Using conventional infant ventilators at unconventional rates

The effect of progressive increases in ventilator rate on delivered tidal and minute volumes, and the effect of changing peak inspiratory pressure (Pmax), positive end-expiratory pressure (PEEP), and inspiration to expiration (I:E) ratio at different ventilator rates were examined. Five different continuous-flow, time-cycled, pressure-preset infant ventilators were studied using a pneumotachograph, an airway pressure monitor, and a lung simulator. As rates increased from 10 to 150 breaths per minute, tidal volume stayed constant until 25 to 30 breaths per minute; then progressively decreased. In all, tidal volume began to decrease when proximal airway pressure waves lost inspiratory pressure plateaus. As rates increased, minute volume increased until 75 breaths per minute, then leveled off, then decreased. Substituting helium for O₂ increased the ventilator rate at which the minute volume plateau effect occurred. Increasing peak inspiratory pressure consistently increased tidal volume. Increasing positive end-expiratory pressure decreased tidal volume. At rates less than 75 breaths per minute, inspiratory time (inspiration to expiration ratio) had little effect on delivered volume. At rates greater than 75 breaths per minute, inspiratory time became an important determinant of minute volume. For any given combination of lung compliance and airway resistance: (1) there is a maximum ventilator rate beyond which tidal volume progressively decreases and another maximum ventilator rate beyond which minute volume progressively decreases; (2) at slower rates, delivered volumes are determined primarily by changes in proximal airway pressures; (3) at very rapid rates, inspiratory time becomes a key determinant of delivered volume.