Cardiac Output Fundamentals

Overview of Cardiac Output

What is cardiac output? Cardiac output is the amount of blood ejected out of the heart into the body over one minute. The formula we use to calculate cardiac output is:

Cardiac output = Heart rate × Stroke volume

Let’s take a look at that heart rate. It’s pretty simple. The heart rate is the number of beats per minute. For the purpose of this discussion, we are going to use a heart rate of about 70 beats per minute (bpm). If you go back look at the algorithms for treating heart rates, generally, they are pretty easy and quick to deploy.

Related Video – What is Cardiac Output?

Now, we get to stroke volume. The stroke volume is the amount of blood ejected by the ventricles with each contraction. On a given day, the average person has a stroke volume of about 70 mL. That’s for one beat.

An example calculation of cardiac output. An average cardiac output is about 4900 mL per minute or rounded to about 5 liters per minute.

Looking at this formula, if we take a heart rate of 70 bpm and a stroke volume of 70 mL, we are going to come up with a cardiac output of about 4900 mL. If you look at some of the texts, they say the average cardiac output is about 5 liters per minute. That’s where they got the number from because they are using averages. A person’s actual cardiac output will differ, but for the purpose of this discussion, we are going to use those averages.

Review of Blood Flow Through the Heart

Before we dive into stroke volume, let’s back up and take a look at how blood actually moves through the heart. We were taught that blood enters the right atrium and then the right atrium contracts and squeezes blood into the ventricles. 

Those statements are about 20% true. Here’s why. As blood is coming in from the superior and inferior vena cava, it is entering the right atrium. Keep in mind that the tricuspid valve is between the atrium and the ventricle. The valve is like a one-way trapdoor. As long as the pressure is greater on the top, the valve is going to open and blood is going to flow through it.

Read: Systems of Care: In-Hospital Cardiac Arrest

In fact, the only time the one-way valve closes is during systole when the ventricles are contracting. The pressure becomes greater in the ventricles, and it slams those valves closed. The blood will either go through the pulmonary artery or out through the aorta if it is coming from the left ventricle. Therefore, all the blood coming into the right atrium actually passes right through the valve and starts filling up the ventricles, too.

Atrial Kick

Blood comes from the superior vena cava and inferior vena cava. The blood enters the atrium, passes right through the valve, and starts filling up the ventricular chambers. The ventricle is getting more and more full of blood, and so is the atrial chamber. Then, the atrial chamber contracts and it squeezes its volume of blood into the ventricle. We called that volume the atrial kick. The atrial kick contributes about 20 to 25% of the additional volume reaching the ventricles. Essentially, it just tops off the tank.

Sometimes the atrial chambers contribute nothing. For example, with atrial fibrillation. Blood is coming into the atrial chamber, but the atrial chamber is just quivering. The blood goes into the ventricles, fills up the ventricles, but the atrial chamber never contracts because it is fibrillating. You lose maybe 25% of that blood that would have been injected into the ventricles. You lose your atrial kick because of the atrial fibrillation.

End-diastolic Volume and End Systolic Volume

During diastole, blood is coming down and filling up the ventricular chamber. The blood is filling, filling, and filling…then, the atrial chamber contracts and kicks its volume into the ventricular chamber right before systole. The amount of volume in the ventricular chamber before the ventricles contract is called the end-diastolic volume. 

The end-diastolic volume is more than the stroke volume. In this scenario, the end-diastolic volume would be around 120 mL. That 120mL is the volume of blood in the ventricle when it is completely full. 

When systole occurs, the ventricle contracts and kicks out a stroke volume of about 70 mL. That leaves us with 50 mL leftover in the ventricular chamber. We call the blood that is left over the end-systolic volume, or the amount of blood that is left in the ventricular chamber at the end of systole.

Related Video – What is the Cardiac Cycle?

The end-diastolic volume is the total volume of blood in the ventricle right before systole.

During contraction, not all of the blood in the ventricles is pumped out. There is blood left over in the ventricle after contraction, which is known as the end-systolic volume.

