Effect of regional inhomogeneity on collateral airway resistance

We studied the interaction of transpulmonary pressure (Pao) and the pressure in an isolated sublobar lung segment (Ps) on collateral resistance (Rcoll) in excised dog lungs. A double-lumen catheter was advanced through the trachea and wedged in a peripheral airway. Gas flowed through the outer lumen of the catheter (Vcoll) to enter the segment while Ps was measured by the inner lumen. Collateral resistance was calculated as Rcoll = (Ps - Pao)/Vcoll. At constant Ps, raising Pao sharply decreased Rcoll, but at constant Pao, raising Ps increased Rcoll. Replotting these data showed that Rcoll was related to the pressure difference between the segment and the remainder of the lobe (Ps - Pao), such that raising Ps - Pao caused no change in Rcoll at lower Pao but increased Rcoll at higher Pao. Similar findings occurred in the lungs of closed-chest anesthetized dogs. We propose that this technique measures the sum of resistances of airways and collateral channels found in the segment body (Rs) and of those passing through the segment-lobar parenchymal interface (Ri). Raising Ps - Pao decreases Rs because of the volume dependency of airway resistance and increases Ri due to tissue distortion at the interface occurring as a result of inhomogeneous segment inflation. The net change in measured Rcoll depends on which of its components change in greatest magnitude. This effect varies with Pao due to the hyperbolic relationship of Pao with airway and collateral resistance.