Am. J. Respir. Crit. Care Med., Volume 163, Number 6, May 2001, 1365-1370

Effect of Imposed Inflation Time on Respiratory Frequency and Hyperinflation in Patients with Chronic Obstructive Pulmonary Disease

FRANCO LAGHI, JEREMY SEGAL, WILLIAM K. CHOE, and MARTIN J. TOBIN

Division of Pulmonary and Critical Care Medicine, Edward Hines, Jr. Veterans Administration Hospital, and Loyola University of Chicago Stritch School of Medicine, Hines, Illinois

Decreases in ventilator inflation time (TI,vent) can cause tachypnea, probably as a response to lung inflation. The response may differ in chronic obstructive pulmonary disease (COPD) because time-constant inhomogeneities could 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 TI,vent causes tachypnea, prolongation of exhalation, and a decrease in intrinsic positive end-expiratory pressure (PEEP_i). Ten patients with stable COPD received assist-control ventilation through a mouthpiece. Decreases in TI,vent, achieved through increases in flow 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 ± 0.2 to 2.3 ± 0.2 s (p < 0.025), and decreased PEEP_i, from 7.0 ± 1.3 to 6.4 ± 1.1 cm H₂O (p < 0.01). Decreases in TI,vent, achieved by decreasing inspiratory pause from 2 to 0 s, increased frequency, from 12.9 ± 0.8 to 18.1 ± 1.6 breaths/min (p < 0.001), time for exhalation, from 2.0 ± 0.2 to 2.6 ± 0.3 s (p < 0.001), and decreased PEEP_i, from 6.4 ± 1.1 to 5.5 ± 0.9 cm H₂O (p < 0.01). In both experiments, decreases in TI,vent reduced inspiratory effort (p < 0.01). In conclusion, strategies to reduce TI,vent in patients with COPD caused tachypnea, yet prolonged the time for exhalation with consequent decrease in PEEP_i.

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