Dynamics of expiration and gas trapping in rabbits during mechanical ventilation at rapid rates

Mechanical ventilation at rapid rates may cause gas trapping by decreasing the time available for expiration. The volume of gas trapped within the lungs depends not only on the expiratory time, but also on the rate of emptying of the lungs. Because newborn infants are frequently ventilated at rapid rates, we examined the factors determining the rate of lung emptying, and thereby the development of gas trapping, at ventilatory rates of 40 to 120 breath/min in anesthetized and paralyzed rabbits. We found that flow and volume were related nonlinearly during the first segment of expiration, and linearly during the second segment. Only the second segment could therefore be described with a single time constant. The expiratory resistance of the respiratory system and endotracheal tube was up to 4.5 times greater than the inspiratory resistance, a finding explained by the higher transmural airway pressures during inspiration and by the dynamic effects of the increase in cross-sectional airway area at the junction of endotracheal tube and trachea. This high expiratory resistance lengthened the time constant of the second segment of expiration which, combined with the delay caused by the slow opening of the exhalation valve during the first segment, promoted gas trapping when the expiratory time was shortened.