Gas exchange efficacy of high-frequency oscillatory ventilation studied by helium washout from dog lungs

The gas exchange efficacy of high-frequency oscillatory ventilation (HFOV) was assessed from an analysis of helium washout from lungs in anesthetized paralyzed supine dogs. Piston stroke volumes (Vs) were varied from 20 to 40 ml, frequencies (f), from 10 to 40 Hz and mean airway opening pressures from 2 to 10 cm H2O. The time course of washout could be described as the sum of three exponential components. Based on a series model comprising a proximal and a distal lung compartment, two component conductances, a 'distal' conductance (Gd) and a 'proximal' conductance (Gp) and an overall conductance of the lung (Gl) could be calculated. Gd, Gp and Gl increased with f up to a maximum value remaining constant or decreasing at higher f; the frequency at which the maximum occurred depending on Vs and on the diameter of the endotracheal tube (ET). With increasing Vs generally the G values increased, but decreased at higher f with the smaller ET. The insoluble inert gas washout is shown to be a useful method for assessing the ventilatory gas exchange conductance of lungs during HFOV.