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RATIONALE The Beta-Blockers for the Prevention of Acute Exacerbations of Chronic Obstructive Pulmonary Disease (BLOCK COPD) study found that metoprolol was associated with a higher risk of severe exacerbation. OBJECTIVES To determine the mechanism underlying these results, we compared changes in lung function over the course of the study between treatment groups and evaluated whether baseline bronchodilator response or early reduction in FEV1 or FVC was associated with exacerbation risk. METHODS We compared changes in lung function (FEV1 and FVC) over the treatment period between treatment groups using linear mixed-effect models. Cox proportional hazards models were used to evaluate the association between baseline bronchodilator responsiveness (FEV1, FVC, and combined FEV1 and FVC), early post-randomization (14 day) change in lung function, and the interaction between treatment assignment and these measures with risk of any or severe/very severe exacerbations. Negative binomial models were used to evaluate the relationship between bronchodilator responsiveness, the interaction between bronchodilator responsiveness and treatment assignment, and exacerbation rate. RESULTS Over the 336 day treatment period, individuals in the metoprolol group had a significantly greater decrease in log-FEV1, from baseline to visit day 28 compared to individuals in the placebo group. Individuals in the metoprolol group had a significantly greater decrease in FVC from baseline to visit day 14 and 28 and additionally a significantly greater decrease in log FVC from baseline to visits 42 and 112 than individuals in the placebo group. There were no associations between early lung function reduction, or interactions between lung function reduction and treatment assignment, and time to any or severe/very severe exacerbations. There were no interactions between treatment arm and baseline bronchodilator responsiveness measures on risk or rate of exacerbations. However, those with baseline FVC bronchodilator responsiveness had a higher rate of severe/very severe exacerbations (adjusted RR = 1.62, 95% CI, 1.04-2.48). CONCLUSION Metoprolol was associated with reduced lung function during the early part of the treatment period, but these effects were modest and did not persist. Early lung function reduction and baseline bronchodilator responsiveness did not interact with the treatment arm to predict exacerbations; however, baseline FVC bronchodilator responsiveness was associated with a 60% higher rate of severe/very severe exacerbations.