High-frequency oscillatory ventilation may increase airway closure

In five seated, normal subjects, we measured closing volumes using 133Xe boluses inhaled at residual volume. High frequency oscillatory ventilation (HFOV) (15 Hz, 2 cc/kg) was applied during either inspiration to total lung capacity or the subsequent expiration. Closing volume was increased (P less than 0.001) when HFOV was applied during the latter half of expiration, but not when HFOV was applied during inspiration or the first half of expiration. Subsequently, in seven subjects, we measured the regional distributions of 133Xe boluses delivered during open-glottis breath-hold at 14% vital capacity after equilibration with N2O. HFOV was applied during bolus delivery for about 16 sec. These distributions were compared with those achieved by intravenous injections of 133Xe in saline. Regional perfusion (injected isotope) exceeded regional N2O uptake at the lung bases and this was significantly accentuated by HFOV, compatible with increased basal closure. We conclude that in normal subjects at low lung volumes, HFOV may enhance airway closure, though other explanations are possible.