What are the methods to obtain cardiac output and index in patients?

Methods to Obtain Cardiac Output and Cardiac Index

Invasive Methods

Pulmonary Artery Catheterization (Gold Standard)

Pulmonary artery catheterization with thermodilution remains the gold standard for direct cardiac output measurement in critically ill patients with hemodynamic instability. 1

• Thermodilution technique: A known volume and temperature of fluid is injected into the right atrium, and the temperature change is measured downstream in the pulmonary artery 1
• Modern PA catheters can measure cardiac output semi-continuously, providing real-time data without requiring repeated manual injections 1
• Measurements should be taken in triplicate for reliability 2
• Limitation: Severe tricuspid regurgitation can cause underestimation of cardiac output with thermodilution, necessitating use of the Fick method instead 2
• PA catheters also provide direct measurements of right atrial pressure, pulmonary artery pressure, and pulmonary artery occlusion pressure 1

Direct Fick Method

• Calculates cardiac output using the formula: Q = V̇O₂ / [C(a-v)O₂], where V̇O₂ is oxygen consumption and C(a-v)O₂ is the arteriovenous oxygen content difference 3, 2
• Requires invasive monitoring with a pulmonary artery catheter to obtain true mixed venous blood samples from the pulmonary artery 3
• Requires measurement of oxygen saturations from superior vena cava, inferior vena cava, right ventricle, pulmonary artery, and systemic artery 2
• Preferred in patients with severe tricuspid regurgitation or intracardiac shunts where thermodilution may be inaccurate 3, 2
• Considered the gold standard but rarely used clinically due to its invasive nature and technical demands 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
• Uses a central venous catheter and arterial catheter (typically femoral) for measurement 4, 5
• Provides continuous cardiac output monitoring through pulse contour analysis after initial calibration 5, 6

Lithium Indicator Dilution

• A minimally invasive technique that uses lithium as an indicator for cardiac output measurement 5, 6
• Requires peripheral venous access and arterial line 5

Minimally Invasive Methods

Pulse Contour/Pulse Wave Analysis

• Calibrated pulse wave analysis enables absolute CO changes to be monitored in real-time during the assessment of fluid responsiveness 7
• 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
• Utilizes the arterial waveform to calculate values for stroke volume, stroke volume variation (SVV), pulse pressure variation (PPV), and cardiac index 4
• Recent validation studies demonstrate that pulse wave analysis may be used reliably as an alternative to the pulmonary artery catheter in different clinical settings 8

Esophageal Doppler Monitoring (ODM)

• Provides minimally invasive cardiac output measurement through Doppler assessment of descending aortic blood flow 5, 6
• In high-risk noncardiac surgical patients, esophageal Doppler should be used for perioperative hemodynamic management 7
• Limitation: High operator dependency limits widespread use 8

Non-Invasive Methods

Echocardiography (Transthoracic and Transesophageal)

Echocardiography should be performed early to assess cardiac output, ventricular function, and guide fluid responsiveness in unstable patients. 1

• 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 4
• Volumetric cardiac output method: Calculated as volumetric stroke volume (end-diastolic volume minus end-systolic volume) multiplied by heart rate 4
• In patients undergoing open-heart and thoracic aortic surgery, transesophageal echocardiography is recommended 7
• Can assess left ventricular and right ventricular contractility, structural/valvular abnormalities, and guide the use of inotropes and vasopressors 4
• Limitation: Operator-dependent and may not provide continuous monitoring 8

CO₂ Rebreathing (Partial CO₂ Rebreathing)

• Uses the formula: Cardiac output = CO₂ output / (mixed venous CO₂ - arterial CO₂ difference) 2
• Should be applied in a precisely defined clinical setting to mechanically ventilated patients only 8
• Major limitation: Accuracy may be compromised in patients with advanced pulmonary disease due to ventilation-perfusion mismatch 3, 2
• Requires patient cooperation, which may be difficult in critically ill patients 3, 2

Foreign Gas Rebreathing

• Uses soluble inspired gases such as acetylene or nitrous oxide for reliable and safe noninvasive cardiac output assessment 2
• Shows better reliability than CO₂ rebreathing for noninvasive monitoring in heart failure, with a coefficient of variation of 7-11% 2

Bioimpedance/Bioreactance

• Thoracic bioimpedance provides information about hemodynamic status without the risk, cost, and skill associated with invasive techniques 5
• Currently provides a valid, fast, and non-invasive measurement of cardiac output 6
• Primarily applied in an investigational setting 8

Cardiac Magnetic Resonance (CMR)

• Offers flow-encoded imaging that can quantify cardiac output accurately 2
• Provides detailed chamber volumes and structure, allowing calculation of stroke volume and cardiac output 2
• Limitation: Not practical for continuous monitoring in critically ill patients

Calculating Cardiac Index

Cardiac index is calculated as: Cardiac Index = Cardiac Output / Body Surface Area 4, 2

• Provides a standardized measurement that accounts for differences in body size, making it more useful for comparing cardiac function between individuals 2
• Target goal: Cardiac Index ≥ 2.2 L/min/m² (or 3.3–6.0 L/min/m² in pediatric septic shock) should be maintained during surgery using appropriate vasopressors and inotropes as needed 4

Clinical Decision Algorithm

When to Use Invasive PA Catheterization:

• Persistent circulatory shock after initial resuscitation requiring monitoring of response to fluids and vasoactive agents 7
• Circulatory shock associated with right ventricular dysfunction, pulmonary artery hypertension, or acute respiratory distress syndrome 7
• Severe mitral regurgitation, severe tricuspid regurgitation, and severe pulmonary arterial hypertension with persistently severe symptoms 1
• Discrepancy between clinical symptoms and non-invasive hemodynamic assessment 1

When to Use Minimally Invasive/Non-Invasive Methods:

• High-risk noncardiac surgical patients requiring perioperative hemodynamic management (use pulse wave analysis or esophageal Doppler) 7
• Early assessment in unstable patients (use echocardiography) 1
• Hemodynamically stable patients without severe valvular disease or intracardiac shunts 2

Common Pitfalls to Avoid:

• Do not use uncalibrated pulse contour methods in ARDS patients or those on vasopressors 1
• Avoid CO₂ rebreathing in patients with advanced pulmonary disease 3, 2
• Do not rely on thermodilution in patients with severe tricuspid regurgitation or intracardiac shunts—use the Fick method instead 3, 2
• Ensure all pressure measurements are determined at the end of normal expiration 3
• Perfusion Index is not a method for assessing cardiac output—it reflects peripheral perfusion and microcirculatory blood flow, not global cardiac output 1