Preferential axial flow during high-frequency oscillations: effects of gas density

Allen et al. (J. Clin. Invest. 76: 620-629, 1985) reported that regional phasic lung distension during high-frequency oscillations (HFO) is substantially and systemically heterogeneous when both frequency (f) and tidal volume (VT) are large. They hypothesized that this phenomenon was attributable to central airway geometry and preferential axial flow induced therein by the momentum flux of the inspiratory gas stream. According to that hypothesis, the observed distribution of phasic lung distension would depend on the ratio VT/VD* (where VD* is an index of anatomic dead space), independent of gas density (rho), when f is scaled in proportion to lung resonant frequency, fo. To test this hypothesis, we used the methods of Allen et al. (ibid.) to study six excised dog lungs during HFO (f = 2-32 Hz; VT = 5-80 ml) using gases of different densities. Alveolar pressure excursions (PA) were measured as rho spanned a 12-fold range using He, air, and SF6. The apex-to-base and right-to-left ratios of PA were used as indexes of regional heterogeneity of phasic lung distension. For each gas at low f, distension of the lung base was favored slightly independent of VT, but at higher f distension of the lung apex was favored when VT was small, whereas distension of the lung base was favored when VT was large. In addition, we observed substantial right-to-left differences in apical lobes during oscillation at high f not seen before.(ABSTRACT TRUNCATED AT 250 WORDS)