The cardiac output equation
\[CO = HR \times SV\] \(CO\) = cardiac output (L/min); \(HR\) = heart rate (beats/min); \(SV\) = stroke volume (mL/beat). At rest, \(CO \approx 5\ \text{L/min}\).
Why the product matters
Heart rate and stroke volume are controlled by different mechanisms, so they can move independently. A heart that cannot eject a normal volume per beat can still maintain output for a while by beating faster. The equation also shows the ceiling: if stroke volume is halved, heart rate must double to keep output constant, and heart rate cannot rise indefinitely.
Two routes to the same output
A trained endurance athlete may have a resting heart rate of 50 beats/min and a stroke volume of 100 mL, giving \(50 \times 100 = 5000\ \text{mL/min}\), or 5 L/min. A person with a weakened ventricle might have a stroke volume of only 35 mL and a heart rate of 140 beats/min, giving \(140 \times 35 = 4900\ \text{mL/min}\), also about 5 L/min. The output is nearly identical, but the second person is working near the limit of what heart rate can compensate for.
Normal range
Cardiac output at rest is usually quoted as 4–8 L/min and scales with body size, so it is often expressed as the cardiac index: cardiac output divided by body surface area, normally about 2.5–4.0 L/min/m².