Cardiac output depends on the interrelated factors of heart rate (HR) and stroke volume (SV):
Cardiac output = HR x SV
Stroke volume is the volume of blood ejected by the ventricle with a single contraction. It can be defined as the difference between end-diastolic and end-systolic volume. SV expressed as a percentage is the ejection fraction. Stroke volume has three major determinants:
This basic physiology is clinically important because, in low cardiac output states leading to hypoperfusion, the heart rate, preload, afterload, and then contractility should be addressed - in that order. There is little point in trying to pharmacologically improve the contractility of a heart when it is either too empty or too distended to eject.
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