Cardiac output and cardiac index

Cardiac output (CO) and cardiac index (CI) are important hemodynamic parameters characterizing cardiac function and reflecting body metabolism.

Usage

Cardiac output and cardiac index are used in the evaluation of patients with heart disease and critically ill patients as well as patients under general anesthesia ¹.

Cardiac output can be used for the quantification of cardiac shunts and the calculation of pulmonary resistance ¹.

Measurement

There are many ways to measure cardiac output invasive and noninvasive.

A simple method is to derive cardiac output from the product of heart rate (HR) and the stroke volume (SV):

CO [L/min] = HR x SV

The cardiac index describes the cardiac output corrected for the body surface area (BSA).

CI [L/min/m²] = CO/BSA

Methods

Non-invasive imaging-based methods include the following ¹:

Echocardiography/Doppler ultrasound

Cardiac output can be derived from the heart rate and flow volume calculated by the velocity-time integral (VTI) and cross-sectional area (CSA) of the valve:

CO [L/min] = HR x VTI x CSA

MRI

Cardiac output can be obtained by measuring flow volume by means of velocity-encoded phase-contrast imaging multiplied with the heart rate (HR). Net volume or stroke volume can then be calculated from the difference of antegrade and retrograde volumes, which are derived from mean velocity and the cross-sectional area of the vessel ^1,2:

CO [L/min] = HR [bpm] x (antegrade - retrograde volume [mL] /1000)

For accurate measurements, Venc should be adjusted to slightly above the true peak velocity ^1,2.

Other non-invasive methods include pulse pressure methods, ultrasonic cardiac output monitor and impedance cardiography ^1,3. Invasive methods include Fick’s principle thermodilution and dye dilution methods.

Interpretation

Normal values for cardiac output obviously depend on height, weight and age. Normal cardiac index slightly decreases with age and weight ⁴. They are usually given within the following ranges ^1,4:

• cardiac output (CO):      4.0-8.0 L/min/m²  and 3.1-6.4 L/min in individuals > 60 years
• cardiac index (CI):          2.2-4.1 L/min/m²  and 2.1-3.2 L/min/m²  in individuals > 60 years

A cardiac index below 2.0 L/min/m² might indicate cardiogenic shock in an acute setting.

See also

• cardiac volumes and measurements