Inspiratory-to-expiratory time ratio and alveolar ventilation during high-frequency ventilation in dogs

It has been suggested that the increase in inspiratory flow rate caused by a decrease in the inspiratory-to-expiratory time ratio (I:E) at a constant tidal volume (VT) could increase the efficiency of ventilation in high-frequency ventilation (HFV). To test this hypothesis, we studied the effect of changing I:E from 1:1 to 1:4 on steady-state alveolar ventilation (VA) at a given VT and frequency (f) and at a constant mean lung volume (VL). In nine anesthetized, paralyzed, supine dogs, HFV was performed at 3, 6, and 9 Hz with a ventilator that delivered constant inspiratory and expiratory flow rates. Mean airway pressure was adjusted so that VL was maintained at a level equivalent to that of resting FRC. At each f and one of the I:E chosen at random, VT was adjusted to obtain a eucapnic steady state [arterial pressure of CO₂ (Pa(CO₂)) = 37 ± 3 Torr]. After 10 min of each HFV, Pa(CO₂), arterial pressure of O₂ (Pa(O₂)), and CO₂ production (V̇CO₂) were measured, and I:E was changed before repeating the run with the same f and VT. V̇A was calculated from the ratio of V̇CO₂ and Pa(CO₂). We found that the change of I:E from 1:1 to 1:4 had no significant effects on Pa(CO₂), Pa(O₂), and V̇A at any of the frequencies studied. We conclude, therefore, that the mechanism or mechanisms responsible for gas transport during HFV must be insensitive to the changes in inspiratory and expiratory flow rates over the VT - f range covered in our experiments.