Effect of lung volume on plateau response of airways and tissue to methacholine in dogs.

We have recently shown in dogs that much of the increase in lung resistance (RL) after induced constriction can be attributed to increases in tissue resistance, the pressure drop in phase with flow across the lung tissues (Rti). Rti is dependent on lung volume (VL) even after induced constriction. As maximal responses in RL to constrictor agonists can also be affected by changes in VL, we questioned whether changes in the plateau response with VL could be attributed in part to changes in the resistive properties of lung tissues. We studied the effect of changes in VL on RL, Rti, airway resistance (Raw), and lung elastance (EL) during maximal methacholine (MCh)-induced constriction in 8 anesthetized, paralyzed, open-chest mongrel dogs. We measured tracheal flow and pressure (Ptr) and alveolar pressure (PA), the latter using alveolar capsules, during tidal ventilation [positive end-expiratory pressure (PEEP) = 5.0 cmH2O, tidal volume = 15 ml/kg, frequency = 0.3 Hz]. Measurements were recorded at baseline and after the aerosolization of increasing concentrations of MCh until a clear plateau response had been achieved. VL was then altered by changing PEEP to 2.5, 7.5, and 10 cmH2O. RL changed only when PEEP was altered from 5 to 10 cmH2O (P < 0.01). EL changed when PEEP was changed from 5 to 7.5 and 5 to 10 cmH2O (P < 0.05). Rti and Raw varied significantly with all three maneuvers (P < 0.05). Our data demonstrate that the effects of VL on the plateau response reflect a complex combination of changes in tissue resistance, airway caliber, and lung recoil.