How is cardiac index (CI) calculated?

How to Calculate Cardiac Index

Cardiac index (CI) is calculated by dividing cardiac output (CO) by body surface area (BSA), expressed as L/min/m². 1

Standard Calculation Formula

CI = Cardiac Output (L/min) ÷ Body Surface Area (m²)

The calculation requires two components:

1. Cardiac Output Measurement

Cardiac output can be determined by two primary methods 1:

Volumetric Method:

• Calculate as: Stroke Volume × Heart Rate 1
• Stroke volume is the difference between end-diastolic and end-systolic left ventricular volumes 1
• This includes total flow (effective forward flow, aortic regurgitation, and mitral regurgitation) 1

Doppler Method:

• Measured as: Cross-sectional area of LVOT × Velocity-time integral of systolic LVOT blood flow 1
• This represents forward flow through the aortic valve (includes effective forward flow and aortic regurgitation but excludes mitral regurgitation) 1
• The velocity-time integral is the integral of the instantaneous flow velocity curve during left ventricular ejection 1

2. Body Surface Area Calculation

BSA is typically calculated using standard formulas (e.g., DuBois formula), though the specific formula used can significantly impact the resulting CI value 2. The choice of actual versus predicted body weight in BSA calculation affects CI values, particularly in overweight patients (BMI ≥25 kg/m²) 2.

Normal Values and Clinical Thresholds

Normal cardiac index: 2.2–4.0 L/min/m² 3

Critical pathologic thresholds:

• CI <2.2 L/min/m² defines cardiogenic shock when combined with systolic BP <90 mmHg and PCWP >15 mmHg 1, 4, 3
• CI <2.0 L/min/m² with hypotension constitutes hemodynamic criteria for shock diagnosis 3
• CI <1.8 L/min/m² without vasopressor/inotropic support indicates severe cardiogenic shock 4, 3

Measurement Methods in Clinical Practice

Invasive Measurement

Thermodilution via pulmonary artery catheter remains the gold standard 3:

• Requires measurements in triplicate for reliability 3
• Severe tricuspid regurgitation causes underestimation; use Fick method instead 3

Fick Method:

• CI = (VO₂) ÷ (arterial O₂ content - venous O₂ content) ÷ BSA 5
• Direct measurement of oxygen consumption (VO₂) is preferable to estimation 1
• Critical pitfall: LaFarge equation for estimated VO₂ systematically overestimates CI compared to thermodilution, particularly in children <3 years and at higher CI values 5

Noninvasive Measurement

Echocardiographic Doppler assessment provides reliable CI in most scenarios 3, 6:

• Normal limits by echocardiography: 1.9–4.3 L/min/m² 3
• Simplified method: Left ventricular outflow tract mean velocity by pulsed-wave Doppler permits accurate CI quantification without requiring cross-sectional area measurement 6
• The regression equation: CI = 172 × (LVOT mean velocity) - 172 showed excellent correlation (r=0.97) with thermodilution 6

Common Pitfalls and Caveats

Body weight considerations:

• Age, height, and actual body weight are independently associated with cardiac output 2
• Each year of age decreases CO by 66 mL/min (95% CI: 47–86 mL/min) 2
• Indexation to actual BSA yields statistically lower CI values compared to predicted BSA in overweight patients 2

Measurement accuracy:

• Cross-sectional area measurement is the main source of error in Doppler flow volume quantification 6
• Direct mean velocity measurement eliminates this error source 6
• Tricuspid regurgitation and right ventricular dilatation cause erroneous thermodilution measurements 7

Clinical context:

• CI must be interpreted alongside filling pressures: normal cardiac function maintains CI >2.0 L/min/m² with PCWP <20 mmHg 3
• In cardiogenic shock, CI <2.2 L/min/m² combined with PCWP >15 mmHg and systolic BP <90 mmHg confirms the diagnosis 1, 4
• Cardiac power output <0.6 W (calculated as CO × MAP ÷ 451) is more predictive of refractory shock than CI alone 4, 3