Study objective: In children with acute lung injury, there is an increase in minute ventilation (V̇e) and inefficient gas exchange due to a high level of physiologic dead space ventilation (Vd/Vt). Mechanical ventilation with positive end-expiratory pressure, when used in critically ill patients to correct hypoxemia, may contribute to increased Vd/Vt. The purpose of this study was to measure metabolic parameters and Vd/Vt in critically ill children.
Design: A cross-sectional study.
Setting: Pediatric ICU of a university hospital.
Patients: A total of 45 mechanically intubated children (mean age, 5.5 years).
Interventions: Indirect calorimetry was used to measure metabolic parameters. Vd/Vt parameters were calculated using the modified Bohr-Enghoff equation. ARDS was defined based on criteria by The American-European Consensus Conference.
Measurements and results: The group mean (± SD) ventilatory equivalent for oxygen (VeqO2) and ventilatory equivalent for carbon dioxide (VeqCO2) were 2.9 ± 1 and 3.3 ± 1 L per 100 mL, respectively. The group mean Vd/Vt was 0.48 ± 0.2. When compared to non-ARDS patients (33 patients), the patients with ARDS (12 patients) had a significantly higher VeqO2 (3.3 ± 1 vs 2.8 ± 1 L per 100 mL, respectively; p < 0.05), a significantly higher VeqCO2 (3.7 ± 1 L/100 vs 3.1 ± 1 L per 100 mL, respectively; p < 0.05), and a significantly higher Vd/Vt (0.62 ± 0.14 vs 0.43 ± 0.15, respectively; p < 0.0005).
Conclusions: Critically ill children with ARDS have increased Vd/Vt. Increased Vd/Vt was the main cause of the excess of V̇e demand in these patients. Increased metabolic demands, as shown by the VeqO2, VeqCO2, and ventilatory support, are the major determinants of V̇e requirements in children with ARDS.