EFFECT OF TIDAL VOLUME AND FREQUENCY ON AIRWAY RESPONSIVENESS IN MECHANICALLY VENTILATED RABBITS

We evaluated the effects of the rate and volume of tidal ventilation o n airway resistance (Raw) during intravenous methacholine (MCh) challe nge in mechanically ventilated rabbits. Five rabbits were challenged a t tidal volumes of 5, 10, and 20 ml/kg at a frequency of 15 breaths/mi n and also under static conditions (0 ml/kg tidal volume). Four rabbit s were subjected to MCh challenge at frequencies of 6 and 30 breaths/m in with a tidal volume of 10 ml/kg and also under static conditions. I n both groups, the increase in Raw with MCh challenge was significantl y greater under static conditions than during tidal ventilation at any frequency or volume. Increases in the volume or frequency of tidal ve ntilation resulted in significant decreases in Raw in response to MCh. We conclude that tidal breathing suppresses airway responsiveness in rabbits in vivo. The suppression of narrowing in response to MCh incre ases as the magnitude of the volume or the frequency of the tidal osci llations is increased. Our findings suggest that the effect of lung vo lume changes on airway responsiveness in vivo is primarily related to the stretch of airway smooth muscle.