Expiratory time constants in mechanically ventilated patients with and without COPD

Objective: In mechanically ventilated patients, the expiratory time constan t provides information about the respiratory mechanics and the actual time needed for complete expiration. As an easy method to determine the time con stant, the ratio of exhaled tidal volume to peak expiratory flow has been p roposed. This assumes a single compartment model for the whole expiration. Since the latter has to be questioned in patients with chronic obstructive pulmonary disease (COPD), we compared time constants calculated from variou s parts of expiration and related these to time constants assessed with the interrupter method. Design: Prospective study. Setting: A medical intensive care unit in a university hospital. Patients: Thirty-eight patients (18 severe COPD, eight mild COPD, 12 other pathologies) were studied during mechanical ventilation under sedation and paralysis. Measurements and results: Time constants determined from flow-volume curves at 100%, the last 75, 50, and 25% of expired tidal volume, were compared t o time constants obtained from interrupter measurements. Furthermore, the t ime constants were related to the actual time needed for complete expiratio n and to the patient's pulmonary condition. The time constant determined fr om the last 75% of the expiratory flow-volume curve (RCfv75) was in closest agreement with the time constant obtained from the interrupter measurement , gave an accurate estimation of the actual time needed for complete expira tion, and was discriminative for the severity of COPD. Conclusions: In mechanically ventilated patients with and without COPD, a t ime constant can well be calculated from the expiratory flow-volume curve f or the last 75% of tidal volume, gives a good estimation of respiratory mec hanics, and is easy to obtain at the bedside.