Cardiac output (CO) is the volume of blood the heart pumps per minute. It is the bottom-line measure of how much oxygen and nutrition the organs receive.
It is made up of two simple parts: stroke volume × heart rate. So the heart can raise output either by a stronger contraction or by speeding up the rhythm — for example, during exertion. It is closely tied to ejection fraction and blood pressure.
Reference ranges
Cardiac output at rest in an adult:
| Normal (rest) | ≈ 4 – 8 L/min |
| Formula | stroke volume × heart rate |
| Cardiac index | CO / body surface area ≈ 2.5 – 4 L/min/m² |
| During exertion | can rise several-fold |
What an elevated level means
A high cardiac output is the heart working harder. Normally it rises with physical exertion and pregnancy.
A pathologically high output occurs in hyperthyroidism, severe anemia, fever and sepsis — when the organs need more blood or vascular resistance falls. Prolonged overload exhausts the heart over time.
What a low level means
A low cardiac output means the organs receive too little blood. This is a dangerous condition.
Causes: a weak heart muscle (a low ejection fraction, heart failure), severe arrhythmias, massive blood loss, shock. It shows itself as weakness, cold skin, confusion and a drop in blood pressure, and it needs emergency care.
How the test is done
Cardiac output is calculated from echocardiography (stroke volume × heart rate) or measured invasively in intensive care. In everyday life it is not measured separately.
In practice the heart's pumping function is judged from the ejection fraction, symptoms, blood pressure and pulse, while cardiac output matters mainly in intensive care.
What affects the value
- Physical exertion — raises cardiac output.
- Fitness — raises stroke volume.
- Hyperthyroidism, anemia, fever, sepsis — raise it.
- Heart failure — lowers it.
- Severe arrhythmias — lower the effective output.
- Blood loss and dehydration — lower it.
Estimate your heart age
The heart's pumping function is assessed on echocardiography — find out your heart age:
Frequently asked questions
What is cardiac output made of?
Of stroke volume (how much blood the heart ejects per contraction) and heart rate. Their product is the minute volume of blood. So output can be raised both by a stronger contraction and by speeding up the rhythm — for example, during exertion.
How does cardiac output differ from ejection fraction?
Ejection fraction is the fraction (percentage) of blood ejected per beat, while cardiac output is the absolute volume of blood per minute. They are related but describe different things: EF assesses the "quality" of the contraction, while cardiac output assesses the overall blood supply.
Why is a low cardiac output dangerous?
The organs get too little blood and oxygen. This shows itself as weakness, cold skin, confusion and a drop in blood pressure, and it occurs in severe heart failure, arrhythmias, blood loss and shock — conditions that need emergency care.
Related terms
Sources
- Cardiac Output — StatPearls (NCBI)
- American Heart Association — How the Heart Works
- MedlinePlus (NIH) — Heart Failure