Cardiac Emergencies

Acute Coronary Syndromes and Myocardial Infarction

Acute coronary syndrome encompasses a spectrum of clinical conditions resulting from acute myocardial ischemia: unstable angina, non-ST-elevation myocardial infarction, and ST-elevation myocardial infarction. Unstable angina presents with cardiac chest pain that occurs at rest or accelerates in severity, notably without elevated cardiac biomarkers. In contrast, myocardial infarction involves measurable myocardial cell injury. Serum troponin is the most specific and sensitive cardiac biomarker for detecting myocardial cell injury. With conventional laboratory assays, troponin typically rises within 3 to 6 hours following injury, though high-sensitivity assays may detect this rise earlier. Depending on the specific type of troponin measured, levels can remain elevated for up to 14 days.

Prompt identification of acute coronary syndrome is critical. A 12-lead electrocardiogram must be obtained and interpreted within 10 minutes of patient arrival for suspected acute coronary syndrome. STEMI is generally identified by new ST-segment elevation meeting lead-specific amplitude thresholds in at least two contiguous leads in a compatible clinical setting; recognized STEMI-equivalent ECG patterns can also warrant urgent reperfusion. Chewable aspirin must be administered as soon as possible unless absolute contraindications exist, such as a serious aspirin allergy or active bleeding. Sublingual nitroglycerin helps relieve ischemic chest pain primarily by reducing cardiac preload; it can also dilate coronary arteries, while substantial afterload reduction generally requires higher doses.

Nitroglycerin requires careful screening for contraindications. It must be withheld if the systolic blood pressure drops below 90 mm Hg or falls more than 30 mm Hg below the patient's baseline. Additionally, nitroglycerin is strictly contraindicated after recent phosphodiesterase-5 inhibitor use due to the danger of severe hypotension; clinicians must wait at least 24 hours following sildenafil or vardenafil use, and at least 48 hours following tadalafil use. If ischemic chest pain persists despite appropriate anti-ischemic treatment, morphine may be considered. However, morphine requires cautious use because it can cause hypotension and may delay the gastrointestinal absorption of oral antiplatelet medications.

Reperfusion therapy is the cornerstone of ST-elevation myocardial infarction care. Primary percutaneous coronary intervention is the preferred reperfusion strategy, with a target device activation time within 90 minutes of first medical contact at a PCI-capable hospital, or within 120 minutes when transfer to a PCI-capable facility is required. If primary percutaneous coronary intervention cannot be achieved within 120 minutes of first medical contact, eligible patients presenting within 12 hours of symptom onset should receive fibrinolytic therapy, provided no contraindications exist. Absolute contraindications to fibrinolytic therapy include active internal bleeding, a history of hemorrhagic stroke, and suspected aortic dissection. Notably, a right ventricular infarction can manifest with hypotension, clear lung fields, and elevated jugular venous pressure; in these hypotensive patients, nurses must avoid nitrates and diuretics and instead consider cautious intravenous fluid administration.