What cardiac output measures
Cardiac output is the volume of blood the heart pumps each minute. It is the product of how often the heart beats and how much it ejects with each beat:
cardiac output (L/min) = heart rate × stroke volume ÷ 1,000
At rest, a healthy adult’s cardiac output is about 4 to 8 L/min. During hard exercise, it can rise to 20 L/min or more in trained athletes.
Adjusting for body size
A large person needs more blood flow than a small one, so clinicians use the cardiac index, cardiac output divided by body surface area (BSA). The calculator estimates BSA with the Mosteller formula, √(height in cm × weight in kg ÷ 3,600).
| Measure | Usual resting range |
|---|---|
| Cardiac output | 4 to 8 L/min |
| Cardiac index | 2.5 to 4.0 L/min/m² |
| Stroke volume | 60 to 100 mL |
| Stroke volume index | 33 to 47 mL/m² |
| Systemic vascular resistance | 800 to 1,200 dyn·s/cm⁵ |
Systemic vascular resistance
The resistance the left ventricle pumps against is:
SVR = 80 × (MAP − CVP) ÷ cardiac output
where MAP is the mean arterial pressure and CVP the central venous pressure. The factor 80 converts mmHg·min/L into dyn·s/cm⁵.
Worked example
A heart rate of 75, stroke volume of 70 mL, a patient 5 ft 10 in and 180 lb, MAP 80 and CVP 5 mmHg:
| Result | Value |
|---|---|
| Cardiac output | 5.25 L/min |
| Body surface area | 2.01 m² |
| Cardiac index | 2.61 L/min/m² |
| Stroke volume index | 34.9 mL/m² |
| Systemic vascular resistance | 1,143 dyn·s/cm⁵ |
All values are in the usual range.
Interpreting patterns
- Low cardiac index with high SVR: the pattern of cardiogenic or hypovolemic shock, with the vessels clamping down.
- High cardiac index with low SVR: the typical picture of early septic shock or other vasodilated states.
- A cardiac index below 2.2 L/min/m² with signs of poor perfusion and pulmonary congestion is part of the definition of cardiogenic shock.
How the numbers are obtained
Stroke volume can be measured by echocardiography, by thermodilution with a pulmonary artery catheter, or estimated by monitors that analyze the arterial pressure waveform. Each method has its own error, and trends over time are often more useful than a single value.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.