Intensive Care Medicine

, Volume 26, Issue 11, pp 1612–1618

Expiratory time constants in mechanically ventilated patients with and without COPD

  • M. Lourens
  • B. van den Berg
  • J. Aerts
  • A. Verbraak
  • H. Hoogsteden
  • J. Bogaard
Original

DOI: 10.1007/s001340000632

Cite this article as:
Lourens, M., van den Berg, B., Aerts, J. et al. Intensive Care Med (2000) 26: 1612. doi:10.1007/s001340000632

Abstract

Objective: In mechanically ventilated patients, the expiratory time constant provides information about the respiratory mechanics and the actual time needed for complete expiration. As an easy method to determine the time constant, the ratio of exhaled tidal volume to peak expiratory flow has been proposed. 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 various 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 to time constants obtained from interrupter measurements. Furthermore, the time constants were related to the actual time needed for complete expiration and to the patient's pulmonary condition. The time constant determined from 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 expiration, and was discriminative for the severity of COPD. Conclusions: In mechanically ventilated patients with and without COPD, a time constant can well be calculated from the expiratory flow-volume curve for the last 75% of tidal volume, gives a good estimation of respiratory mechanics, and is easy to obtain at the bedside.

Mechanical ventilation Chronic obstructive pulmonary disease Respiratory mechanics Flow-volume curve Time constant Airflow obstruction 

Copyright information

© Springer-Verlag 2000

Authors and Affiliations

  • M. Lourens
    • 1
  • B. van den Berg
    • 1
  • J. Aerts
    • 1
  • A. Verbraak
    • 1
  • H. Hoogsteden
    • 1
  • J. Bogaard
    • 1
  1. 1.Department of Pulmonary and Intensive Care Medicine, Erasmus Medical Centre Rotterdam, Dr Molewaterplein 40, 3015 6D Rotterdam, The NetherlandsThe Netherlands