How To Calculate Cardiac Output And Why It’s Important

Many people are worried about heart health, or cardiovascular health. And why wouldn't they be? Even though you may not know all of the technical jargon, you can still benefit from knowing a little bit more about the topic. This is why it could help to learn how to calculate cardiac output.

Cardiac output is a good indicator of how well your heart works. If your heart pumps blood efficiently, this means that you don't have much to worry about in terms of cardiovascular problems. If it doesn't, that's just a sign that you may need to make some lifestyle changes.

Learning how to calculate cardiac output could be very helpful just by creating a stronger understanding of the cardiovascular system. As they say, knowledge is power. This is why learning everything you need to know about cardiovascular health can help you ensure your own.

What Is Cardiac Output?

In order to understand how to calculate cardiac output, you first need to understand what this is. Simply put, cardiac output is the amount of blood that your heart pumps in one minute.

Cardiac output is expressed in the unit of liters per minute. A lot of people might think that you just focus on the heart to figure out this quantity, but this is not the case. There are many factors that influence cardiac output outside of the heart, and you need to consider the other components of the circulatory system as well.

There are many factors that have an influence on cardiac output. These include heart rate, preload, afterload, and contractility.

Heart Rate

It's pretty easy to see how heart rate affects cardiac output. Obviously, the faster your heart beats, the more blood it can pump in a minute or any other period of time.

However, it's not as simple as thinking the faster your heart beats, the higher the cardiac output will be. If your heart beats too quickly for too long, it'll end up tiring out and slowing down out of exhaustion. There's an optimal heart rate that'll give you optimal cardiac output.

Preload

In order to do their work, muscle cells typically expand and contract. This is true of myocardial cells within the ventricle of the heart as well. Preload is the degree of myocardial distension before they contract.

In this case, distension is how far the cells stretch out before they contract. One significant property of myocardial cells is that the force with which they contract depends on how far they were stretched.

The more the cells stretch, the more the ventricle becomes distended, and the more force with which the cells will contract. This will lead to higher cardiac output.

Afterload

The afterload is the force that the ventricles need to act against in order to get blood out of the heart. This is very dependent on arterial blood pressure and vascular tone. Reducing the afterload can increase cardiac output by making it easier for the heart to pump blood.

Contractility

Just like the word would imply, contractility is the ability of the heart muscle to contract. If the heart muscle is more able to contract, this will result in a higher cardiac output.

However, as is the case with heart rate, too much contractility will result in fatigue of the heart. In some cases, this could lead to a complete collapse of the heart muscle. This could be dangerous or even fatal.

How To Calculate Cardiac Output

​Now that you know a little bit more about cardiac output and what determines this quantity, you can learn more about how to calculate cardiac output.

Calculating Cardiac Output

You can calculate cardiac output by multiplying two different variables. These would be heart rate and stroke volume.

The variables

You can easily determine your own heart rate by feeling the pulse rate on either your neck or wrist. You can count the number of pulses per minute or per 10 seconds and multiply that number by six. If you have a doctor or scientist perform this measurement for you, they'll likely determine it by attaching electrodes to your chest and recording electrical activity.

Stroke volume is the volume of blood that your ventricles (the large muscular chambers of your heart) push out with every beat of your heart. Doctors can take echocardiograms, which allow them to view and measure your heart to calculate stroke volume.

So as long as you know your heart rate and stroke volume, you can calculate cardiac output pretty easily. How to calculate cardiac output just involves multiplying these two quantities.

The equation

Professionals write the equation as follows:

Cardiac Output = Stroke Volume * Heart Rate, or CO = SV * HR

As long as you know all of the variables, it can be pretty easy to calculate cardiac output. However, this doesn't mean that you'll be able to learn how to calculate cardiac output in your living room. It's not easy to calculate stroke volume without a doctor and all of their machines, for example.

As previously stated, however, you can calculate heart rate pretty easily on your own. You can do this either by physically putting your fingers on your wrist or neck and tracking your pulse or by using a heart rate monitor or fitness tracker.

