BACKGROUND: In mechanically ventilated patients with COPD, the response of the expiratory resistance of the respiratory system (expiratory R RS ) to bronchodilators is virtually unknown. OBJECTIVE: To examine the effect of inhaled albuterol on expiratory R RS , and the correlation of albuterol-induced changes in expiratory R RS with end-inspiratory resistance and the expiratory flow-volume relationship. METHODS: We studied 10 mechanically ventilated patients with COPD exacerbation, before and

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... min after administration of albuterol. We obtained flow-volume curves during passive expiration, divided the expired volume into 5 equal volume slices, and then calculated the time constant and dynamic effective deflation compliance of the respiratory system (effective deflation C RS ) of each slice via regression analysis of the volume-flow and post-occlusion volume-tracheal pressure relationships, respectively. For each slice we calculated expiratory R RS as the time constant divided by the effective deflation C RS . RESULTS: Albuterol significantly decreased the expiratory R RS (mean expiratory R RS 42.68 ؎ 17.8 cm H 2 O/L/s vs 38.08 ؎ 16.1 cm H 2 O/ L/s) and increased the rate of lung emptying toward the end of expiration (mean time constant 2.51 ؎ 1.2 s vs 2.21 ؎ 1.2 s) . No correlation was found between the albuterol-induced changes in expiratory R RS and that of end-inspiratory resistance. Only at the end of expiration did albuterolinduced changes in the expiratory flow-volume relationship correlate with changes in expiratory R RS in all patients. CONCLUSIONS: In patients with COPD, albuterol significantly decreases expiratory resistance at the end of expiration. In mechanically ventilated patients, neither inspiratory resistance nor the whole expiratory flow-volume curve may be used to evaluate the bronchodilator response of expiratory resistance.