Cardiac Output

What is cardiac output?

Cardiac output is how many liters of blood your heart pumps in one minute. Your healthcare provider can figure this out with this cardiac output equation: multiply stroke volume by heart rate.

• Stroke volume (the amount of blood your heart sends to your body in one heartbeat) can vary based on how hard your heart muscles have to work (and the force they need to use) to push your blood out to your body. Stroke volume can go up or down based on your heart health and whether you’re at rest or moving.
• Heart rate (number of heartbeats per minute) is normally 60 to 100 beats per minute. Your heart rate can go up or down depending on whether you’re resting or exercising.

Sometimes, like when you’re exercising, your body needs more oxygen. At that time, your body can change its cardiac output by adjusting your heart rate and stroke volume. Blood delivers oxygen to your cells, so you need more cardiac output when your active body is using more oxygen than usual.

When do you need to know cardiac output?

Your healthcare provider may use the cardiac output formula to find out why you’re having trouble exercising. They may measure your cardiac output if they think you have heart failure. They may suspect it if you’re not able to exercise as much as you did in the past.

They may want to know your cardiac output to make a diagnosis or find out if a medicine, device or procedure they gave you is working.

Decreased cardiac output symptoms

Symptoms of decreased cardiac output include:

• Not being able to exercise much.
• Feeling very tired.
• Swelling in your arms and legs.
• Shortness of breath.
• Nausea and vomiting.
• Abdominal pain.

How to calculate cardiac output

Your healthcare provider can use several different methods to figure out your cardiac output. These methods include:

• Find out the amount of oxygen an organ or tissue uses by subtracting how much oxygen is in your veins from how much oxygen is in your arteries. Then you multiply it by blood flow rate.
• Compare the temperature of blood in your vena cava or right atrium with the temperature of blood in your pulmonary arteries.
• Take mean arterial pressure (your arteries’ average pressure) and divide it by systemic vascular resistance (the amount of force or pressure your vessels put on your blood).
• Use a portable device that involves breathing certain gases through a mouthpiece.
• Use a device that involves putting six electrodes on your chest. These measure your heart rate and stroke volume and calculate cardiac output from those.
• Transthoracic echocardiography to estimate stroke volume.
• Heart MRI.
• Inject dye into your blood when it goes into your heart’s right side and measure how much dye comes out of your heart’s left side.

What to expect during the test

For the most accurate method, your provider will need to measure how much oxygen you’re breathing in and get blood samples from an artery and from a catheter in your pulmonary artery. Your provider can do this during right heart catheterization. You will be able to find out your cardiac output the same day along with information regarding pressures inside your heart.

What type of results do you get and what do the results mean?

Normal cardiac output ranges from 5 to 6 liters per minute in a person at rest. While exercising, an athlete can have a cardiac output of more than 35 liters per minute. A non-athlete’s cardiac output will be lower than an athlete’s but higher than when the non-athlete is at rest. Your cardiac output also goes up during pregnancy.

Some factors affecting cardiac output

Factors that affect your cardiac output include:

• High blood pressure: Your heart has to pump with more force to get blood into your arteries. Like walking into a wind gust, your heart is pushing blood against arteries’ higher pressure. Your heart also has to work harder to move blood to your lungs if their blood pressure is high (because of scars or emphysema).
• Older age: As you age, your heart’s walls get stiffer, keeping your heart from filling with blood as quickly or as well as it once did. This makes your heart pump out less blood to your body.
• Heart attack that damaged your heart: It may not be able to pump as much blood as it did before the heart attack. An infection (myocarditis) can lower your heart’s ability to pump, too.
• Abnormal heart rhythm like atrial fibrillation: Your heart’s upper chambers can’t pump enough blood into your heart’s lower chambers and out to your body.
• Aortic stenosis (narrow aortic valve): Because the valve leading to your aorta is narrow, you can’t get enough blood to it. This is a problem because your aorta is the biggest blood vessel that sends blood to your body.
• Constrictive pericarditis: When the sac (pericardium) around your heart gets stiff, your heart can’t relax, fill with blood and pump as it should.

How to improve cardiac output

Getting more blood to your body depends on a strong heart and healthy blood vessels. You can help your cardiovascular system work better in these ways:

• Exercise regularly.
• Keep your cholesterol numbers low, except for the good kind of cholesterol (HDL).
• Follow your treatment plan if you have diabetes.
• Avoid fried foods and other foods high in fat.
• Stay at a healthy weight.
• Don’t use tobacco products.
• Control your blood pressure with medicine if necessary.

A note from Cleveland Clinic

Knowing how much blood your heart is able to pump can help your healthcare provider make a diagnosis or find out if a treatment they gave you is working. Once they have that information, they can consider the available options to help you. Be sure to ask questions if you aren’t clear about your choices. Being informed can help you make good decisions for the best outcome.