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