Regional lung mechanics and gas transport in lungs with inhomogeneous compliance

The effect of respiratory frequency (f) on the distributions of ventilation, regional gas transport, lung volume, and regional impedance was assessed with positron imaging in lungs with nonuniform lung mechanics after unilateral lung lavage. Supine dogs were studied during eucapnic oscillatory ventilation at f between 1 and 15 Hz and at a constant mean airway pressure of 5 cmH2O. Substantial differences in mean lung volume and tidal volume (VT) between lavaged and control lungs were found at all f values, but pendelluft never exceeded 2% of mouth flow. For f < or = 10 Hz, VT distributed in direct proportion to lung volume, whereas gas transport per unit of lung volume, measured from washout maneuvers, was reduced by 20% in the lavaged lung. At 15 Hz, however, the distributions of VT and gas transport approached equality between both lungs. Regional impedance was analyzed with a model that included a Newtonian resistance, an inertance, and Hildebrandt's model of tissue viscoelasticity. The data obtained from this work provide useful insights with respect to the mechanisms of gas transport during high-frequency ventilation and suggest the impact of operating frequency in clinical situations where substantial interregional heterogeneity in lung compliance could be expected.