A medical student in the coronary intensive care unit asked whether Marvin should also be treated with propranolol (a -adrenergic antagonist). The student reasoned that propranolol would reduce the myocardial O₂ requirement and possibly prevent another infarction. Why does propranolol decrease the myocardial O₂ requirement? The attending physician pointed out that there could be a risk associated with the use of propranolol. What is this risk?