To assess the effect of a decrease in respiratory rate on dynamic hyperinflation, as determined by changes in plateau airway pressure, in patients with status asthmaticus whose baseline minute ventilation approximated 10 L/min.
Observational descriptive study. Setting: Medical intensive care unit.
Twelve patients with severe asthma mechanically ventilated in the assist control mode with a tidal volume of 613 ± 100 mL and an inspiratory flow rate of 79 ± 4 L/min.
A decrease in respiratory rate from 18 to 12 and 6 breaths/min.
Measurements and Main Results:
Plateau airway pressure decreased by approximately 2 cm H2O (25.4 ± 2.8 vs. 23.3 ± 2.6 cm H2O, p < .01) when respiratory rate was decreased from 18 to 12 breaths/min (increase in expiratory time 1.7 secs) and by a similar amount (23.3 ± 2.6 vs. 21.3 ± 2.9 cm H2O, p < .01) when respiratory rate was decreased from 12 to 6 breaths/min (increase in expiratory time 5 secs). Peak airway pressure was similar at the three respiratory rates (66.8 ± 8.7 vs. 66.4 ± 9.5 vs. 67.8 ± 11.1 cm H2O at 18, 12, and 6 breaths/min, respectively). End-expiratory flow rates (n = 7) were 61.4 ± 12.6, 38.6 ± 4.5, and 23.1 ± 5.8 mL/sec at respiratory rates of 18, 12, and 6 breaths/min, respectively.
Prolongation of expiratory time decreases dynamic hyperinflation in patients with status asthmaticus, as evidenced by a reduction in plateau airway pressure, but the magnitude of this effect is relatively modest when baseline minute ventilation is ≤10 L/min, because of the low end-expiratory flow rates. Since flow progressively decreases throughout expiration, the reduction in dynamic hyperinflation resulting from a given prolongation of expiratory time will depend on the baseline respiratory rate (i.e., less reduction in dynamic hyperinflation at a lower respiratory rate). Changes in peak airway pressure may not always reflect the changes in dynamic hyperinflation that result from prolongation of expiratory time.