Effect of frequency on pressure cost of ventilation and gas exchange in newborns receiving high-frequency oscillatory ventilation

BACKGROUND: We hypothesized that ventilating at the resonant frequency of the respiratory system optimizes gas exchange while limiting the mechanical stress to the lung in newborns receiving high-frequency oscillatory ventilation (HFOV). We characterized the frequency dependence of oscillatory mechanics, gas exchange, and pressure transmission during HFOV. METHODS: We studied 13 newborn infants with a median (interquartile range) gestational age of 29.3 (26.4-30.4) weeks and body weight of 1.00 (0.84-1.43) kg. Different frequencies (5, 8, 10, 12, and 15 Hz) were tested, keeping carbon dioxide diffusion coefficient (DCO2) constant. Oscillatory mechanics and transcutaneous blood gas were measured at each frequency. The attenuation of pressure swings (Delta P) from the airways opening to the distal end of the tracheal tube (TT) and to the alveolar compartment was mathematically estimated. RESULTS: Blood gases were unaffected by frequency. The mean (SD) resonant frequency was 16.6 (3.5) Hz. Damping of Delta P increased with frequency and with lung compliance. Delta P at the distal end of the TT was insensitive to frequency, whereas Delta P at the peripheral level decreased with frequency. CONCLUSION: There is no optimal frequency for gas exchange when DCO2 is held constant. Greater attenuation of oscillatory pressure at higher frequencies offers more protection from barotrauma, especially in patients with poor compliance.