Dynamics of respiration during rapid rate mechanical ventilation in anesthetized and paralyzed rabbits

Infants with respiratory failure are often ventilated at rates exceeding 60 breath min^-1. To obtain insight into the factors controlling the delivery and distribution of tidal volume at these ventilatory rates, we analyzed the inspiratory and expiratory pressure-flow relationships of the respiratory system and its components (lungs, endotracheal tube, and chest wall) in anesthetized, paralyzed rabbits ventilated at 60, 90, and 120 breath min^-1. Inspiratory times were 0.3,0.2, and 0.1 s. Driving pressure was maintained constant. We expressed the effect of ventilatory rate and flow direction on the pressure-flow relationships as changes in compliance, mean resistance, and inertance. We found a nonlinear pressure-flow relationship in both endotracheal tube and respiratory system. This nonlinearity could be accurately described as a function of gas flow and volume acceleration with similar coefficients for tube and respiratory system. Although the inspiratory and expiratory compliance and the mean inspiratory resistance of the lungs were lower at the higher ventilatory rates, the constant resistive behavior of the endotracheal tube and the constant elastic behavior of the chest wall caused a relatively rate-independent pressure-flow behavior of the respiratory system. The lower inspiratory resistance of the lungs was offset by the higher inspiratory resistance of the endotracheal tube, resulting in the resistance of the respiratory system being independent of the direction of gas flow. The rate-dependency of compliance and inspiratory resistance of the lungs suggests an heterogeneous distribution of inspiratory flow at rapid ventilatory rates. At these ventilatory rates, the high resistance of the respiratory system and the nonlinearity of the pressure-flow relationship decrease the tidal volume delivered to the lungs in a magnitude that can be predicted from the time course of the ventilatory pressure.