Relationship between respiratory mechanics and postinspiratory muscle activity during muscarinic challenge in rabbits

In six spontaneously breathing New Zealand rabbits (weight 2.36 +/- 0.29 kg) anesthetized with pentobarbital sodium (35 mg/kg i.v.) airflow, tidal volume, and tracheal and esophageal pressures were measured. These allowed the determination of breathing pattern parameters, respiratory system and lung resistances, elastances, and time constants, total postinspiratory muscle pressure and its timing parameters. The measurements were performed: (a) under control conditions; (b) after increasing concentrations of aerosolized methacholine (8, 16, 32, and 64 mg/ml); and (c) after aerosolized atropine (1 mg/ml). Methacholine progressively increased intrinsic mechanical load (resistance and elastance) of the respiratory system. Total postinspiratory muscle pressure (Pmus,0) increased in relation to the increase in elastic load. The duration of postinspiratory activity increased mainly because of the decrease in expiratory time. In conclusion, increase in inspiratory activity during expiration is due to two independent mechanisms controlling the amplitude and the relative duration.