Cardiac Output - Definition, Calculation , Measurement

Cardiac output or heart output is an important concept in cardiac physiology. It is defined as the amount of blood pumped out of each heart ventricle in per minute duration. Also, it is often equal to the product of heart rate and stroke volume. Explore this article further to know more about the calculation of cardiac output and the factors affecting this process.

Cardiac Output Definition

Cardiac output refers to the amount or volume of blood pumped from each ventricle. It typically refers to the left ventricular outflow via the aorta. Furthermore, it is the most crucial component of the cardiovascular system as it determines how quickly blood flows through various body organs.

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Cardiac Output Calculation

Cardiac output (CO) is the volumetric flow rate of the heart’s pumping output that is often denoted by Q. Usually, it is expressed with the help of heart rate and stroke volume.

1. Heart rate (HR) - It is the frequency of the heartbeat determined by the number of heart contractions per minute. Normally it is between 60 to 100 beats/minute.
2. Stroke Volume (SV) - It is the volume of blood pumped out in each ventricle during each heartbeat. If the heart rate is normal (72/minute), then the usual SV value is 70 mL (60 to 80 mL).

Thus the final cardiac output (CO) formula is as follows:

CO  =  HR ✕ SV

Cardiac Output  =  Heart Rate  ✕  Stroke Volume

This cardiac output is often expressed as L/min. The average cardiac output at rest is 5 L/min, with the normal range being 4.0 to 8.0 L/min. For athletes, it can go up to 40 L/min.

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Cardiac Index

The cardiac index is a parameter that relates CO or cardiac output and body surface area. It represents the amount of blood pumped out per ventricle/ minute/ square meter of the body's surface area. In this way, it connects an individual's size to their heart's performance.

CI = CO (Cardiac Output) ➗ BSA (Body Surface Area)

It is expressed as liters per minute per square meter (L/min/m 2 ).

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Measurement of Cardiac Output

There are different indirect as well as direct methods of measuring cardiac output. Only animals are often treated with direct methods. Indirect methods are used for both humans and animals.

• Direct Methods - Cardiometer and Flowmeter
• Indirect Methods - Doppler echocardiography, doppler ultrasound, thermodilution technique, etc.

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Factors Determining Cardiac Output

The cardiac output is determined or maintained by 4 factors:

1. Venous Return
2. Heart Rate
3. Peripheral Resistance
4. The force of Contraction

Venous Return

Venous return is the amount of blood that returns to the heart via the veins from various body segments. When the venous return raises, the ventricular filling as well as the cardiac output also raises. Thus venous return is directly proportional to cardiac output as long as the remaining 3 factors i.e, heart rate, the force of contraction, and peripheral resistance remain constant.

Heart Rate

There is also a direct correlation between cardiac output and heart rate. Thus if the heart rate increases, the CO also increases if the other three variables remain constant. Also, a slight variation in heart rate has no impact on cardiac output. Cardiac output only increases when the heart rate increases noticeably and also vice versa.

Peripheral Resistance

The resistance to blood flow present in peripheral blood arteries is known as peripheral resistance. It is the resistance or load against which the heart must pump blood. Therefore, the relationship between peripheral resistance and cardiac output is inverse. As peripheral resistance increases, the cardiac output decreases, and vice versa.

The force of Contraction

Cardiac output is directly proportional to contraction force. This force of contraction depends on preload and afterload. Preload is the amount of blood preloading in the ventricles. Thus the stretching of ventricles increases preload. This increases the blood flow inside the heart. In the end, it has an impact on stroke volume and then raises cardiac output.

Afterload is defined as the force against which the ventricles must contract and expel blood to circulate it throughout the body. Increased afterload results from increased resistance. Thus an increase in afterload and a decrease in stroke volume are mutually related.