What are the methods to measure cardiac output?

Methods to Measure Cardiac Output

Invasive Methods

Pulmonary artery catheterization with thermodilution remains the gold standard for cardiac output measurement, particularly in patients with cardiac pathology and when performed by experienced users. 1, 2

Thermodilution Technique

• Inject cold solution into the right atrium and measure temperature change in the pulmonary artery, with measurements taken in triplicate for reliability. 3
• This method is most accurate in patients without severe tricuspid regurgitation or intracardiac shunts. 3
• Critical pitfall: Severe tricuspid regurgitation causes underestimation of cardiac output with thermodilution, necessitating use of the Fick method instead. 3
• In patients with intracardiac shunts, thermodilution may be inaccurate due to early recirculation. 4

Direct Fick Method

• Calculate cardiac output using the formula: Cardiac Output = V̇O₂ / [C(a-v)O₂], where V̇O₂ is oxygen consumption and C(a-v)O₂ is the arteriovenous oxygen content difference. 3
• Requires invasive monitoring with a pulmonary artery catheter to obtain true mixed venous blood samples from the pulmonary artery. 4
• Obtain oxygen saturations from superior vena cava, inferior vena cava, right ventricle, pulmonary artery, and systemic artery. 3
• This is the preferred method when severe tricuspid regurgitation is present. 3
• Major limitation: Not widely available in clinical practice because it requires specialized equipment for gas exchange analysis that many institutions do not possess. 4

Transpulmonary Thermodilution

• Recommended for complex situations, particularly in ARDS patients, as it provides cardiac output along with extravascular lung water and pulmonary vascular permeability index. 1, 3
• Serves to calibrate the pulse contour method that allows real-time cardiac output monitoring from a femoral artery pressure curve. 1
• Avoid in ARDS patients with patent foramen ovale, as this technique has been reported as inaccurate in this population. 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. 3
• Critical limitation: Uncalibrated pulse contour methods cannot be recommended for ARDS patients as validity has been seriously questioned in the presence of sepsis and/or vasopressor use. 1

Noninvasive Methods

Echocardiography with Doppler

• Measure cardiac output as the product of cross-sectional area of the left ventricular outflow tract and velocity of the systolic LVOT blood flow velocity. 3
• Provides stroke volume and cardiac output through velocity-time integral measurements. 3
• Should be used to assess cardiac output, ventricular function, and guide fluid responsiveness in unstable patients. 3

CO₂ Rebreathing Method

• Uses the formula: Cardiac Output = V̇CO₂ / (mixed venous CO₂ - arterial CO₂ difference). 1, 3
• End-tidal CO₂ is taken as a measure of arterial blood CO₂. 1
• Major pitfalls to avoid:
  • Requires patient cooperation, which may be difficult in some patients. 1, 3
  • High concentrations of inhaled CO₂ may cause lightheadedness or feelings of suffocation. 1, 3
  • Accuracy is compromised in patients with advanced pulmonary disease due to ventilation-perfusion mismatch. 1, 3, 4

Foreign Gas Rebreathing

• Uses soluble inspired gases such as acetylene or nitrous oxide for reliable and safe noninvasive cardiac output assessment. 1, 3
• Shows coefficient of variation of 7-11% in patients with and without heart failure. 1, 3
• More reliable than CO₂ rebreathing for noninvasive monitoring in heart failure patients. 3
• Recent technology using portable infrared absorption spectrometry may facilitate point-of-care monitoring. 1

Cardiac Magnetic Resonance (CMR)

• Offers flow-encoded imaging that can quantify cardiac output accurately. 3
• Provides detailed chamber volumes and structure, allowing calculation of stroke volume and cardiac output. 3

Clinical Application Algorithm

When selecting a cardiac output measurement method, follow this approach:

1. For patients with severe tricuspid regurgitation or intracardiac shunts: Use direct Fick method, not thermodilution. 3, 4

2. For complex ARDS patients: Use transpulmonary thermodilution to obtain additional data on extravascular lung water and pulmonary vascular permeability. 1, 3

3. For unstable patients requiring rapid assessment: Use echocardiography with Doppler for immediate bedside evaluation. 3

4. For patients with advanced pulmonary disease: Avoid CO₂ rebreathing; consider foreign gas rebreathing or invasive methods instead. 1, 3, 4

5. For patients on vasopressors or with sepsis: Avoid uncalibrated pulse contour methods; use pulmonary artery catheter thermodilution. 1

Calculating Cardiac Index

• Calculate as: Cardiac Index = Cardiac Output / Body Surface Area to account for body size differences. 3
• Target goal: Cardiac Index ≥ 2.2 L/min/m² should be maintained during surgery using appropriate vasopressors and inotropes as needed. 3