A 75-year-old male with angina pectoris lasting 4 hours, unresponsive to nitroglycerin. ECG shows 6mm ST-segment elevation in leads II, III, aVF and 4mm horizontal ST depression in V1-3, with occasional ventricular premature beats. Diagnosed with acute myocardial infarction (AMI), the patient develops restlessness, low blood pressure (BP), heart rate of 45 bpm, and third-degree atrioventricular block (AV block) with frequent atrial premature beats after 10 hours. What is the first-line treatment?