What is the formula for calculating cardiac output (CO) in patients?

Cardiac Output Formula

Cardiac output is calculated as stroke volume multiplied by heart rate (CO = SV × HR), and can also be determined using the Fick equation: CO = V̇O₂ / [C(a-v)O₂], where V̇O₂ is oxygen consumption and C(a-v)O₂ is the arteriovenous oxygen content difference. 1

Primary Calculation Methods

Basic Formula

• CO = Stroke Volume (SV) × Heart Rate (HR) 1, 2
• This represents the volume of blood ejected by the heart per minute 2, 3
• Cardiac output is dependent on four key components: heart rate, contractility, preload, and afterload 3

The Fick Equation

• CO = V̇O₂ / [C(a-v)O₂] 1, 4
• V̇O₂ represents oxygen consumption (oxygen uptake) 1, 5
• C(a-v)O₂ is the arteriovenous oxygen content difference, reflecting oxygen extraction 1, 5
• Maximal oxygen extraction is approximately 75% of arterial oxygen content in healthy non-athletic individuals 1

Measurement Approaches

Volumetric Method (Echocardiography)

• Volumetric CO = (End-Diastolic Volume - End-Systolic Volume) × Heart Rate 1
• This calculates total flow including effective flow, aortic regurgitation, and mitral regurgitation 1

Doppler Method (Echocardiography)

• Doppler CO = (LVOT Cross-Sectional Area × LVOT Velocity-Time Integral) × Heart Rate 1, 4
• LVOT refers to left ventricular outflow tract 1
• This measures forward flow (effective forward flow plus aortic regurgitation) but excludes backward flow from mitral regurgitation 1

Cardiac Index Calculation

• Cardiac Index = Cardiac Output / Body Surface Area 1, 4, 6
• Expressed in L/min/m² 1
• This standardizes cardiac output for body size, making it more useful for comparing cardiac function between individuals 4
• Target goal during surgery is Cardiac Index ≥ 2.2 L/min/m² 4

Clinical Measurement Techniques

Invasive Gold Standard

• Thermodilution using pulmonary artery catheter remains the gold standard for direct cardiac output measurement 4, 6, 2
• Measurements should be taken in triplicate for reliability 4, 5
• The Fick method is preferred when severe tricuspid regurgitation is present, as thermodilution may underestimate cardiac output in this setting 4, 5

Noninvasive Alternatives

• CO₂ rebreathing: CO = V̇CO₂ / (CvCO₂ - CaCO₂) 4, 5
• Foreign gas rebreathing using acetylene or nitrous oxide shows coefficient of variation of 7-11% 4
• Echocardiography with Doppler provides stroke volume and cardiac output through velocity-time integral measurements 1, 4
• Transpulmonary thermodilution is recommended for complex situations, particularly in ARDS 4, 6

Important Clinical Caveats

• Severe tricuspid regurgitation causes underestimation with thermodilution, necessitating the Fick method instead 4
• CO₂ rebreathing accuracy is compromised in patients with advanced pulmonary disease due to ventilation-perfusion mismatch 1, 5
• Uncalibrated pulse contour methods cannot be used in ARDS patients as their validity is questioned in sepsis and/or vasopressor use 6
• The indirect Fick method (using estimated oxygen consumption) is more widely available but less reliable than direct measurement 4, 5
• All pressure measurements should be taken at end of normal expiration when using invasive methods 4, 5