Is there a fixed oxygen consumption (VO₂) value for infants to use with the indirect Fick method?

No, there is no reliable fixed oxygen consumption constant for infants when using the indirect Fick method

The indirect Fick method using assumed VO₂ values introduces clinically significant errors in infants and young children, and direct measurement of oxygen consumption should be used whenever possible. 1

Why Assumed VO₂ Values Are Problematic in Infants

The evidence clearly demonstrates that estimating oxygen consumption rather than measuring it directly leads to substantial inaccuracy:

• The LaFarge equation (the most commonly used predictive formula) introduces considerable errors, particularly in children younger than 3 years of age 2
• In a validation study of young children, assumed VO₂ showed weak concordance with reference VO₂ (ρc = 0.28, r² = 0.31) with a mean bias of +27.5% (SD ± 30.0%) 3
• Many prediction models perform poorly in patients < 36 months of age 3
• The 2025 Chest guidelines explicitly warn that "the indirect Fick method uses an estimated value for the VO₂, which can introduce a clinically significant error" 1

Clinical Impact of Inaccurate VO₂ Measurement

Any error in VO₂ measurement translates directly into equivalent percentage errors in:

• Calculated cardiac output
• Pulmonary vascular resistance (PVR)
• Systemic vascular resistance

This is particularly critical since inaccurate cardiac output values result in clinically important misclassification of pulmonary hypertension 1, which can lead to inappropriate treatment decisions affecting morbidity and mortality.

The Superior Alternative: Direct VO₂ Measurement

Recent evidence strongly supports direct measurement:

• Measured VO₂ using modern equipment (e.g., CARESCAPE E-sCAiOVX module) is significantly more accurate than assumed VO₂ when compared to reference standards (mean bias -3.2% vs +27.5%) 3
• Direct measurement showed satisfactory concordance (ρc = 0.73, r² = 0.63) with reference VO₂ 3
• The accuracy of measured VO₂ in patients < 36 months was not significantly different from older patients 3
• Real-time gas-exchange systems are now feasible for ventilated neonates and infants, with 97% of measurements within 25% of calculated values 4

Practical Recommendations

For cardiac catheterization in infants:

• Use direct VO₂ measurement with respiratory mass spectrometry or modern gas-exchange systems whenever available
• If direct measurement is unavailable, recognize that assumed values will overestimate VO₂ by approximately 27.5% on average in young children
• The direct Fick method with measured VO₂ remains the gold standard for cardiac output assessment 1

Critical Caveat

Even indirect calorimetry (a form of direct measurement) has been shown to overestimate oxygen consumption by 14.7% in young children compared to reverse Fick calculations 5. This underscores that any method using assumed rather than directly measured VO₂ should be interpreted with extreme caution in the infant population, as errors compound when calculating hemodynamic parameters critical for diagnosis and treatment decisions.