Pressure-flow relationships in a collaterally ventilating dog lung segment.

We evaluated the pressure (P)-flow (V) relationship in collaterally ventilating dog lung segments by passing He, N2, and SF6 through a bronchoscope (5 mm OD) wedged in a peripheral airway. Measurements were made at functional residual capacity (FRC) and two higher lung volumes, keeping segment-to-airway opening pressure constant (3 cmH2O) in five anesthetized, paralyzed, vagotomized, supine dogs. Average flows ranged from 5.0 to 8.0 ml/s for He, 4.5 to 7.5 ml/s for N2, and 3.4 to 4.7 ml/s for SF6. When these data were fitted as P = K1/3/3 mu V + K2 rho V2, density-dependent pressure losses were unimportant when He and N2 were used, suggesting laminar flow with these gases. A dimensionless plot of the total pressure drop relative to a reference dynamic pressure as a function of Reynolds number at the bronchoscope tip suggested that flow through the segment behaved as if it were laminar at Reynolds numbers less than 100. Furthermore, when the airway diameter used to compute the normalized pressure and Reynolds number was scaled as the cubic root of lung volume, curves for all three gases were superimposed, suggesting that the dimensions of intrasegmental/collateral airways scale as lung volume 1/3.