Expiratory flow-volume curves in mechanically ventilated patients with chronic obstructive pulmonary disease

Background: Forced expiratory flow-volume curves are commonly used to asses s the degree of airflow obstruction in patients with chronic obstructive pu lmonary disease (COPD). In mechanically ventilated subjects, expiratory air ways obstruction can only be estimated from relaxed expirations. The aim of this study was to quantify the degree of airways obstruction from relaxed expiratory flow-volume curves in mechanically ventilated patients with COPD . Methods: As measure of airflow obstruction, the effective time constant dur ing the last 50% of expired volume (tau) was calculated. For bedside monito ring, tau was recalculated as the slope of the flow during the last 50% of expired volume (SF50). In order to study reproducibility, the variables wer e calculated from consecutive breaths and at different levels of end-expira tory lung volume (EEV). The SF50 and the tau were correlated with the force d expiratory volume in 1 s (FEV1) measured prior to the start of ventilator y support. Results: Twenty-seven patients were studied with a FEV1 expressed as percen tage predicted of 31+/-12% (mean+/-SD). The SF50 amounted to 19+/-10 degree s. A positive linear correlation was established between SF50 and the FEV1, (%pred), (r=0.90, P<0.0001). The tau showed an exponential relationship wi th FEV1 (%pred), (r(2)=0.78) From 5 consecutive breaths the mean variation coefficient of SF50 was 5+/-2%. Changes of Delta EEV from 0.05 to 1.00 L di d not affect the SF50-values. In 12 patients, mechanically ventilated for r espiratory diseases other than COPD, mean tau and SF50 were significantly d ifferent from the COPD-patients (P<0.0001). Conclusions: This study indicates that relaxed expiratory flow-volume curve s can be used to assess airflow obstruction in mechanically ventilated pati ents with COPD. This information can be used to adapt ventilatory settings.