Monday Apr 28, 2025

Cardiac Physiology Basics - Output, Index, Preload, Afterload, Contractility | Clinical Medicine

Cardiac output and cardiac index are two critical concepts to understand. We tie together preload, afterload, contractility, heart right, end diastolic volumes, the cardiac cycle, Frank Starling curves, Anrep Affect, and more in a clear and concise manner! We also talk about the physiologic things that contribute to these different variables.

Cardiac output is the amount of blood the heart squeezes out every minute. It is equal to the stroke volume times the heart rate. What contributes to these two variables though? The stroke volume is the end diastolic volume minus the end systolic volume and is affected by three things: preload, afterload, and contractility. The preload is the end diastolic volume, which is essentially the amount of blood in the ventricle right before is starts to contract in systole. As it increases, stroke volume tends to increase up until the ventricle becomes overstretched as demonstrated by the Frank Starling curve. The after load is the pressure that the ventricle has to contract against. Typically, as the after load increases the stroke volume decreases. The exception to this is the Anrep Affect, which we explain. Contractility is how powerful of a contraction the heart muscles make. As it increases, the strove volume increases. It is intimately connected to and affected by preload and afterload. Heart rate is more straightforward, but remember that there is a point where continued increases in heart rate actually decrease cardiac output given too short of time for diastolic filling. Cardiac index then is the cardiac output divided by the body surface area, which provides a more informative look at how well the body is being supplied with blood.

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