SUSTAINED INFLATIONS IMPROVE RESPIRATORY COMPLIANCE DURING HIGH-FREQUENCY OSCILLATORY VENTILATION BUT NOT DURING LARGE TIDAL VOLUME POSITIVE-PRESSURE VENTILATION IN RABBITS

Objective: To determine whether volume recruitment maneuvers that indu ce significant lung reexpansion during high-frequency oscillatory vent ilation are also of value during conventional positive-pressure ventil ation. Design: Crossover comparison of volume recruitment maneuvers ad ministered during high-frequency oscillatory ventilation and positive- pressure ventilation in normal and surfactant-deficient adult rabbits. Setting: Laboratory. Subjects: Nineteen adult New Zealand white rabbi ts (weight 2.3 to 3.3 kg). Methods: Respiratory system compliance was measured plethysmographically before and after sustained inflations in six normal and five saline-lavaged anesthetized rabbits, using both v entilators over a range of mean and end-expiratory pressures. Results: Under conditions where sustained inflations during high-frequency osc illatory ventilation at 15 Hz increased respiratory system compliance 50 +/- 28%, sustained inflations during conventional positive-pressure ventilation at a rate of 30 to 40 breaths/min and tidal volumes of 14 to 17 mL/kg did not change respiratory system compliance (mean change 3 +/- 9%). Sustained inflations during conventional positive-pressure ventilation could not be made effective by increasing the positive en d-expiratory pressure level to equal the mean pressure during high-fre quency oscillatory ventilation. Sustained inflations on conventional p ositive-pressure ventilation remained ineffective up to positive end-e xpiratory pressure levels of 17.5 cm H2O. In lavaged rabbits, sustaine d inflations increased respiratory system compliance 49 +/- 14% during high-frequency oscillatory ventilation and 0 +/- 3% during convention al positive-pressure ventilation. Sustained inflations increased compl iance significantly during conventional positive-pressure ventilation only when ventilating with tidal volumes of 7 mL/kg and low end-expira tory pressure. Conclusions: Active recruitment of lung volume during h igh-frequency oscillatory ventilation appears necessary, because small pressure/volume cycles adequate to support high-frequency gas transpo rt are not able to reexpand atelectatic lung units without the aid of a sustained inflation. We conclude that volume recruitment maneuvers i mprove respiratory system compliance substantially during high-frequen cy oscillatory ventilation at 15 Hz, but these maneuvers offer potenti al risk and no benefit during conventional positive-pressure ventilati on with large tidal volumes or when using smaller tidal volumes and hi gh levels of positive end-expiratory pressure.