SIMPLE METHOD TO MEASURE TOTAL EXPIRATORY TIME CONSTANT BASED ON THE PASSIVE EXPIRATORY FLOW-VOLUME CURVE

Objective: In intubated, mechanically ventilated patients, inspiration is forced by externally applied positive pressure, In contrast, exhal ation is passive and depends on the time constant of the total respira tory system, The expiratory time constant is thus an important determi nant of mechanical ventilation, The aim of this study was to evaluate a simple method for measuring the expiratory time constant in ventilat ed subjects. Design: Prospective study using a lung simulator and ten dogs. Setting: University hospital. Subjects: Commercially available l ung simulator and ten greyhound dogs. Interventions: Different expirat ory time constants were set on the lung simulator, In the dogs, the en dotracheal tube was clamped to increase airways resistance by 22.5 cm H2O/(L/sec) and the lungs were injured with hydrochloric acid to decre ase total respiratory compliance by 16 mL/cm H2O. This procedure resul ted in a wide range of expiratory time constants. Measurements and Mai n Results: Pneumotachography was used to measure flow and volume, The ratio of exhaled volume and peak flow was calculated from these signal s, corrected for the limited exhalation time yielding the ''calculated expiratory time constant'' and compared with the actual expiratory ti me 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.