Measurement of interrupter resistance in rabbits exposed to methacholine aerosols

We studied the effect of increasing airway resistance on equilibration of airway and alveolar pressure during passive expiratory airflow interruption. In 10 anesthetized and paralyzed rabbits, airway and alveolar pressures were compared before and after airway resistance was increased with methacholine. In all studies, airway pressure rose to equilibrate with alveolar pressure immediately after the interruption (ΔPinit) regardless of increases in airway resistance. The pressures then remained equal during the interruption while gradually increasing to plateau (ΔPdiff). Before methacholine exposure, ΔPdiff was small (0.6 ± 0.3 cmH₂O). Steady-state resistance calculated from the sum of ΔPinit and ΔPdiff was similar to airway resistance calculated from ΔPinit alone. After methacholine, increased airway resistance was accompanied by increased ΔPdiff (2.0 ± 0.5 cmH₂O), causing disproportionate increase in steady-state resistance. ΔPdiff increases were equal in the airway and alveoli, implying resistive changes distal to the sampled alveoli. Thus increasing airway resistance did not delay pressure equilibration across airways. However, increases in airway resistance were accompanied by tissue resistive changes that were greater than the increases in airway resistance.