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Cardiac Output – Definition and Clinical Significance

Cardiac output describes the amount of blood the heart pumps into the circulation per minute. It is a key measure of cardiac function and overall circulatory health.

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Things worth knowing about "Cardiac Output"

Cardiac output describes the amount of blood the heart pumps into the circulation per minute. It is a key measure of cardiac function and overall circulatory health.

What Is Cardiac Output?

Cardiac output (CO) refers to the total volume of blood pumped by the heart into the circulatory system per minute, expressed in liters per minute (L/min). In a healthy adult at rest, cardiac output typically ranges between 4.5 and 5.5 L/min. It is one of the most important parameters used to assess heart function and overall cardiovascular health.

Cardiac output is calculated by multiplying the stroke volume (the amount of blood ejected per heartbeat) by the heart rate (beats per minute): CO = Stroke Volume x Heart Rate.

Physiological Basis

The heart consists of two pumps working in synchrony: the right side sends blood to the lungs for oxygenation (pulmonary circulation), while the left side pumps oxygenated blood throughout the body (systemic circulation). Both sides normally eject equal volumes of blood. Cardiac output continuously adapts to meet the metabolic demands of the body.

Stroke volume: Blood ejected per heartbeat (approximately 60–100 mL at rest)
Heart rate: Number of heartbeats per minute (approximately 60–80 at rest)
Cardiac output: Stroke volume x Heart rate

Factors Affecting Cardiac Output

Several factors regulate cardiac output:

Preload: The degree of ventricular filling before contraction. A higher preload increases stroke volume through the Frank-Starling mechanism.
Afterload: The resistance the heart must overcome to eject blood. Elevated afterload, such as in hypertension, can reduce cardiac output.
Contractility: The intrinsic pumping strength of the heart muscle, which can be enhanced by hormones such as adrenaline (epinephrine).
Heart rate: An increase in heart rate raises cardiac output, as long as adequate filling time is maintained.

Clinical Relevance

Measurement of cardiac output is essential in intensive care medicine and cardiology. A reduced cardiac output may indicate heart failure, a condition in which the heart can no longer pump sufficient blood to meet the body requirements. Common causes of reduced cardiac output include:

Coronary artery disease (narrowing of the coronary arteries)
Myocardial infarction (heart attack causing muscle damage)
Heart valve disorders
Cardiac arrhythmias
Cardiomyopathies (diseases of the heart muscle)

An elevated cardiac output can occur during physical exercise, fever, pregnancy, or in conditions such as hyperthyroidism (overactive thyroid).

Diagnosis and Measurement

Several methods are available to measure cardiac output:

Echocardiography: A non-invasive ultrasound examination of the heart used to estimate stroke volume and cardiac output. It also allows calculation of the ejection fraction (EF), which represents the percentage of blood ejected from the left ventricle with each heartbeat.
Thermodilution method: An invasive technique using a pulmonary artery catheter (Swan-Ganz catheter), in which a cold saline solution is injected and the resulting temperature change in blood flow is measured.
Fick principle: Calculation of cardiac output based on oxygen consumption and the difference in oxygen content between arterial and venous blood.
Pulse contour analysis: A minimally invasive continuous monitoring method using an arterial line.

Treatment of Reduced Cardiac Output

Treatment depends on the underlying cause. General approaches include:

Medications to reduce cardiac workload (e.g., ACE inhibitors, beta-blockers, diuretics)
Treatment of cardiac arrhythmias
Interventional procedures for valve disease or coronary artery stenosis
In severe cases: mechanical circulatory support devices or heart transplantation

References

Hall, J.E., Hall, M.E.: Guyton and Hall Textbook of Medical Physiology. 14th Edition. Elsevier, Philadelphia 2020.
McDonagh, T.A. et al.: 2021 ESC Guidelines for the diagnosis and treatment of acute and chronic heart failure. European Heart Journal, 42(36), 3599-3726, 2021.
World Health Organization (WHO): Cardiovascular diseases (CVDs) – Key Facts. Geneva, 2023. Available at: https://www.who.int/news-room/fact-sheets/detail/cardiovascular-diseases-(cvds)

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