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Stroke volume, ejection fraction, and cardiac output
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cardiac output p. 291
cardiac output and p. 292
equation for p. 732
exercise and p. 692
in pregnancy p. 657
variables in p. 291
V /Q mismatch and p. 691
cardiac output p. 291
Now, let’s zoom into the left ventricle. There’s a moment when the left ventricle is fully relaxed. It occurs at the end of filling or diastole, also called the end-diastolic point, and the volume of blood within the left ventricle is called the end-diastolic volume, and it’s about 120 milliliters. Then the left ventricle contracts, forcing blood through the aorta and into the whole arterial system. After that is another moment when the left ventricle is fully contracted. It occurs at the end of contraction or systole, also called the end-systolic point, and the volume of blood within the left ventricle is called end-systolic volume, and it’s about 50 milliliters. So, end-diastolic volume minus end-systolic volume, gives us the stroke volume, which is the volume of blood that the left ventricle ejects with every heartbeat, or stroke. In this case, the stroke volume is 120 minus 50, which equals 70 milliliters.
Stroke volume is a useful measurement, but it can vary based on the size of a person. For example, a stroke volume of 50 milliliters might be absolutely fine for a small person with a small heart volume, but may be low for a large person with a bigger heart volume. So another helpful measurement is the ejection fraction, which is the stroke volume divided by the end-diastolic volume, Ejection fraction = Stroke Volume / End- Diastolic Volume. In a normal individual that’s 70/120, or about 58%, but it can fluctuate between 50 and 65% and still be considered normal. In other words, at least half of the blood volume in the left ventricle should get pumped out during each heartbeat. In hearts that have a low contractility - a low force of contraction - the ejection fraction can fall below 50%.
Stroke volume is the amount of blood the heart pumps with each beat. It is the difference between the end-diastolic volume and the end-systolic volume. The ejection fraction is the proportion of the end-diastolic volume that is pumped out with each beat. It is calculated as stroke volume divided by the end-diastolic volume. Cardiac output is the amount of blood the heart pumps in one minute and is equal to the heart rate multiplied by the stroke volume.
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