Simple method to measure total expiratory time constant based on the passive expiratory flow-volume curve

Abstract

Objective: In intubated, mechanically ventilated patients, inspiration is forced by externally applied positive pressure. In contrast, exhalation is passive and depends on the time constant of the total respiratory system. The expiratory time constant is thus an important determinant of mechanical ventilation. The aim of this study was to evaluate a simple method for measuring the expiratory time constant in ventilated subjects.

Design: Prospective study using a lung simulator and ten dogs.

Setting: University hospital.

Subjects: Commercially available lung simulator and ten greyhound dogs.

Interventions: Different expiratory time constants were set on the lung simulator. In the dogs, the endotracheal tube was clamped to increase airways resistance by 22.5 cm H2O/(L/sec) and the lungs were injured with hydrochloric acid to decrease total respiratory compliance by 16 mL/cm H2O. This procedure resulted in a wide range of expiratory time constants.

Measurements and main results: Pneumotachography was used to measure flow and volume. The ratio of exhaled volume and peak flow was calculated from these signals, corrected for the limited exhalation time yielding the "calculated expiratory time constant" and compared with the actual expiratory time constant. The typical error was +/- 0.19 sec for the lung simulator and +/- 0.15 sec for the dogs.

Conclusions: The volume and peak flow corrected for limited exhalation time is a good estimate of the total expiratory time constant in passive subjects and may be useful for the titration of mechanical ventilation.