Dead Space Ventilation in Critically Ill Children With Lung Injury

Study objective: In children with acute lung injury, there is an increase in minute ventilation (V̇ e ) and inefficient gas exchange due to a high level of physiologic dead space ventilation (V d /V t) . Mechanical ventilation with positive end-expiratory pressure, when used in critically ill patients to correct hypoxemia, may contribute to increased V d /V t . The purpose of this study was to measure metabolic parameters and V d /V t 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. V d /V t 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 (VeqO 2 ) and ventilatory equivalent for carbon dioxide (VeqCO 2 ) were 2.9 ± 1 and 3.3 ± 1 L per 100 mL, respectively. The group mean V d /V t was 0.48 ± 0.2. When compared to non-ARDS patients (33 patients), the patients with ARDS (12 patients) had a significantly higher VeqO 2 (3.3 ± 1 vs 2.8 ± 1 L per 100 mL, respectively; p < 0.05), a significantly higher VeqCO 2 (3.7 ± 1 L/100 vs 3.1 ± 1 L per 100 mL, respectively; p < 0.05), and a significantly higher V d /V t (0.62 ± 0.14 vs 0.43 ± 0.15, respectively; p < 0.0005). Conclusions: Critically ill children with ARDS have increased V d /V t . Increased V d /V t was the main cause of the excess of V̇ e demand in these patients. Increased metabolic demands, as shown by the VeqO 2 , VeqCO 2 , and ventilatory support, are the major determinants of V̇ e requirements in children with ARDS.