What are the methods for calculating cardiac output?

Methods for Calculating Cardiac Output

Cardiac output can be measured using several established methods, with the Fick technique and thermodilution being the gold standard invasive approaches, while noninvasive methods including echocardiography, CO2 rebreathing, and foreign gas rebreathing provide alternatives with varying degrees of accuracy. 1

Invasive Methods

Thermodilution Technique

• The thermodilution method involves injecting cold solution into the right atrium and measuring temperature change in the pulmonary artery, with measurements taken in triplicate for reliability. 2
• This is performed using a pulmonary artery catheter (Swan-Ganz catheter) and remains the most popular dilution method in clinical settings. 3
• The technique is adequate in pulmonary hypertension patients provided there is not severe tricuspid regurgitation or intracardiac shunt. 1
• A critical pitfall: severe tricuspid regurgitation can cause underestimation of cardiac output with thermodilution, necessitating use of the Fick method instead. 1

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. 1
• This can be performed as direct Fick (measuring actual oxygen consumption) or indirect Fick (estimating oxygen consumption). 2
• The direct Fick method is more reliable than indirect estimation, though the indirect method is more widely available. 2
• Requires measurement of oxygen saturations from superior vena cava, inferior vena cava, right ventricle, pulmonary artery, and systemic artery. 1

Transpulmonary Thermodilution

• Uses a thermistor-tipped catheter placed in the aorta via femoral artery access, making it less invasive than pulmonary artery catheterization. 4
• Shows very good correlation (R = 0.968) with pulmonary artery thermodilution but yields slightly lower values (bias of -4.7%). 4
• Can be used in very small children where pulmonary artery catheter placement may be difficult or impossible. 4

Pressure-Volume Relations

• The gold standard catheterization approach for comprehensive cardiac function analysis uses properly calibrated multi-electrode conductance or impedance catheters that provide simultaneous pressure-volume signals without need for imaging. 1
• These techniques allow calculation of cardiac power, dP/dt, systemic resistance, aortic input impedance, and essentially any mechanical parameter derived from pressures, volumes, and flows. 1

Noninvasive Methods

CO2 Rebreathing (Indirect Fick)

• Uses CO2 as the indicator gas, with the formula: Cardiac output = CO2 output / (mixed venous CO2 - arterial CO2 difference). 1
• Mixed venous PCO2 is estimated from rebreathing a gas mixture (typically 5% CO2 and 95% O2), while end-tidal CO2 represents arterial CO2. 1
• Major limitations include requirement for patient cooperation, potential for lightheadedness or suffocation from high CO2 concentrations, and compromised accuracy in patients with advanced pulmonary disease. 2, 1

Foreign Gas Rebreathing

• Uses soluble inspired gases such as acetylene or nitrous oxide for reliable and safe noninvasive cardiac output assessment. 1
• Assumes mixed venous content of the gas is zero and arterial partial pressure equals end-tidal air. 1
• Newer portable infrared absorption spectrometers may facilitate point-of-care monitoring, though traditionally required respiratory mass spectrometry. 1
• Reproducibility shows coefficient of variation of 7-11% in patients with and without heart failure. 1

Echocardiography with Doppler

• Provides measures of stroke volume and cardiac output through velocity-time integral measurements across the left ventricular outflow tract. 1
• Can be combined with pressure data from implanted sensors or catheters for more complete cardiac function assessment. 1

Cardiac Magnetic Resonance (CMR)

• Offers flow-encoded imaging that can quantify cardiac output accurately. 1
• Provides detailed chamber volumes and structure, allowing calculation of stroke volume and cardiac output. 1

Clinical Considerations

Cardiac Index Calculation

• Cardiac index relates cardiac output to body surface area, providing standardized measurement that accounts for body size differences: Cardiac Index = Cardiac Output / Body Surface Area. 1, 2

Measurement Technique

• When performing right heart catheterization, zero the external pressure transducer at mid-thoracic line in supine position, measure pressures in pulmonary artery, wedge position, right ventricle, and right atrium, and take all measurements at end of normal expiration. 2

Method Selection Algorithm

• For patients without severe tricuspid regurgitation or intracardiac shunt: use thermodilution (triplicate measurements). 1, 2
• For patients with severe tricuspid regurgitation: use Fick method with direct oxygen consumption measurement. 1
• For pediatric patients or when pulmonary artery catheterization is contraindicated: consider transpulmonary thermodilution. 4
• For noninvasive monitoring in heart failure: foreign gas rebreathing shows better reliability than CO2 rebreathing. 1
• Avoid CO2 rebreathing in patients with advanced pulmonary disease due to compromised accuracy. 2, 1