What is the normal range for the Fick cardiac index (FCI)?

Normal Range for Fick Cardiac Index

The normal range for the Fick cardiac index in adults is 2.2-4.0 L/min/m².

Understanding Cardiac Index

The cardiac index is a hemodynamic parameter that relates cardiac output to body surface area 1. It provides a standardized measurement that accounts for differences in body size, making it more useful for comparing cardiac function between individuals.

Measurement Methods

There are two primary methods for measuring cardiac output, which is then indexed to body surface area to calculate cardiac index:

1. Thermodilution Method

   • Considered the routine clinical standard for cardiac output measurement 2
   • Involves injecting a cold solution into the right atrium and measuring temperature change in the pulmonary artery 1
   • Measurements should be taken in triplicate for reliability 1

2. Fick Method

   • Can be direct (measuring oxygen consumption) or indirect (estimating oxygen consumption) 1
   • The direct Fick method requires:
     • Measurement of oxygen uptake (VO₂)
     • Arterial and mixed venous oxygen content difference 1
   • Formula: Cardiac output = oxygen consumption ÷ arterio-venous oxygen difference 1

Clinical Significance and Interpretation

• Low cardiac index (<2.2 L/min/m²) is associated with higher mortality compared to normal values 3
• Normal cardiac index (2.2-4.0 L/min/m²) indicates adequate cardiac function 3
• High cardiac index (>4.0 L/min/m²) may be seen in high-output states

Comparison of Measurement Methods

• Thermodilution and Fick methods show modest correlation (r = 0.65) but can have significant differences in individual patients 3
• Thermodilution cardiac index is a better predictor of mortality than estimated Fick cardiac index 3
• The median absolute percent error between Fick and thermodilution methods can be as high as 17.5% 2
• In patients with intracardiac shunts, thermodilution may be inaccurate due to early recirculation of the injectate 1

Important Considerations

• When performing right heart catheterization, it's recommended to:

  • Zero the external pressure transducer at the mid-thoracic line in a supine patient 1
  • Make pressure measurements in the pulmonary artery, pulmonary artery wedge position, right ventricle, and right atrium 1
  • Take all pressure measurements at the end of normal expiration 1

• For patients with elevated mean pulmonary arterial pressure, both thermodilution and Fick methods should be performed, with preference given to thermodilution values when there is disagreement 4

Common Pitfalls

• Indirect Fick method (using estimated values of oxygen uptake) is more widely available but less reliable than direct measurement 1
• CO₂ rebreathing methods require patient cooperation, which may be difficult in some patients 1
• High concentrations of inhaled CO₂ may cause lightheadedness or feelings of suffocation 1
• Accuracy of CO₂ rebreathing technique may be compromised in patients with advanced pulmonary disease 1