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Measurement of oxygen consumption (Vȯ2) is difficult in children but is essential to calculate cardiac index and systemic vascular resistance.To compare measurements of Vȯ2 using respiratory mass spectrometry and the breath-by-breath method.Vȯ2 was measured simultaneously and continuously for 10 minutes by using respiratory mass spectrometry and the breath-by-breath method in children receiving mechanical ventilation via cuffed endotracheal tubes.Sixteen children (7 boys; median [range]: age, 1.5 [0.2-6] years; weight, 11.5 [2.8-23.5] kg; body surface area, 0.55 [0.18-0.98] m2) were studied. The correlation between measurements of Vȯ2 by the 2 methods was good (R = 0.924). Mean Vȯ2 measured by mass spectrometry was 63 (95% CI, 47-78) mL/min vs 65 (95% CI, 47-83) mL/min measured by the breath-by-breath method. The mean Vȯ2 difference between the 2 methods was 3 (95% CI, -9 to 5) mL/min and statistically insignificant. Bland-Altman analysis showed that the 95% limits of agreement were between -28 and + 23. Cardiac index did not differ significantly when calculated using Vȯ2 measured with one method or the other (mean difference, 0.1; 95% CI, -0.2 to 0.3).Measurements of Vȯ2 did not differ between mass spectrometry and the breath-by-breath method. Use of the breath-by-breath method may facilitate calculation of cardiac index and systemic vascular resistance in critically ill children.