Effects of mean airway pressure on gas transport during high-frequency ventilation in dogs

In 10 anesthetized, paralyzed, supine dogs, arterial blood gases and CO₂ production (V̇CO₂) were measured after 10-min runs of high-frequency ventilation (HFV) at three levels of mean airway pressure (P̄aw) (0, 5, and 10 cmH₂O). HFV was delivered at frequencies (f) of 3, 6, and 9 Hz with a ventilator that generated known tidal volumes (VT) independent of respiratory system impedance. At each f, VT was adjusted at P̄aw of 0 cmH₂O to obtain a eucapnia. As P̄aw was increased to 5 and 10 cmH₂O, arterial PCO₂ (Pa(CO₂)) increased and arterial PO₂ (Pa(O₂)) decreased monotonically and significantly. The effect of P̄aw on Pa(CO₂) and Pa(O₂) was the same at 3, 6, and 9 Hz. Alveolar ventilation (V̇A), calculated from V̇CO₂ and Pa(CO₂), significantly decreased by 22.7 ± 2.6 and 40.1 ± 2.6% after P̄aw was increased to 5 and 10 cmH₂O, respectively. By taking into account the changes in anatomic dead space (VD) with lung volume, V̇A at different levels of P̄aw fits the gas transport relationship for HFV derived previously: V̇A = 0.13 (VT/VD)^1.2VTf (J. Appl. Physiol. 60: 1025-1030, 1986). We conclude that increasing P̄aw and lung volume significantly decreases gas transport during HFV and that this effect is due to the concomitant increase of the volume of conducting airways.