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To evaluate the effects of arm exercise with or without support of mechanical ventilation on breathing pattern, respiratory muscle pressure output, and ratings of dyspnea and arm discomfort in difficult-to-wean patients with COPD.Prospective, controlled, physiological study in a respiratory ICU.Eight tracheostomized difficult-to-wean patients.Patients performed an incremental and an endurance arm exercise while breathing through a trach collar or while receiving pressure support ventilation.Breathing pattern, mechanics, arterial saturation, heart rate, and subjective ratings of dyspnea and arm discomfort were measured at baseline, at the end, and 30 min after exercise. Exercise during pressure support ventilation was found to result in higher peak workload (incremental testing) than exercise during trach collar. Moreover, compared to incremental and endurance testing during trach collar, incremental and endurance testing during pressure support ventilation resulted in greater tidal volume, and lower respiratory rate, lower pressure output from the respiratory muscles, and lower work of breathing. Exercise-induced worsening of dyspnea and arm discomfort during trach collar was similar to the corresponding values recorded during pressure support ventilation.In tracheostomized difficult-to-wean patients with COPD arm exercise performed during unassisted respiration (trach collar) causes greater increases in respiratory rate and in respiratory muscle pressure output than arm exercise performed during pressure support ventilation. Exercise-induced dyspnea and arm discomfort are similar during assisted and nonassisted respiration.