Cardiac Index Measurement and Interpretation

Understanding Cardiac Index

The cardiac index is a hemodynamic parameter that relates cardiac output to body surface area, providing a standardized measurement that accounts for differences in body size, making it more useful for comparing cardiac function between individuals, as stated by the American College of Cardiology 1

The thermodilution method involves injecting a cold solution into the right atrium and measuring temperature change in the pulmonary artery, with measurements taken in triplicate for reliability, according to the European Respiratory Society 2
The Fick method can be direct (measuring oxygen consumption) or indirect (estimating oxygen consumption), and requires measurement of oxygen uptake and arterial and mixed venous oxygen content difference, as per the European Respiratory Society 2
The formula for cardiac output using the Fick method is: Cardiac output = oxygen consumption ÷ arterio-venous oxygen difference, as stated by the European Respiratory Society 2

When performing right heart catheterization, it's recommended to zero the external pressure transducer at the mid-thoracic line in a supine patient, make pressure measurements in the pulmonary artery, pulmonary artery wedge position, right ventricle, and right atrium, and take all pressure measurements at the end of normal expiration, according to the European Respiratory Society 2

The indirect Fick method (using estimated values of oxygen uptake) is more widely available but less reliable than direct measurement, as stated by the European Respiratory Society 2
The CO₂ rebreathing method requires patient cooperation, which may be difficult in some patients, and high concentrations of inhaled CO₂ may cause lightheadedness or feelings of suffocation, according to the American Heart Association 3
The accuracy of the CO₂ rebreathing technique may be compromised in patients with advanced pulmonary disease, as stated by the American Heart Association 3

The American College of Cardiology recommends the thermodilution method for measuring cardiac output in patients without severe tricuspid regurgitation or intracardiac shunt, with measurements taken in triplicate for reliability 4
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, and is recommended for patients with severe tricuspid regurgitation 5
Severe tricuspid regurgitation can cause underestimation of cardiac output with thermodilution, necessitating use of the Fick method instead 4
The direct Fick method requires measurement of oxygen saturations from superior vena cava, inferior vena cava, right ventricle, pulmonary artery, and systemic artery 4

The CO2 rebreathing method uses the formula: Cardiac output = CO2 output / (mixed venous CO2 - arterial CO2 difference) 6
Foreign gas rebreathing uses soluble inspired gases such as acetylene or nitrous oxide for reliable and safe noninvasive cardiac output assessment, with a coefficient of variation of 7-11% in patients with and without heart failure 6
Echocardiography with Doppler provides measures of stroke volume and cardiac output through velocity-time integral measurements across the left ventricular outflow tract 7, 8
Cardiac Magnetic Resonance (CMR) offers flow-encoded imaging that can quantify cardiac output accurately, and provides detailed chamber volumes and structure, allowing calculation of stroke volume and cardiac output 7, 9

The American College of Cardiology recommends calculating cardiac index as: Cardiac Index = Cardiac Output / Body Surface Area, to account for body size differences 10
For noninvasive monitoring in heart failure, foreign gas rebreathing shows better reliability than CO2 rebreathing 6
Avoid CO2 rebreathing in patients with advanced pulmonary disease due to compromised accuracy 6

Transpulmonary thermodilution is recommended for complex situations, particularly in ARDS, as it provides cardiac output along with extravascular lung water and pulmonary vascular permeability index, according to the American College of Critical Care Medicine 11
Pulse contour/pulse wave analysis utilizes the arterial waveform to calculate values for stroke volume, stroke volume variation (SVV), pulse pressure variation (PPV), and cardiac index, as suggested by the World Journal of Emergency Surgery 12

Echocardiography should be used to assess cardiac output, ventricular function, and guide fluid responsiveness in unstable patients, with the Doppler cardiac output method measured as the product of cross-sectional area of the left ventricular outflow tract and velocity of the systolic LVOT blood flow velocity, as recommended by the American Heart Association 13
The cardiac index is calculated as Cardiac Index = Cardiac Output / Body Surface Area, and a target goal of Cardiac Index ≥ 2.2 L/min/m² should be maintained during surgery using appropriate vasopressors and inotropes as needed, according to the American College of Critical Care Medicine 11 and the World Journal of Emergency Surgery 12

Pulmonary artery catheterization with thermodilution is considered the gold‑standard method for cardiac output measurement in patients with cardiac pathology when performed by experienced clinicians. 14
Transpulmonary thermodilution is recommended for complex cases such as acute respiratory distress syndrome (ARDS) because it provides cardiac output together with extravascular lung water and pulmonary vascular permeability index. 14
The transpulmonary thermodilution technique can be used to calibrate pulse‑contour analysis, enabling real‑time cardiac output monitoring from a femoral arterial pressure waveform. 14
In ARDS patients who have a patent foramen ovale, transpulmonary thermodilution should be avoided because it has been reported to give inaccurate results. 14
Uncalibrated pulse‑contour (pulse‑wave) analysis is not recommended for ARDS patients, especially when sepsis or vasopressor therapy is present, due to serious concerns about validity. 14
For patients receiving vasopressors or who have sepsis, the preferred approach is pulmonary‑artery‑catheter thermodilution rather than uncalibrated pulse‑contour methods. 14

The CO₂ rebreathing method estimates cardiac output using the formula CO = V̇CO₂ / ( mixed‑venous CO₂ – arterial CO₂ ), with end‑tidal CO₂ serving as a surrogate for arterial CO₂. 15
Accurate CO₂ rebreathing requires patient cooperation, which may be difficult to obtain in certain clinical situations. 15
High concentrations of inhaled CO₂ during the test can cause light‑headedness or a sensation of suffocation. 15
In patients with advanced pulmonary disease, the CO₂ rebreathing method is unreliable because ventilation‑perfusion mismatch compromises accuracy. 15
Foreign‑gas rebreathing (using soluble gases such as acetylene or nitrous oxide) provides a reliable, non‑invasive cardiac output assessment. 15
In both heart‑failure and non‑heart‑failure populations, foreign‑gas rebreathing demonstrates a coefficient of variation of approximately 7–11 %, indicating good reproducibility. 15
Recent portable infrared‑absorption spectrometry technology can facilitate point‑of‑care cardiac output monitoring with foreign‑gas rebreathing. 15
For patients with advanced pulmonary disease, clinicians should avoid CO₂ rebreathing and consider foreign‑gas rebreathing or an invasive method instead. 15