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.
ASJC Scopus subject areas
- Physiology (medical)