High frequency oscillatory ventilation near resonant frequency of the respiratory system in rabbits with normal and surfactant depleted lungs

Abstract

It has been suggested that high frequency oscillatory ventilation (HFV) might improve gas exchange and reduce the risk of pressure-related side-effects compared to conventional mechanical ventilation (CMV). Whereas most studies have used arbitrarily set frequencies for HFV, we evaluated the effects of HFV near resonant frequency (f_r). Anaesthetised and tracheotomised adult rabbits (n=10; 3.8–5.1 kg body weight) were ventilated by alternating periods of CMV and HFV near f_r. Negative ventilator resistance was used for complete resistive unloading of the respiratory system before each HFV period. This enabled a continuous swinging at resonance thus allowing measurement of f_r and selection of exactly that frequency for the HFV run. Intra-animal CMV-HFV comparisons (n=4) were performed on each animal: with healthy lungs at a mean airway pressure (MAP) of 0.5 kPa and after saline lung lavage at MAPs of <1.5 kPa; 1.5–1.8 kPa; >1.8 kPa. Surfactant removal caused total respiratory system compliance (C_tot) to decrease from 44±5 to 22±3 ml/kPa. Corresponding f_r was 244±48 and 360±30 min⁻¹, respectively. HFV produced effective pulmonary gas exchange but did not improve arterial oxygenation in comparison with CMV at matched MAPs both before and after surfactant depletion. Volume amplitudes of oscillation necessary to achieve normocapnia were slightly above the natural plus equipment (2 ml) dead space. Maximum intra-alveolar pressure (P_max) was calculated for the HFV runs from MAP, C_tot, and the volume amplitude of oscillation. P_max during CMV was nearly twice that during HFV at equivalent PaCo₂ and equivalent MAPs throught the experiments.