Medical Equations

Cardiac Output Calculator

Cardiac Output equals Stroke Volume times Heart Rate

Solution

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Worked Examples

Normal Resting Example

Calculate CO from 70 mL stroke volume and 72 bpm

This is the classic bedside cardiac output example for a healthy adult at rest.

  1. Choose Solve for Cardiac Output.
  2. Enter 70 for stroke volume and 72 for heart rate.
  3. Read the result of about 5.04 L/min.
  4. Notice that the result falls inside the normal 4 to 8 L/min range.
  5. Add BSA if you also want the cardiac index interpretation.

This example is useful for checking the math before comparing the result with monitored values in the ICU or cath lab.

Low Output Assessment

Evaluate a patient with 45 mL stroke volume, 80 bpm, and BSA 1.9

This example shows how a modest stroke volume can produce a low-normal or low output picture once you also index it to body size.

  1. Choose Solve for Cardiac Output.
  2. Enter 45 for stroke volume and 80 for heart rate.
  3. Enter 1.9 for body surface area.
  4. Read the cardiac output of 3.6 L/min and the cardiac index of about 1.9 L/min/m².
  5. Use the indexed result to recognize that perfusion is likely inadequate for this patient's size.

Cardiac index often clarifies whether a raw cardiac output value is adequate for a given patient.

Target Heart Rate

Find the heart rate needed for 5.6 L/min with a 70 mL stroke volume

Reverse solving is helpful when you know the desired output and estimated stroke volume but need the heart rate target.

  1. Choose Solve for Heart Rate.
  2. Enter 5.6 for cardiac output and 70 for stroke volume.
  3. Read the calculated heart rate of about 80 bpm.
  4. Confirm that the result is physiologically plausible for the clinical setting.
  5. Use the number as a target, then reassess with real monitored data rather than relying on the equation alone.

This can help frame pacing or response-to-therapy discussions, but real hemodynamics still depend on preload, afterload, and contractility.

Solve for Cardiac Output

Calculate cardiac output by multiplying stroke volume (mL) by heart rate (bpm). The result in mL/min is converted to L/min by dividing by 1000. Normal CO ranges from 4 to 8 L/min.

Q = SV × HR

Solve for Stroke Volume

Determine stroke volume by dividing cardiac output by heart rate. Useful for assessing ventricular performance when CO and HR are known from monitoring.

SV = Q / HR

Solve for Heart Rate

Calculate the required heart rate from cardiac output and stroke volume. Helps assess pacing needs or chronotropic adequacy.

HR = Q / SV

How It Works

Cardiac output (CO) measures the total volume of blood the heart pumps per minute. It reflects how effectively the heart delivers oxygenated blood to tissues and organs throughout the body. The equation multiplies stroke volume (SV) by heart rate (HR). Stroke volume is the amount of blood ejected from the left ventricle with each heartbeat, typically 60 to 80 mL in a healthy adult at rest. Their product yields cardiac output in mL/min, which is converted to L/min by dividing by 1000.

Example Problem

A resting adult has a stroke volume of 70 mL and a heart rate of 72 beats per minute.

  1. Select the cardiac output form because stroke volume and heart rate are the known values.
  2. Multiply stroke volume by heart rate: 70 mL/beat × 72 beats/min = 5040 mL/min.
  3. Convert 5040 mL/min to liters per minute by dividing by 1000, which gives 5.04 L/min.
  4. Compare 5.04 L/min with the usual resting adult range of about 4 to 8 L/min.
  5. If body surface area is available, divide 5.04 L/min by BSA to estimate the cardiac index as a size-adjusted check.

The calculator can also solve the equation in reverse when cardiac output is known and you need either stroke volume or heart rate.

Formula Guide

The cardiac output equation links the amount of blood pumped per beat with how often the heart beats each minute.

Q = Cardiac Output (L/min)

The total blood flow ejected by the ventricle each minute.

SV = Stroke Volume (mL)

The amount of blood ejected from the ventricle with each heartbeat.

HR = Heart Rate (bpm)

The number of heartbeats per minute.

CI = Cardiac Index (L/min/m²)

Optional size-adjusted output, calculated when body surface area is entered.

When to Use Each Variable

  • Solve for CO — when you know stroke volume and heart rate and need to determine the total cardiac output.
  • Solve for SV — when cardiac output and heart rate are known (e.g., from a PAC or Doppler) and you need stroke volume.
  • Solve for HR — when you know cardiac output and stroke volume and need the heart rate to achieve a target CO.

Key Concepts

Four primary factors determine cardiac output: preload (venous return), afterload (arterial resistance), contractility (myocardial force), and heart rate. A healthy adult at rest typically has a cardiac output between 4 and 8 L/min. Cardiac Index (CI) normalizes CO to body size by dividing by body surface area (BSA). Normal CI ranges from 2.5 to 4.0 L/min/m².

Applications

  • Hemodynamic monitoring in the ICU
  • Guiding fluid resuscitation and vasoactive medication titration
  • Differentiating types of shock
  • Evaluating cardiac performance during exercise testing
  • Pre- and post-operative cardiac assessment

Common Mistakes

  • Forgetting to convert mL/min to L/min (divide by 1000)
  • Using resting stroke volume when the patient is exercising or stressed
  • Not accounting for body size - use Cardiac Index for cross-patient comparison
  • Assuming normal CO means adequate tissue perfusion (distribution matters too)

Frequently Asked Questions

What affects cardiac output?

Four primary factors determine cardiac output: preload (venous return), afterload (arterial resistance), contractility (myocardial force), and heart rate. Changes in any of these variables alter the volume of blood the heart delivers each minute.

How is cardiac output measured clinically?

The gold standard is thermodilution via a pulmonary artery catheter. Less invasive methods include echocardiography (Doppler-based measurements), arterial pulse contour analysis, and bioimpedance or bioreactance techniques. Each method has trade-offs in accuracy, invasiveness, and continuous monitoring capability.

Why use Cardiac Index instead of Cardiac Output?

A cardiac output of 4 L/min might be adequate for a small adult but insufficient for a large individual. By dividing CO by body surface area, the Cardiac Index provides a size-normalized value that enables more meaningful comparison across patients of different body sizes.

What is a normal cardiac output at rest?

Most healthy adults at rest have a cardiac output of about 4 to 8 L/min. Athletes, pregnant patients, septic patients, or people with major illness can fall outside that range, so context matters when interpreting a single number.

Can cardiac output be normal even when tissue perfusion is poor?

Yes. A normal global cardiac output does not guarantee adequate microcirculatory blood flow or oxygen delivery to every tissue bed. Conditions such as sepsis, severe vasoconstriction, or regional ischemia can produce poor perfusion despite a seemingly acceptable output.

Why does the calculator ask for stroke volume in mL but return cardiac output in L/min?

Stroke volume is conventionally recorded in milliliters per beat, while cardiac output is easier to interpret clinically in liters per minute. The calculator performs the mL-to-L conversion automatically after multiplying stroke volume by heart rate.

Reference: Guyton AC, Hall JE. Textbook of Medical Physiology. 13th ed. Elsevier, 2016.

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