We sought to investigate the differences in assumed and measured oxygen consumption values for the determination of cardiac output by using the Fick principle in a pediatric population with congenital heart disease.
Methods
The patient population consisted of 143 patients with a mean age of 11.3 years (age range, 2 days to 23.8 years) undergoing cardiac catheterization during general anesthesia and with mechanical ventilation. Oxygen consumption was measured with a standard commercial analyzing system (Deltatrac II; Datex, Engström, Helsinki, Finland). Assumed oxygen consumption values were calculated according to the formulas of Krovetz and Goldbloom and LaFarge and Miettinen. Comparisons between measurements and assumptions were performed by Bland-Altman plots. Two-sided paired t tests were used to assess a difference of the assumed and measured values.
Results
The range of measured oxygen consumption values was between 55.2 and 249 mL · min−1 · m−2. The Krovetz-Goldbloom formula led to systematically larger values compared with the measured values (P = .0001; mean difference of −53.3 mL · min−1 · m−2; 95% confidence interval, −56.7 to −49.8 mL · min−1 · m−2). The use of the LaFarge-Miettinen formula tends to overestimate oxygen consumption (P = .0037; mean difference of −15.9 mL · min−1 · m−2; 95% confidence interval, −26.5 to −5.4 mL · min−1 · m−2). A similarly poor agreement was found when analyzing a subgroup of 25 patients with Fontan-type circulation.
Conclusion
The use of assumed instead of measured oxygen consumption values introduces large errors in the determination of cardiac output.