Cardiac output (CO) is calculated by multiplying stroke volume by heart rate. The result tells you how many liters of blood the heart pumps per minute, and a normal resting value falls between 4 and 8 liters per minute, with most healthy adults sitting around 5 to 6.
The Core Formula
The standard cardiac output equation is straightforward:
CO = Stroke Volume × Heart Rate
Stroke volume is the amount of blood pushed out of the left ventricle with each heartbeat, measured in milliliters. Heart rate is beats per minute. Multiply them together and you get milliliters per minute, which is then expressed in liters per minute.
For example, if your heart ejects 70 mL of blood per beat and beats 72 times per minute, cardiac output is 70 × 72 = 5,040 mL/min, or about 5 L/min. That falls squarely in the normal resting range.
How Stroke Volume Is Calculated
Stroke volume itself comes from two measurements of the heart’s left ventricle:
SV = End-Diastolic Volume (EDV) − End-Systolic Volume (ESV)
End-diastolic volume is how much blood fills the ventricle when it’s fully relaxed, just before it contracts. End-systolic volume is how much blood remains after the contraction. The difference is the volume actually pumped out. In a typical heart, EDV is about 120 mL and ESV is about 50 mL, giving a stroke volume of roughly 70 mL.
These volumes are usually measured with an echocardiogram (an ultrasound of the heart). If you’re working through a physiology problem rather than a clinical scenario, the values are normally provided for you.
Adjusting for Body Size: Cardiac Index
Raw cardiac output doesn’t account for how big someone is. A person who weighs 50 kg and a person who weighs 100 kg could both have a CO of 5 L/min, but that number means something very different for each of them. To adjust for this, clinicians use the cardiac index:
CI = Cardiac Output ÷ Body Surface Area
Body surface area (BSA) is expressed in square meters and is calculated from height and weight. A normal cardiac index is around 2.5 to 4.0 L/min/m². This adjusted number gives a more accurate picture of whether the heart is pumping enough blood relative to the body’s needs.
The Fick Method
In clinical settings, there’s a second way to calculate cardiac output that doesn’t rely on measuring ventricular volumes directly. The Fick principle uses oxygen consumption instead:
CO = Oxygen Consumption ÷ Arteriovenous Oxygen Difference
The idea is simple: if you know how much oxygen the body uses per minute and how much oxygen the blood loses as it circulates (the difference between oxygen in arterial blood and oxygen in venous blood), you can work backward to figure out how much blood must be flowing. This method is considered a gold standard for accuracy, though it requires blood samples from both an artery and the pulmonary artery, so it’s typically reserved for patients in catheterization labs or intensive care.
What Changes Cardiac Output
Because CO depends on both stroke volume and heart rate, anything that shifts either variable changes the final number. Heart rate is the more intuitive one: exercise, stress, fever, and stimulants all push it up, directly increasing cardiac output. But stroke volume is influenced by three mechanical factors that are worth understanding.
Preload is how much blood fills the ventricle before it contracts. More filling stretches the heart muscle fibers, and up to a point, more stretch produces a stronger contraction. This is the basis of a principle called the Frank-Starling mechanism: when more blood returns to the heart, the heart automatically pumps harder to match. Dehydration or significant blood loss reduces preload, which drops stroke volume and, in turn, cardiac output.
Afterload is the pressure the heart has to push against to eject blood into the aorta. High blood pressure increases afterload, which means the ventricle can’t empty as completely. More blood stays behind after each beat, end-systolic volume rises, and stroke volume falls.
Contractility refers to the inherent strength of the heart muscle itself, independent of how much it’s stretched. Conditions like heart failure reduce contractility, meaning the heart squeezes less forcefully even when filling is normal. The result is a lower stroke volume and reduced cardiac output.
How Cardiac Output Is Measured Clinically
Outside of textbook calculations, cardiac output is measured in hospitals using several techniques. Echocardiography is the least invasive: an ultrasound probe captures images of the heart, allowing measurement of ventricular volumes and blood flow velocity, from which stroke volume and CO can be derived.
For critically ill patients, thermodilution is a common method. A small amount of cool saline is injected into a large vein, and a temperature sensor downstream in the pulmonary artery tracks how quickly the blood temperature changes. The faster the temperature normalizes, the higher the cardiac output. The relationship is described by the Stewart-Hamilton equation, though in practice a computer attached to the catheter handles the math and displays the result.
Putting the Numbers Together
If you’re solving a physiology problem, the steps are:
- Find stroke volume. If given EDV and ESV, subtract: SV = EDV − ESV. If SV is provided directly, skip this step.
- Multiply by heart rate. CO = SV × HR. Convert to liters by dividing by 1,000 if your SV is in milliliters.
- Adjust for body size if needed. Divide CO by BSA to get cardiac index.
A quick reference: a resting CO below 4 L/min generally signals that the heart isn’t meeting the body’s demands, while values above 8 L/min at rest could indicate a high-output state from conditions like severe anemia, hyperthyroidism, or sepsis. During intense exercise, cardiac output in a healthy person can climb to 20 to 25 L/min as both heart rate and stroke volume increase.