What is Cardiac Output?

Tricia Christensen
Tricia Christensen

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.

Cardiac output measures how much blood the heart pumps in a minute's time.
Cardiac output measures how much blood the heart pumps in a minute's time.

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.

When blood doesn't pump as expected during exercise, there may be an underlying heart problem.
When blood doesn't pump as expected during exercise, there may be an underlying heart problem.

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.

A stress echocardiogram could be one means for measuring the heart at work and determining that output is healthy and appropriate.
A stress echocardiogram could be one means for measuring the heart at work and determining that output is healthy and appropriate.

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.

An irregular cardiac output may be a sign of an underlying heart problem.
An irregular cardiac output may be a sign of an underlying heart problem.

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.

After completing cardiovascular exercises, an athlete typically experiences lower blood pressure than he had prior to starting the workout.
After completing cardiovascular exercises, an athlete typically experiences lower blood pressure than he had prior to starting the workout.
Tricia Christensen
Tricia Christensen

Tricia has a Literature degree from Sonoma State University and has been a frequent wiseGEEK contributor for many years. She is especially passionate about reading and writing, although her other interests include medicine, art, film, history, politics, ethics, and religion. Tricia lives in Northern California and is currently working on her first novel.

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