Abstract
Rapid ventilatory rates are commonly used during conventional mechanical ventilation of infants with respiratory distress syndrome to improve pulmonary gas exchange while supposedly reducing barotrauma to the lungs. To understand the effects of this method of ventilation on pulmonary mechanics, we measured inspiratory (RI) and expiratory (RE) airway resistance; pulmonary compliance (CL); tidal volume (VT); and functional residual capacity (FRC) at ventilatory rates of 30, 60, and 90 breaths/min in anesthetized and paralyzed rabbits lungs had been saline-lavaged with 40 ml/kg of normal saline. The rabbits were ventilated with tracheal pressures of 30/6 cm H2O (mean = 11.2), and had a PaO2 and PaCO2, of 40–60 torr and 35–45 torr respectively (FIO=1). I:E ratio was Kept constant at 1:3. We found that RI was approximately 50% lower than RE (P=0.01). RI decreased at higher ventilatory rates (0.046±0.019 vs. 0.019±0.008 at rates of 30 and 90 breaths/min, P=0.05). CL did not change with ventilatory rate (0.30±0.10 vs. 0.28±0.15 ml/cmH2O/kg at rates of 30 and 90 breaths/min). VT and FRC were also rate-independent (6.3±2.7 and 12.4±4.4 ml/kg at a rate of 30 vs. 5.1±0.5 and 11.9±3.5 ml/kg at a rate of 90, respectively). These results demonstrate a rate-independent behavior of the saline-lavaged lung, except for the decrease in RI. The decrease in RI suggests dynamic dilatation of the conducting airways during inspiration at rapid ventilatory rates. Dynamic dilatation of the airways may represent a potential mechanism of barotrauna, reducing the benefit derived from the lower tracheal pressures needed to achieve adequate gas exchange at rapid ventilatory rates. RI may be a sensitive measurement of this effect.
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