Ejection Fraction

Stroke volume is the end diastolic volume minus the end systolic volume. The percentage of the stroke volume compared to the end diastolic volume is called the ejection fraction (EF). The ejection fraction is the percentage of blood that the ventricle pumps out with each contraction. For example, of that 120mL of blood in the ventricle when it’s full, we kicked out 70 mL. 70 mL divided by 120 mL is about 58%. The ejection fraction in this case is 58%.

Related Video – What is Stroke Volume?

A normal ejection fraction is usually between 50 and 70%. On average, we like to see an ejection fraction above 60%. An ejection fraction less than 50% is a cause for alarm. Essentially, ejection fraction speaks to how well the heart is contracting. If it is less than 50% you need to wonder if it is it a systolic problem. Is there not good squeeze out of that ventricular chamber?

Related Video – What is Ejection Fraction?

Effect of Bradycardia and Tachycardia on Cardiac Output

Let’s look at the end diastolic volume and how it fits back into our cardiac output equation. If we have an extremely bradycardic patient with a very slow heart rate, that slow heart rate allows for adequate ventricular filling time. The patient will be getting a good end diastolic volume. In a bradycardic patient, the bradycardic heart rate probably won’t impact my stroke volume directly.

However, if you have an extremely tachycardic patient, the heart contracts before the blood can fill up the ventricular chamber adequately. The heart spits out less volume that it usually would. That’s why extremely tachycardic heart rates can directly impact your stroke volume. The heart just doesn’t have enough time for those ventricles to fill.

Now, take tachycardia and combine it with atrial fibrillation. Let’s say you have a patient in atrial fibrillation with a rapid ventricular response. Now the atrial kick is lost and there is reduced end diastolic filling time. Volume is lost from both atrial and ventricular chambers. That combination of heart conditions will directly reduce the cardiac output.

Frank-Starling Law

Another factor when considering cardiac output is called the Frank-Starling Law. The Frank-Starling Law says the greater the stretch of the myocardium, the greater the force of contraction. It’s easy to understand by using a balloon analogy. If you have a flat balloon with no volume in it, there’s no stretch and no contraction.

Now, let’s put a little volume into this balloon. It stretched a little bit, which means there is a little more strength of contraction. As more volume goes into the balloon there will be greater stretch.

To better understand the Frank-Starling law, think of the stretch of a balloon as an analogy to the stretch of the heart.

Let’s think back to our patient who has atrial fibrillation with rapid ventricular response. Since the patient had atrial fibrillation,you need to account for that by reducing the volume of the balloon. So, take a little volume out. If the patient is also tachycardic with reduced filling time, take even more volume out of the balloon.

Now you can see the impact of Frank-Starling’s Law on a patient that has atrial fibrillation with rapid ventricular response. You’ve lost more volume coming into the heart, and now there is less stretch to the myocardium. Therefore, you’ll have less force of contraction and less cardiac output. That is Frank Starling’s Law very simplified.

Summary

To review our formula, cardiac output equals heart rate times stroke volume. Say we’ve got a heart rate of 70 bpm and a stroke volume of 70 mL. If the patient becomes bradycardic and that heart rate drops to 35 bpm, my cardiac output is cut in half. Now, let’s say that the patient is extremely tachycardic. That directly reduces the stroke volume, which reduces the cardiac output.

What is the takeaway from this segment? Always treat the rate first. 

Why? One, because it’s quick and easy. Go back and look at the treatment algorithms. You can give a drug, use pacing, or cardiovert them. It’s quick and easy, which is a good thing. For example, if I’m 40,000 feet in the aircraft and my patient starts to decompensate, I want a treatment that is quick and easy.

Treat the rate first.

So, always treat the rate first. Such as in bradycardia because the stroke volume might be fine, but it’s the rate that is impacting the cardiac output. In tachycardia, the rate is directly reducing the stroke volume, which is reducing their cardiac output.