CPAP does not abolish inspiratory threshold load due to intrinsic positive airways pressure (IPEEP) in severe stable copd

Introduction: iPEEP can be a major contributor to acute respiratory failure in COPD, constituting an inspiratory threshold load (ITL) on muscles already at a severe mechanical disadvantage. iPEEP has been documented in patients with severe stable COPD. Extrinsic PEEP has been advocated to relieve the ITL caused by iPEEP. As part of a larger study of non-invasive ventilation, we sought to document the effect of incrementally increasing extrinsic PEEP (as CPAP) on ITL. Methods: Four patients with severe stable COPD (mean FEV1 24% pred) had iPEEP measured as the change in oesophageal pressure from the onset of inspiratory effort to the start of flow. CPAP was increased in 1cmH20 increments to a maximum of 8cmH20. Gastric pressure and abdominal girth (via Respitrace) were also measured to exclude a contribution of expiratory muscles to measured iPEEP. Results: Mean (+/- SE) iPEEP over ten breaths at O CPAP was 2.84 (0.97) cmH20. In all patients, increasing CPAP significantly reduced but did not abolish measured ITL. Mean (+/- SE) minimum ITL was 1.56 (0.65) cm H20. (p=0.03) Conclusion: In severe stable COPD it is not possible to abolish the ITL due to iPEEP using CPAP. This suggests not all the measured iPEEP is due to dynamic compression of airways. Possible other factors include i) stress relaxation due to tissue viscoelastic forces and pendeluft, ii) fixed airflow obstruction which does not produce the "waterfall" effect, or iii) undetected expiratory muscle activity.