A patient with a history of angina pectoris develops chest pain during a dental procedure and does not respond to nitroglycerin. What is the priority first action?

Nitroglycerin-unresponsive chest pain in a patient with known angina should be treated as an acute myocardial infarction (MI) until proven otherwise. The highest priority action is to activate EMS immediately (call 911). Concurrently, supplemental oxygen (if SpO2 <90%), non-enteric coated aspirin 325 mg (chewed), and positioning the patient comfortably (usually semi-reclined, not fully supine) are administered. However, calling EMS is the single most critical first action because definitive treatment (thrombolytics, PCI) cannot be provided in the dental office.