Influence of lung volume history and increased surface forces on collateral resistance.

In order to investigate the major route for collateral ventilation, the influences of lung volume history and increased surface forces on collateral resistance (Rcoll) were studied in excised canine lower lobes. Measurements were made during inflation and deflation in two states, i.e. before (control) and after stiffening by large tidal ventilation raising peak recoil pressure (Ptp) by 8-10 cmH2O. Air flowed into a wedged segment via a double lumen catheter (Vcoll) keeping segmental pressure (PS) constant at 2 cmH2O as monitored through the other lumen, giving Rcoll = Ps/Vcoll. Rcoll vs lung volume (VL) showed little hysteresis, in sharp contrast to Rcoll vs Ptp (Rcoll higher during inflation). Thus, in each state, Gcoll (= 1/Rcoll) depended primarily on VL, but the stiffed state appeared to have permanently enlarged channels or recruited new ones. The relations between Gcoll and VL or Ptp somewhat resemble reported bronchial diameter behaviour. The ventilation process itself, rather than higher surface forces per se, appeared to distend collateral channels in the stiff lung. The results are consistent with the major component of Gcoll being located at the bronchiole level.