A patient with known angina pectoris is given sublingual nitroglycerin in the dental office but shows no improvement after 5 minutes. What is the most appropriate next step?

If sublingual nitroglycerin fails to relieve angina within 5 minutes, this may indicate an acute myocardial infarction rather than stable angina. The immediate priority is to activate Emergency Medical Services (call 911) so that advanced cardiac life support can be initiated. While awaiting EMS, administering aspirin 325 mg (if not contraindicated) and supplemental oxygen are appropriate adjunct measures, but EMS activation is the critical first step. Waiting and giving additional nitroglycerin without activating EMS risks dangerous delays in definitive care.