How can cardiac output be measured in clinical practice, including both invasive and non‑invasive methods?

Measuring Cardiac Output in Clinical Practice

Pulmonary artery catheter thermodilution remains the gold standard for direct cardiac output measurement in critically ill patients with hemodynamic instability, providing the most accurate and reliable measurements despite its invasive nature. 1

Invasive Methods (Gold Standard)

Pulmonary Artery Catheterization with Thermodilution

• The thermodilution method involves injecting a known volume and temperature of cold solution into the right atrium and measuring the temperature change downstream in the pulmonary artery, with measurements taken in triplicate for reliability. 1, 2
• Modern PA catheters can measure cardiac output semi-continuously, providing real-time data without requiring repeated manual injections. 1
• This method is most accurate in patients without severe tricuspid regurgitation or intracardiac shunts. 2
• Critical pitfall: Severe tricuspid regurgitation causes underestimation of cardiac output with thermodilution, necessitating use of the Fick method instead. 2

Direct Fick Method

• The Fick equation calculates cardiac output as: Q = V̇O₂ / [C(a-v)O₂], where V̇O₂ is oxygen consumption and C(a-v)O₂ is the arteriovenous oxygen content difference. 2
• This method is preferred when severe tricuspid regurgitation or intracardiac shunts are present. 2
• Requires measurement of oxygen saturations from superior vena cava, inferior vena cava, right ventricle, pulmonary artery, and systemic artery. 2
• Major limitation: Rarely used clinically due to specialized equipment requirements and technical demands, despite being considered the gold standard. 3

Transpulmonary Thermodilution

• Recommended for complex situations, particularly in ARDS, as it provides cardiac output along with extravascular lung water and pulmonary vascular permeability index. 1, 2
• Offers additional hemodynamic parameters beyond simple cardiac output measurement. 1

Minimally Invasive Methods

Pulse Contour/Pulse Wave Analysis

• Utilizes the arterial waveform to calculate stroke volume, stroke volume variation (SVV), pulse pressure variation (PPV), and cardiac index. 2
• Critical limitation: Uncalibrated pulse contour methods cannot be used in ARDS patients as their validity has been seriously questioned in the presence of sepsis and/or vasopressor use. 1

Lithium Indicator Dilution

• Provides minimally invasive cardiac output measurement through indicator dilution principles. 4
• Less commonly used than other minimally invasive techniques. 4

Non-Invasive Methods

Echocardiography (Recommended First-Line Non-Invasive)

• Should be performed early to assess cardiac output, ventricular function, and guide fluid responsiveness in unstable patients. 1, 2
• The Doppler cardiac output method measures the product of cross-sectional area of the left ventricular outflow tract and velocity of the systolic LVOT blood flow velocity. 2
• Provides additional diagnostic information about ventricular function and structural abnormalities. 1
• Limitation: Requires proper training for accurate measurements and interpretation. 5

Foreign Gas Rebreathing Methods

• Uses soluble inspired gases such as acetylene or nitrous oxide for reliable and safe noninvasive cardiac output assessment. 6, 2
• Shows reproducibility with coefficient of variation of 7-11% in patients with and without heart failure. 6, 2
• Advantage over CO₂ rebreathing: Better reliability in heart failure patients. 2

CO₂ Rebreathing Method

• Uses the formula: Cardiac output = CO₂ output / (mixed venous CO₂ - arterial CO₂ difference). 6, 2
• Major pitfalls: Requires patient cooperation which may be difficult in critically ill patients; high concentrations of inhaled CO₂ may cause lightheadedness or feelings of suffocation. 6, 2
• Avoid in patients with advanced pulmonary disease due to compromised accuracy from ventilation-perfusion mismatch. 6, 2, 3

Cardiac Magnetic Resonance (CMR)

• Offers flow-encoded imaging that can quantify cardiac output accurately with detailed chamber volumes and structure. 2
• Provides calculation of stroke volume and cardiac output non-invasively. 2
• Limited by availability, cost, and inability to use in unstable patients. 2

Electrical Cardiometry/Bioimpedance

• Despite low bias, the mean percentage error (48% in adults, 42% in pediatrics) is not clinically acceptable and cannot replace thermodilution or echocardiography for absolute cardiac output values. 7
• May have limited utility for trending rather than absolute measurements. 7

Clinical Decision Algorithm

For critically ill patients with hemodynamic instability: Use PA catheter thermodilution as the gold standard. 1, 5

For patients with severe tricuspid regurgitation or intracardiac shunts: Use the direct Fick method instead of thermodilution. 2

For ARDS patients or when additional lung water assessment is needed: Use transpulmonary thermodilution. 1, 2

For initial assessment in unstable patients: Perform echocardiography early to assess cardiac output and guide management. 1, 2

Avoid: Uncalibrated pulse contour methods in sepsis or with vasopressor use; CO₂ rebreathing in advanced pulmonary disease; electrical cardiometry for absolute cardiac output values. 1, 2, 7