The latter option might be better if you want to see your heart rate over a period of time; it's particularly helpful if you want to compare your resting heart rate to your heart rate during exercise.

What's A Normal Cardiac Output?

Now that you know more about how to calculate cardiac output, you might be wondering what a normal result would be. On the average, if you're a healthy adult, your heart should pump about 5 liters of blood per minute. This would make the average cardiac output for a healthy person about 5 liters per minute.

It's also worth knowing that the typical adult has about 5 liters of blood in his or her body. This means that every minute, even when you're completely at rest, your heart will circulate your entire blood volume. That's quite a bit of work for an organ that's really not that big at all, isn't it?

However, you have to remember that any deviation from 5 liters per minute does not necessarily mean that a person is unhealthy. Everyone's different, and not every healthy person is going to have exactly the same cardiac output. You should keep in mind that a larger person will have a higher cardiac output than a smaller person in most cases, given that a bigger person is just going to have more blood in their body.

When you're thinking about how to calculate cardiac output, it's important to first calculate cardiac output for any given person at rest. Naturally, the heart will beat more quickly and vigorously when the person is exercising. This is because it's doing a lot more work; in some cases, cardiac output during exercise can be about five times what it is during rest.

Typically, when talking about how to calculate cardiac output, we're referring to this quantity when the person is at rest. This is the baseline that professionals can use for comparison to other healthy individuals or for any other purpose.

Why Measure Cardiac Output?

someone checking blood pressure

​Image source: Pixabay

You may wonder why exactly it's so important to know about how to calculate cardiac output. After all, it's not the only measure of heart health.

There are a few reasons why it's important to know how to calculate cardiac output. Medical professionals, such as cardiologists, study the heart so they can see whether the patient is in good cardiac health or not.

In The Short Term

One way to measure heart function in general lies in knowing how to calculate cardiac output. This is a good indication to doctors of how well your heart is doing its job. In the short term, cardiac output is important because it predicts how quickly your cells will receive oxygen through the pumped blood.

The primary function of your cardiovascular system is to provide the tissues of your body with enough oxygen. These tissues need all this oxygen so that they can perform their necessary functions. If cardiac output isn't sufficient, your entire body can end up suffering.

In The Long Term

Doctors often use people's cardiac output and changes in the quantity over time to see their cardiovascular functioning over time. If someone has changes in cardiac output over a period of years, this could reflect a change in their heart's ability to pump blood and do its job. Doctors can use this quantity to see whether a person's cardiovascular health is improving or declining.

In Case Of Disease

A very low cardiac output can be very dangerous. It can be an indicator of severe disease in a patient. Patients who are critically ill with very low cardiac output generally are at much greater risk of adverse health effects.

In some of these patients, the cause of low cardiac output is an inability of the heart to fill with blood. Doctors can address this with fluid resuscitation, but they need the cardiac output measurement in order to know to do this.

Doctors not only see cardiac output as significant but they also significantly note changes in cardiac output. There's no official agreement as to how much of a change needs to take place in order to be significant, but doctors typically note changes of 15 percent or more to be relevant.

In the case of a patient who is considered hemodynamically unstable, clinicians typically need to take measurements of cardiac output over and over again. They have many measuring devices and techniques for this.

​Pumping In, Pumping Out...

As you can probably see, there's way more to cardiac output than you would have thought. You may or may not have known what cardiac output was before, but you probably didn't know just how important it is. Now you know just how important it can be to learn how to calculate cardiac output.

You may not be able to calculate cardiac output right in the comfort of your own home. Although you can pretty easily calculate heart rate, you won't necessarily be able to calculate stroke volume without more sophisticated equipment than most people have in their homes. However, it's good to know the equation and process nevertheless.

Just having a stronger understanding of cardiac output can help you monitor your own health more conscientiously. You can have more informed conversations with doctors when you know about these sorts of things. They may even be impressed that you know how to calculate cardiac output!

What do you think of everything you've just learned about how to calculate cardiac output? Share your thoughts in the comments section!

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