Effects of respiratory variables on regional gas transport during high-frequency ventilation

The regional effects of tidal volume (VT), respiratory frequency, and expiratory-to-inspiratory time ratio (TE/TI) during high-frequency ventilation (HFV) were studied in anesthetized and paralyzed dogs. Regional ventilation per unit of lung volume (spV̇r) was assessed with a positron camera during the washout of the tracer isotope ¹³NN from the lungs of 12 supine dogs. From the washout data, functional images of the mean residence time (MRT) of ¹³NN were produced and spV̇r was estimated as the inverse of the regional MRT. We founds that at a constant VT x f product (where f represents frequency), increasing VT resulted in higher overall lung spV through the local enhancement of the basal spV̇r and with little effect in the apical spV̇r. In contrast, increasing VT x f at constant VT increased overall ventilation without significantly affecting the distribution of spV̇r values. TE/TI had no substantial effect in regional spV̇r distribution. These findings suggest that the dependency of gas transport during HFV of the form VT² x f is the result of a progressive regional transition in gas transport mechanism. It appears, therefore, that as VT increases, the gas transport mechanism changes from a relative inefficient dispersive mechanism, dependent VT x f, to the more efficient mechanism of direct fresh gas convection to alveoli with high regional tidal volume-to-dead-space ratio. A mathematical model of gas transport in a nonhomogeneous lung that exhibits such behavior is presented.