Among all topics taught in cardiac physiology, the concept of cardiac output seems to be the most important. This is because understanding the cardiac output gives an idea of how the cardiac muscle is doing its job pumping blood to the body.

In addition, cardiac output has many clinical implications since understanding heart failure, one of the most common heart diseases, is all about understanding the concept of cardiac output.

The cardiac output formula is straightforward, and it won’t require lots of effort to interpret or memorize. Nevertheless, we will be spending lots of time here digging deeper into the concept of cardiac output, the factors affecting it, and how doctors use it in real life.

How to calculate cardiac output?

Cardiac output is defined as the volume of blood the heart pumps through the circulatory system per minute. Normally, it is around 5 L/min at rest. Here is the cardiac output equation:

Cardiac output (CO) = Heart rate (HR) x Stroke volume (SV)     Where:

Heart rate (HR) is the number of beats per minute

Stroke volume (SV) is the volume of blood pumped by the left ventricle in a single heartbeat. Stroke volume can also be defined as the end-diastolic volume (EDV) minus end-systolic volume (ESV).

What is the relation between stroke volume and cardiac output?

Most students confuse cardiac output with stroke volume. This is expected since these two concepts tend to go together. Also, you need to figure out the stroke volume before you can calculate the cardiac output. Let’s make this concept crystal clear.

Stroke volume is used to calculate the amount of blood pumped in a single heartbeat. In contrast, cardiac output is used to calculate the amount of blood pumped for 1 minute. So, if your heart rate is 70/min, then your cardiac output is the amount pumped every beat for 70 beats.

Here is how to use the cardiac output equation in a real example: Suppose John has a resting heart rate of 80 beats per minute, and his stroke volume is 90mL. What is the cardiac output?

Solution:

• The heart rate (# of beats per minute) is 80 beats per minute
• The stroke volume (the amount of blood pumped each beat) is 80 mL
• Cardiac output = heart rate (80) x stroke volume (90) = 7200 ml/min

Factors affecting cardiac output

There are multiple variables like the size of the heart, the physical condition of the individual, sex, contractility, preload (EDV), and afterload that control the cardiac output. Let’s learn more about these factors and see what factors can increase/decrease cardiac output.

Heart rate

Out heart rate varies during the day. At rest, it can range from 60 to 100 bpm. During physical activity, it can increase up to 200 bpm. Therefore, the heart rate increases during physical activity and, consequently, the cardiac output increases.

If someone has a sick heart, they will develop cardiac output abnormalities. For example, suppose someone is in severe shock, and their heart rate goes down. In that case, their cardiac output can significantly decrease.

Stroke volume

As in heart rate, stroke volume variations can lead to changes in cardiac output. What makes stroke volume hard is that many factors control it. Changes in these factors lead to changes in stroke volume and, as a result, cardiac output. These factors are:

Preload

Preload is the amount of blood pumped to the heart before it contracts. The higher the preload, the higher the stroke volume and, of course, the cardiac output. Preload increases during exercise because of contraction of your muscles leading to a faster venous return to the heart.

Think of your heart as a rubber band. The more you stretch it, the stronger it will rebound. Similarly, higher preload à greater stretch in the heart muscle à high stroke volume and cardiac output.

Afterload

Afterload is the resistance against your heart pumping blood into the circulation. This resistance is in the form of constriction of blood vessels. The more these vessels constrict, the higher resistance is put on your heart.

Think of it as someone pushing against a wall. If the wall is thick and rigid, they will have to make a greater effort to move this wall forward. Higher afterload à high resistance à the heart can’t pump blood enough à decreased stroke volume à reduced cardiac output.

Contractility

The last factor that controls your stroke volume is contractility. It refers to how strong is each contraction. You can have stronger contractions during sympathetic activation. This leads to increased catecholamines in the blood. Catecholamines make each contraction more powerful and increase the amount of blood pumped each beat, leading to a rise in cardiac output.

The frank-starling mechanism and cardiac output

Before discussing heart failure, we need to visit a very important concept related to cardiac output: the frank-starling mechanism. It is a law that describes the relationship between end-diastolic volume, stroke volume, and cardiac output.

It states that an increase in the end-diastolic volume (preload) causes the heart muscle (myocardium) to stretch more (↑ end-diastolic length of cardiac muscle fibers). This increases contractility and results in an increase in stroke volume and cardiac output.

The frank-starling mechanism maintains cardiac output by increasing myocardial contractility in response to an increased preload (end-diastolic volume).

Have a look at the image below. If we increase blood volume, that will increase the stretch of the heart muscle and increase cardiac output. Similarly, injecting any sympathomimetic medication like epinephrine will increase excitability and contractility, leading to a powerful contraction.

Heart failure and cardiac output

Now into the most important clinical application of the concept of cardiac output: heart failure. Heart failure is a disease in which the heart cannot maintain enough cardiac output to meet the metabolic needs of the body.

Heart failure is related to cardiac output because it can be classified based on where it is associated with reduced ejection fraction (HFrEF) or preserved ejection fraction (HFpEF).

The ejection fraction is the percentage of the blood pumped by the left ventricle during each contraction (stroke volume divided by end-diastolic volume; normally ∼ 55%). Therefore, think of it as another way of saying stroke volume.

What is the difference between HFrEF and HFpEF? In HFrEF, there is reduced ventricular contractility leading to systolic dysfunction. Basically, the ventricle can’t pump enough blood anymore. On the other hand, in HFpEF, the contractility is normal, but there is decreased ventricular compliance leading to diastolic dysfunction. The presence of diastolic dysfunction makes the ventricle unable to relax and fill with blood initially. This is why the ejection fraction here is preserved. It is just that the amount of blood entering and leaving the ventricle is too small (and causing decreased cardiac output for sure).

Therefore, we can say that in HFrEF, CO is small because the ventricle is not pumping enough, while in HFpEF, the CO is small because the ventricle is not filling sufficiently with blood.

Conclusion

Good job! We are impressed by your dedication. You have learned a lot about the concept of cardiac output. In this article, we went through the definition and formula of cardiac output. We also learned about the factors that affect it and some clinical points like heart failure. Here is a summary of what you have learned:

Definition of cardiac output	Cardiac output (CO) is the amount of blood pumped by the heart minute
Equation	Cardiac output (CO) = heart rate (HR) x stroke volume (SV)
Factors that control cardiac output	Preload	The stretch of the myocardium or end-diastolic volume of the ventricles. Think of it as the volume in a ventricle just before the start of systole. Higher preload means higher cardiac output.
	Afterload	The amount of pressure that the heart needs to exert to eject the blood during ventricular contraction. Higher afterload means decreased cardiac output.
	Contractility	The innate ability of the heart muscle (cardiac muscle or myocardium) to contract. Stronger contraction means higher cardiac output.
	Heart rate	The number of heart contractions per minute (beats per minute, bpm). The greater it is, the higher the cardiac output.