Indicators of Optimal Lung Volume During High-Frequency Oscillatory Ventilation in Infants*

OBJECTIVES: During high-frequency oscillatory ventilation, an understanding of the relationship between lung volume and lung mechanics may help clinicians better apply ventilation. The objectives of this study were: 1) to describe the relationship between lung volume and lung function parameters during mapping of the deflation limb of the pressure-volume relationship in infants receiving high-frequency oscillatory ventilation, and 2) to determine whether these parameters might be useful in targeting an optimal volume to apply ventilation. DESIGN: Observational physiological study. SETTING: Tertiary neonatal intensive care unit in a pediatric hospital. PATIENTS: Fifteen infants receiving high-frequency oscillatory ventilation and muscle relaxants. INTERVENTIONS: The deflation limb of the pressure-volume relationship was mapped in each infant, after recruitment to total lung capacity, using stepwise airway pressure decrements. Total lung capacity and closing volume were defined by oxygenation response. MEASUREMENTS AND MAIN RESULTS: Lung volume (respiratory inductive plethysmography), oxygen saturation, transcutaneous carbon dioxide, and indicators of lung mechanics were recorded at each pressure. A distinct bell-shaped relationship between lung volume and carbon dioxide, minute ventilation, and tidal volume (both at airway opening and by inductive plethysmography) could be identified on the deflation limb, with an improvement of 21.6 mm Hg (CO2), 168 mL/sec (minute ventilation), 0.25 mL/kg (airway opening tidal volume), and 13.7% (plethysmography tidal volume) compared with total lung capacity levels. The mean (SD) optimal volumes and pressures for these parameters were significantly lower than total lung capacity, occurring at volumes between 38.6 (39.8)% and 62.8 (31.1)% of total lung capacity, and 28 (36.3)% and 41.3 (38.7)% of pressure at total lung capacity (p < 0.05; Bonferroni post-test). These coincided with the lowest pressure and volumes that maintained the oxygenation benefit of recruitment. CONCLUSIONS: Transcutaneous carbon dioxide, tidal volume, and minute ventilation may assist in refining strategies to identify optimal lung volume.