Effect of imposed inflation time on respiratory frequency and hyperinflation in patients with chronic obstructive pulmonary disease

Decreases in ventilator inflation time (Tl,vent) can cause tachypnea, proba bly as a response to lung inflation. The response may differ in chronic obs tructive pulmonary disease (COPD) because time-constant inhomogeneities cou ld foster overdistention of some lung units during early inflation, causing neural inspiratory time to be shorter than in healthy subjects. We tested the hypothesis that a decrease in Tl,vent causes tachypnea, prolongation of exhalation, and a decrease in intrinsic positive end-expiratory pressure ( PEEPi). Ten patients with stable COPD received assist-control ventilation t hrough a mouthpiece. Decreases in Tl,vent, achieved through increases in fl ow from 30 to 90 L/min, increased frequency, from 16.1 +/- 1.0 (SE) to 20.8 +/- 1.5 breaths/min (p < 0.001), time for exhalation, from 2.1 <plus/minus > 0.2 to 2.3 +/- 0.2 s (p < 0.025), and decreased PEEPi, from 7.0 <plus/min us> 1.3 to 6.4 +/- 1.1 cm H2O (p < 0.01). Decreases in Tl,vent, achieved by decreasing inspiratory pause from 2 to 0 s, increased frequency, from 12.9 <plus/minus> 0.8 to 18.1 +/- 1.6 breaths/min (p < 0.001), time for exhalat ion, from 2.0 <plus/minus> 0.2 to 2.6 +/- 0.3 s (p < 0.001), and decreased PEEPi, from 6.4 <plus/minus> 1.1 to 5.5 +/- 0.9 cm H2O (p < 0.01). In both experiments, decreases in Tl,vent reduced inspiratory effort (p < 0.01). In conclusion, strategies to reduce Tl,vent in patients with COPD caused tach ypnea, yet prolonged the time for exhalation with consequent decrease in PE EPi.