Myocardial Infarction (MI)

Both STEMI and NSTEMI destroy heart muscle, but one demands the cath lab within 90 minutes while the other may not. Knowing which is which changes everything about urgency.

A myocardial infarction means myocardial tissue death from prolonged ischemia. The critical distinction between STEMI and NSTEMI is the degree of coronary occlusion and how it appears on the 12-lead ECG. A STEMI results from a complete thrombotic occlusion of a coronary artery, producing ST-segment elevation in two or more contiguous leads. This is classically associated with transmural injury, and the treatment clock starts immediately — door-to-balloon time must be ≤90 minutes for PCI. An NSTEMI results from a partial or intermittent occlusion. The ECG may show ST depression, T-wave inversion, or no acute changes at all, which is why troponin elevation is essential to confirm myocardial necrosis. NSTEMI still causes permanent damage, but the artery is not completely blocked, so reperfusion strategy is guided by risk stratification rather than an emergent cath lab activation. Both types present with chest pain, diaphoresis, dyspnea, and nausea, but STEMI pain is more likely unrelenting and unresponsive to nitroglycerin. Women, older adults, and patients with diabetes may present atypically — fatigue, jaw pain, epigastric discomfort, or isolated dyspnea without classic chest pain.

Key Distinctions

Don't confuse NSTEMI with unstable angina — both can show ST depression, but NSTEMI has elevated troponins confirming actual cell death, while unstable angina does not. Students often assume NSTEMI is 'less serious' than STEMI; NSTEMI carries significant mortality risk and may still require urgent intervention. A normal-looking ECG does not rule out NSTEMI — serial troponins are the differentiator.

Clinical Pearl

STEMI = ST Elevation = total blockage = time is muscle. If the ST segments go up, the patient goes to the cath lab — no waiting on serial labs.

MONA

MONA stands for Morphine, Oxygen, Nitroglycerin, and Aspirin, but the priority order is actually reversed: Aspirin first, then Nitroglycerin, then Oxygen (if indicated), then Morphine (used cautiously). Aspirin 162–325 mg chewed (not swallowed whole) is the single highest-impact first intervention — chewing accelerates platelet inhibition within minutes. Nitroglycerin 0.4 mg sublingual is given up to 3 doses, 5 minutes apart, while monitoring blood pressure; hold if systolic BP < 90 mmHg or if the patient took a PDE5 inhibitor (sildenafil within 24 hours, tadalafil within 48 hours), as the combination can cause fatal hypotension. Oxygen is administered only if SpO2 falls below 90% (some textbooks use 94%) — routine O2 for all MI patients is no longer recommended because hyperoxia may worsen ischemic injury. Morphine, once a standard, is now reserved for pain unrelieved by nitroglycerin because it can cause hypotension and respiratory depression, and some evidence links it to worse outcomes in NSTEMI. Throughout, you monitor vital signs continuously, establish IV access, obtain a 12-lead ECG, and keep the patient on bedrest to reduce myocardial oxygen demand.

Key Distinctions

Don't confuse the mnemonic order (M-O-N-A) with the action order (A-N-O-M). Students assume oxygen is given to every MI patient — current guidelines restrict it to SpO2 < 90% (some textbooks use 94%). Nitroglycerin is contraindicated with recent PDE5 inhibitor use AND with right ventricular infarction (these patients are preload-dependent); students often remember one but forget the other.

Clinical Pearl

Think 'A-N-O-M, not MONA': Aspirin chewed first, Nitro sublingual next, Oxygen only if sats drop, Morphine only as a last resort.