title: Effect of prolongation of expiratory time on dynamic hyperinflation in mechanically ventilated patients with severe asthma.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  Observational descriptive study.
  
  
  METHODS
  Medical intensive care unit.
  
  
  METHODS
  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.
  
  
  METHODS
  A decrease in respiratory rate from 18 to 12 and 6 breaths/min.
  
  
  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.
  
  
  CONCLUSIONS
  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 < or = 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.
authors.affiliation: Division of Pulmonary and Critical Care Medicine, Hennepin County Medical Center, Minneapolis, MN, USA.
authors.forename: James W
authors.initials: JW
authors.lastname: Leatherman
authors.forename: Charles
authors.initials: C
authors.lastname: McArthur
authors.forename: Robert S
authors.initials: RS
authors.lastname: Shapiro
mesh_terms: D000328:Adult; D003422:Critical Care; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D012119:Respiration; D012121:Respiration, Artificial / Q000009:adverse effects* / Q000379:methods; D013224:Status Asthmaticus / Q000628:therapy*; D013997:Time Factors