5 Myocardial Infarction and Acute Coronary Syndromes

Understanding acute coronary syndromes

Acute coronary syndrome (ACS) is a group of urgent conditions caused by a sudden reduction in blood flow to the heart muscle. It includes unstable angina, non–ST-elevation myocardial infarction (NSTEMI), and ST-elevation myocardial infarction (STEMI). A common cause is a coronary-artery plaque becoming disrupted and a blood clot forming over it.

A myocardial infarction (MI) occurs when heart muscle is injured or dies because of inadequate blood flow. Unstable angina causes acute ischemia but, unlike MI, does not produce the characteristic rise or fall in cardiac troponin that indicates heart-muscle injury. STEMI and NSTEMI are distinguished primarily by the ECG; both are heart attacks and require urgent assessment.

Recognizing symptoms and responding

Typical symptoms include chest pressure, tightness, squeezing, fullness, or pain that lasts several minutes or comes and goes. Discomfort may spread to one or both arms, the back, neck, jaw, or upper abdomen. Shortness of breath, sweating, nausea, unusual fatigue, or lightheadedness can occur with or without chest discomfort.

Symptoms may be less typical or subtle, particularly in older adults, people with diabetes, and women.

Treat possible heart-attack symptoms as an emergency. Call 911; do not drive yourself or wait to see if symptoms pass. Emergency medical services can begin care and coordinate urgent hospital treatment. If the person becomes unresponsive and is not breathing normally, call 911, start CPR if able, and use an AED if available.

Initial assessment and diagnosis

For healthcare professionals, begin by assessing airway, breathing, circulation, vital signs, oxygen saturation, and signs of instability. Place the patient on cardiac monitoring, obtain IV access as appropriate, and promptly obtain a 12-lead ECG—ideally within 10 minutes of arrival.

Repeat ECGs if symptoms continue, recur, or change. An initially nondiagnostic tracing does not rule out ACS. Do not wait for a troponin result before acting on a diagnostic STEMI ECG.

Measure cardiac troponin, preferably with a high-sensitivity assay, and repeat it according to the validated local testing pathway. A rise or fall in troponin supports acute injury; diagnosis of MI also requires evidence of ischemia, such as compatible symptoms, ECG changes, or imaging findings. Consider other dangerous causes of chest pain as clinically appropriate.

Initial treatment

Initial treatment depends on the clinical picture, ECG, contraindications, and local emergency protocols.

• Aspirin: Give promptly when ACS is suspected unless contraindicated. Emergency personnel or a dispatcher can advise on its use. It must not delay the 911 call or transport.

• Oxygen: Use when the patient is hypoxemic or in respiratory distress. Routine oxygen is not needed for everyone with normal oxygen levels.

• Nitrates: These may relieve ongoing ischemic chest pain when appropriate. Avoid them or use particular caution with low blood pressure, suspected right-ventricular infarction, or recent use of certain erectile-dysfunction medicines.

• Antiplatelet and anticoagulant medicines: Hospital clinicians select these according to the ACS type, planned procedures, bleeding risk, and other patient factors.

Reperfusion and invasive assessment

For STEMI, blood flow must be restored urgently. Primary percutaneous coronary intervention (PCI) is preferred when it can be delivered promptly. If timely PCI is not feasible, eligible patients may receive fibrinolytic treatment, followed by transfer for further care. Fibrinolysis is not a treatment for NSTEMI or unstable angina.

Care for NSTE-ACS is risk-based. Ongoing ischemia, instability, or other high-risk features can require urgent invasive assessment.

Complications and emergency escalation

ACS can lead to dangerous arrhythmias, acute heart failure, cardiogenic shock, or cardiac arrest. Hypotension, altered mental status, cool or clammy skin, severe breathlessness, or loss of consciousness are emergency signs requiring immediate escalation and resuscitation according to protocol.

Care after the acute phase

After the acute phase, care commonly includes individualized antiplatelet and lipid-lowering therapy, management of cardiovascular risk factors, and cardiac rehabilitation.

The 2025 U.S. ACS guideline recommends dual antiplatelet therapy as the default for at least 12 months for many patients who are not at high bleeding risk. Clinicians adjust treatment for bleeding risk and individual circumstances.