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Effects of PEEP on inspiratory resistance in mechanically ventilated COPD patients

¹ Medical Intensive Care Unit, Lyon Sud Hospital and Claude Bernard Lyon I University, Lyon, France, and ² Meakins-Christie Laboratories, McGill University, Montreal, Quebec, Canada

CORRESPONDENCE: C. Guérin, Service de Reanimation Medicale et d'Assistance Respiratoire, Hopital de la Croix-Rousse, 103 Grande Rue de la Croix-Rousse, 69004, Lyon, France. Fax: 33 472071774

Keywords: chronic obstructive pulmonary disease, interrupter technique, positive end-expiratory pressure, pulmonary hyperinflation, tissue resistance, volume dependence of respiratory resistance

Received: August 11, 2000
Accepted May 7, 2001

This work was supported by a grant from the Hospices Civils de Lyon.

This study aimed to investigate the effect of increased lung volume with positive end-expiratory pressure (PEEP) on respiratory resistance in patients with chronic obstructive pulmonary disease (COPD).

Ten patients with COPD were mechanically ventilated for acute respiratory failure. PEEP was set at 0, 5, 10 and 15 cmH₂O. Using the rapid airway occlusion technique, the total inspiratory resistance of the respiratory system was partitioned into interrupter (R_int,rs) and additional effective (R_rs) resistances. At each level of PEEP, at constant inflation flow, the inflation volume (V) was varied from 0.2–1 L, and, at constant V, the inflation flow was varied from 0.2–1.2 L·s^–1. The changes in end-expiratory lung volume (EELV) induced by PEEP were also measured.

The difference between the EELV and the relaxation volume of the respiratory system (FRC) increased significantly with PEEP of 10 and 15 cmH₂O as compared to a PEEP of 0, the increase being associated with a significant reduction of R_int,rs. By contrast, R_rs was independent of FRC. At constant V, R_int,rs fitted Rohrer's equation (R_int,rs =K₁+K₂xflow). While K₂ significantly declined with FRC, K₁ did not change. At all levels of PEEP, R_rs was not influenced by FRC.

With increasing lung volume induced by positive end-expiratory pressure, the inspiratory airway resistance decreased, whereas the viscoelastic behaviour of the respiratory system, as reflected by additional effective resistance, did not change.