Acute Coronary Syndromes and Reperfusion

Classification, Biomarkers, and Electrocardiographic Localization

Acute coronary syndrome encompasses unstable angina, non-ST-elevation myocardial infarction, and ST-elevation myocardial infarction. Unstable angina presents with ischemic symptoms, sometimes with dynamic ST depression or T-wave inversion, without an acute troponin rise or fall attributable to myocardial infarction. Non-ST-elevation myocardial infarction is diagnosed when a rise or fall in cardiac troponin indicates acute myocardial injury in the setting of clinical evidence of ischemia, without persistent ST-segment elevation. ST-elevation myocardial infarction usually reflects acute coronary artery occlusion or near-occlusion causing ongoing myocardial ischemia and injury, although the extent of necrosis is not necessarily transmural. For any patient presenting with suspected acute coronary syndrome, an initial 12-lead electrocardiogram must be obtained and interpreted within 10 minutes of arrival to avoid treatment delays.

Twelve-lead electrocardiography identifies the ischemic territory and guides immediate intervention. ST-segment elevation in leads II, III, and aVF indicates an inferior wall myocardial infarction, which most commonly involves the right coronary artery. ST-segment elevation in leads V1 through V4 indicates an anteroseptal myocardial infarction, most commonly involving the left anterior descending coronary artery. ST-segment elevation in leads I, aVL, V5, and V6 suggests a lateral wall infarction, which may involve the left circumflex artery or a diagonal branch of the left anterior descending artery. Ischemia can also manifest reciprocally; ST-segment depression in leads V1 through V3, particularly when accompanied by tall R waves and upright T waves, suggests a posterior myocardial infarction, and posterior leads V7 through V9 can help confirm it.

Whenever ST elevation is identified in the inferior leads, obtaining a right-sided 12-lead electrocardiogram is indicated to evaluate for concurrent right ventricular infarction. ST-segment elevation of at least 1 mm in right-sided lead V4R represents a key finding supporting right ventricular infarction. Clinically, right ventricular infarction classically manifests as a distinct triad: hypotension, jugular venous distention, and clear lung fields. Right ventricular infarction can impair forward flow and reduce left ventricular filling; assess preload and perfusion closely because both inadequate filling and excessive fluid administration can worsen hemodynamics.

Serial cardiac troponin measurements help identify an acute rise or fall consistent with myocardial injury; clinical status, ECG findings, and hemodynamics guide assessment of recovery. Cardiac troponin I and troponin T are the preferred biomarkers for detecting myocardial necrosis due to their high cardiac specificity and sensitivity. High-sensitivity cardiac troponin may become elevated within 1 to 3 hours of acute myocardial injury and can remain elevated for several days, sometimes lasting up to 10 to 14 days. Creatine kinase-MB generally returns to baseline within 48 to 72 hours, but suspected reinfarction is primarily assessed with symptoms, ECG changes, and serial cardiac troponin changes; CK-MB may be an adjunct if ordered.