pressure and venous return in human septic shock." Critical care medicine 40.12 (2012): 3146-3153.

Henderson, William R., et al. "Clinical review: Guyton-the role of mean circulatory filling pressure and right atrial pressure in controlling cardiac output." Critical care 14.6 (2010): 1-6.

Berlin, David, Vivek Moitra, and Jan Bakker. "Guyton at the Bedside." Monitoring Tissue Perfusion in Shock. Springer, Cham, 2018. 25-34.

Andrew, Philip. "CrossTalk proposal: Guyton's venous return curves should be taught." The Journal of Physiology 591.Pt 23 (2013): 5791.

Beard, Daniel A., and Eric O. Feigl. "CrossTalk opposing view: Guyton's venous return curves should not be taught." The Journal of physiology 591.Pt 23 (2013): 5795.

Sunagawa, Kenji. "Guyton’s venous return curves should be taught at medical schools (complete English translation of Japanese version)." The Journal of Physiological Sciences 67.4 (2017): 447-458.

Levy, MATTHEW N. "The cardiac and vascular factors that determine systemic blood flow." Circulation Research 44.6 (1979): 739-747.