PASSIVE MECHANICS OF LUNG AND CHEST-WALL IN PATIENTS WHO FAILED OR SUCCEEDED IN TRIALS OF WEANING

In an accompanying article (Jubran, et al., Am. J. Respir. Crit Care M ed. 155:906-915), we report that patients with chronic obstructive pul monary disease (COPD) who failed a trial of weaning from mechanical ve ntilation developed worsening of pulmonary mechanics compared with pat ients who tolerated the trial and were extubated. We wondered whether the greater derangements in pulmonary mechanics in the weaning failure patients are evident ever before undertaking the weaning trial. We me asured mechanics of the respiratory system, lung, and chest wall durin g passive ventilation at usual ventilator settings in 12 patients who went on to fail a weaning trial and in 12 patients who were successful ly weaned. No differences in the resistances of the respiratory system , lung, and chest wall were observed between the two groups or when th e resistances were separated into the components derived from ohmic re sistance and viscoelastic behavior/time-constant inhomogeneities. Like wise, the groups did not differ in terms of static elastance and dynam ic intrinsic positive end-expiratory pressure (PEEP(i)) of the respira tory system and the respective lung and chest wall components or in te rms of dynamic elastances of the respiratory system and chest wall. Th e failure group had a higher dynamic elastance of the lung than the su ccess group (p < 0.01), but the individual values showed considerable overlap among the patients in the two groups so limiting its usefulnes s in signaling a patient's ability to sustain spontaneous ventilation. Thus, mechanics of the respiratory system and its lung and chest wall components during passive ventilation did not satisfactorily discrimi nate between patients who failed a weaning trial and those successfull y weaned, and, thus, are unlikely to be useful in signaling a patient' s ability to tolerate the discontinuation of mechanical ventilation.