ABSTRACT
The purpose of this study was to demonstrate that ventilation of rabbit lungs (whose
mechanics are similar to those of human infants) at rapid rates will lead to large
alterations in tracheal airway pressures, tidal volume, and functional residual capacity
(FRC) with only minor changes in arterial blood gases. Thirteen rabbits were ventilated
at rates of 30, 60, 90, and 120 breaths per minutes (BPM) with pressures of 17/2 cm
H2O. Tracheal peak inspiratory pressure (PIP) was always lower than ventilator PIP and
decreased to 11 ± 1 cm H2O at 120 BPM. Positive end-expiratory pressure (PEEP) in the trachea was always greater
than 2 cm H2O and increased with rate (3.5 cm H2O at 120 BPM). Tidal volume decreased as rates were increased such that rates above
60 BPM resulted in insignificant changes in minute ventilation and arterial blood
gases. However, the FRC increased from 16 (30 BPM) to 25 ml/kg (120 BPM), a 56% increase,
suggesting large increases in end-expiratory alveolar pressure. We conclude that rapid-rate
ventilation (> 60 BPM) of healthy rabbits results in significant increases in both
tracheal PEEP and FRC without significantly affecting arterial blood gases. The increased
tracheal PEEP and FRC are manifestations of inadvertent PEEP. The increased FRC without
concomitant increase in PaO2 implicates alveolar overdistention. We speculate that rapid-rate ventilation of human
infants having lung mechanics similar to rabbits, will also result in inadvertent
PEEP and alveolar overdistention.