Lung inflation during high-frequency ventilation.

We investigated the relationship between mean airway pressure and lung volume during low-tidal-volume, high-frequency ventilation (HFV). Eight patients requiring mechanical ventilatory support for treatment of respiratory insufficiency were studied by imposing rapid (60 to 600 breaths/min) oscillations with low tidal volumes (50 to 150 ml) at a constant mean airway pressure of 5 cm H2O. Despite this constant mean airway pressure, lung volume increased substantially during the oscillation period in 7 of 8 subjects, as indicated both by an increase in thoracoabdominal dimensions and by an increase in respiratory system relaxation pressures after the oscillations were stopped. For each patient in whom these changes occurred, the degree of lung inflation rose progressively with increases in either frequency or tidal volume. Given this dissociation between lung volume and mean airway pressure, some index of lung volume or alveolar pressure should be monitored to minimize the likelihood of adverse effects during HFV.