Validity of the LaFarge equation for estimation of oxygen consumption in ventilated children with congenital heart disease younger than 3 years—A revisit

The accurate hemodynamic assessment of patients with congenital heart disease is a fundamental part of the work of the cardiac catheterization laboratory. The direct Fick principle remains the gold standard for calculation of hemodynamic indices in which oxygen consumption (Vo2) must be known.1 Although techniques for metabolic monitoring using indirect calorimetry or respiratory mass spectrometry are available for the measurement of Vo2,2-4 it is still common practice to estimate Vo2 values from tables or published predictive equations. Considerable errors introduced by using assumed Vo2 have been reported.5-8 We have previously examined the validity of 4 equations in estimating Vo2 by comparison with directly measured Vo2 by respiratory mass spectrometry and found poor agreement in all.8 Among these, the LaFarge equation,9 the most commonly used equation, gave the closest estimation with the least bias and limits of agreement. However, our previous study excluded patients whose ages fell outside the range originally used to generate the LaFarge equation, that is, younger than 3 years. Despite the LaFarge equation was restricted to patients between 3 and 40 years of age, it is applied in patients of all ages undergoing cardiac catheterization. With advances in surgical techniques and perioperative management, increasing numbers of patients with complex congenital heart defects undergo cardiac surgery at a younger age. This has resulted in an increasing need for diagnostic cardiac catheterization in children younger than 3 years, often with the single goal of accurate evaluation of systemic and pulmonary blood flows and vascular resistances, particularly pulmonary vascular resistance. If Vo2 needs to be measured at all, then accuracy is vital.10,11 A discrepancy between estimated and measured Vo2 will translate directly into an equivalent percentage underestimation or overestimation of blood flow and vascular resistance. Any error in hemodynamic assessment may significantly mislead surgical and other clinical treatment strategies. Therefore, we revisited our previous data to examine the validity of the LaFarge equation with emphasis on children less than 3 years of age with congenital heart disease undergoing cardiac catheterization.