ORIGINAL ARTICLE

Effect of CPAP on intrinsic PEEP, inspiratory effort, and lung volume in severe stable COPD

F J O'Donoghue^1,2, P G Catcheside¹, A S Jordan¹, A D Bersten^2,3, R D McEvoy^1,2

¹ Sleep Disorders Unit, Repatriation General Hospital, Daw Park, South Australia 5041
² School of Medicine, Flinders University of South Australia, South Australia 5042
³ Department of Critical Care Medicine, Flinders Medical Centre, South Australia 5042

Correspondence to:
Correspondence to:
Dr F O'Donoghue, Institute for Breathing and Sleep, Austin and Repatriation Medical Centre, Heidelberg, Victoria 3084, Australia;
Fergal.O'Donoghue{at}armc.org.au

Background: Intrinsic positive end expiratory pressure (PEEPi) constitutes an inspiratory threshold load on the respiratory muscles, increasing work of breathing. The role of continuous positive airway pressure (CPAP) in alleviating PEEPi in patients with severe stable chronic obstructive pulmonary disease is uncertain. This study examined the effect of CPAP on the inspiratory threshold load, muscle effort, and lung volume in this patient group.

Methods: Nine patients were studied at baseline and with CPAP increasing in increments of 1 cm H₂O to a maximum of 10 cm H₂O. Breathing pattern and minute ventilation (I), dynamic PEEPi, expiratory muscle activity, diaphragmatic (PTPdi/min) and oesophageal (PTPoes/min) pressure-time product per minute, integrated diaphragmatic (EMGdi) and intercostal EMG (EMGic) and end expiratory lung volume (EELV) were measured.

Results: Expiratory muscle activity was present at baseline in one subject. In the remaining eight, PEEPi was reduced from a mean (SE) of 2.9 (0.6) cm H₂O to 0.9 (0.1) cm H₂O (p<0.05). In two subjects expiratory muscle activity contributed to PEEPi at higher pressures. There were no changes in respiratory pattern but I increased from 9.2 (0.6) l/min to 10.7 (1.1) l/min (p<0.05). EMGdi remained stable while EMGic increased significantly. PTPoes/min decreased, although this did not reach statistical significance. PTPdi/min decreased significantly from 242.1 (32.1) cm H₂O.s/min to 112.9 (21.7) cm H₂O.s/min). EELV increased by 1.1 (0.3) l (p<0.01).

Conclusion: High levels of CPAP reduce PEEPi and indices of muscle effort in patients with severe stable COPD, but only at the expense of substantial increases in lung volume.

Keywords: chronic obstructive pulmonary disease; continuous positive airway pressure; positive end expiratory pressure

Abbreviations: Cdyn, dynamic lung compliance; CPAP, continuous positive airway pressure; EELV, end expiratory lung volume; EMGdi; diaphragmatic electromyography; EMGic, intercostal electromyography; PEEP, positive end expiratory pressure; PEEPi, intrinsic PEEP; PEEPe, extrinsic PEEP; PTPdi, diaphragmatic pressure time product; PTPoes, oesophageal pressure time product; Poes, oesophageal pressure. Pga, gastric pressure; Pao, airway opening pressure; Pdi, transdiaphragmatic pressure; Ptp, transpulmonary pressure; Rrs, total lung resistance; TI, inspiratory time; TE, expiratory time; TTOT; respiratory cycle time; I, minute ventilation; VT, tidal volume; VT/TI, mean inspiratory flow; VT; TE, mean expiratory flow

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