From the Guidelines
Cardiac index is determined by dividing cardiac output by body surface area, with a normal range of 2.5-4.0 L/min/m², as supported by the most recent guidelines 1. To calculate cardiac index, first measure cardiac output using methods such as thermodilution with a pulmonary artery catheter, transthoracic echocardiography, or non-invasive techniques like impedance cardiography. Then calculate body surface area using height and weight in formulas like the DuBois formula. Divide the cardiac output (in L/min) by the body surface area (in m²) to get the cardiac index. This index is more useful than cardiac output alone because it accounts for body size differences, allowing for more accurate assessment of cardiac function across different patients. For example, a cardiac output of 5 L/min might be normal for a smaller person but insufficient for a larger individual. Cardiac index helps clinicians better evaluate heart function, guide treatment decisions for conditions like heart failure, and monitor responses to interventions such as fluid administration or medication adjustments, as highlighted in the 2021 ACC/AHA key data elements and definitions for heart failure 1. Key factors to consider when measuring cardiac index include:
- Using accurate and reliable methods for measuring cardiac output and body surface area
- Considering the patient's individual characteristics, such as size and body composition
- Interpreting the results in the context of the patient's overall clinical condition, as noted in the pediatric pulmonary hypertension guidelines 1. By following these steps and considering these factors, clinicians can use cardiac index to make informed decisions about patient care and improve outcomes.
From the Research
Determining Cardiac Index
To determine cardiac index, several methods can be employed, including:
- Pulmonary artery catheterization 2, 3, 4, 5
- Transpulmonary thermodilution technique 2, 3, 4
- Echocardiography 6
- Arterial thermodilution 4
Methods Comparison
Comparisons between these methods have been made in various studies:
- Pulmonary artery catheterization and transpulmonary thermodilution technique have been compared, showing close agreement between the two methods 3
- Transpulmonary thermodilution technique has been shown to be a less invasive alternative to pulmonary artery catheterization in patients with burns 4
- Echocardiography has been compared to thermodilution, showing no significant differences between the two methods 6
Factors Affecting Precision
The precision of cardiac output measurements can be affected by various factors, including:
- Ventilatory mode 5
- Injectate temperature 5
- Clinical situation 5
- Type of echocardiography and site of Doppler use 6
Calculation
The cardiac index can be calculated using the formula: cardiac index = cardiac output / body surface area. The cardiac output can be measured using the methods mentioned above. For example, in a study, the mean cardiac index was 3.93 l/min/m2 and ranged from 0.96 to 9.58 l/min/m2 as measured by the pulmonary artery catheter, and 4.0 l/min/m2 with a range of 0.96 to 9.61 l/min/m2 as measured by the transpulmonary thermodilution technique 4