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What is Cardiac Output?

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  • Written By: Tricia Ellis-Christensen
  • Edited By: O. Wallace
  • Last Modified Date: 30 August 2016
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There are a number of measurements of how well a heart functions and one of these is to determine the volume of blood the heart pumps in a minute’s time. This is called cardiac output, and measuring it is a common feature of several familiar heart tests. It may be measured when people have echocardiograms, and it is often even more accurately calculated if a person has an angiogram or cardiac catheterization.

To understand cardiac output, which is often designated as Q, it helps to understand some other key terms. One of these is stroke volume or SV. SV would be the most precise as possible measurement of the amount of blood that leaves the heart with each stroke. Equally important is the heart rate or HR; this is the number of beats per minute.

Presuming people know or can guess at stroke volume, it then becomes fairly easy to calculate cardiac output. It is merely the product of heart rate and stroke volume in a single minute. This amount can vary among individuals. Those with heart problems might have decreased output and very well trained athletes may have a higher volume output. Interestingly, people with significant athletic training don’t usually have a higher heart rate, but they tend to have a higher stroke volume, which means their hearts pump slower but pump more blood with each beat.

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There can be variety in what is considered normal cardiac output. Many people cite that the volume of 5 L is normal, which is like saying the body circulates its entire supply of blood in one minute. This measurement isn’t always the “normal” one for people, and many suggest that a range of cardiac output from between 4-8 L is a more appropriate statistic, or that any numbers received are best interpreted by a doctor. It should be understood that some account for variation is based on things like level of fitness and it is generally expected that output is higher in those in greater heart health.

As could be predicted, in the healthy heart cardiac output increases with exercise. The heart beats more and circulates the blood more quickly, theoretically. When blood pumped doesn’t increase as would be expected, this could be a significant health finding. It might suggest failure of the heart to properly respond to exercise, and this could result in the heart becoming overwhelmed or being more at risk with exercise. A stress echocardiogram could be one means for measuring the heart at work and determining that output is healthy and appropriate. People who have had heart problems in the past often have these periodically to determine what forms of exercise they can perform safely.

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