Myocardial infarction

Myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow decreases or stops in one of the coronary arteries of the heart, causing infarction (tissue death) of the heart muscle. This typically arises from the rupture of an atherosclerotic plaque, leading to thrombus formation and arterial blockage.

Signs and Symptoms

Chest pain is the most typical symptom, often described as a sensation of tightness, pressure, or squeezing, and may radiate to the left shoulder, arm, jaw, or back. Other symptoms include shortness of breath, nausea, dizziness, cold sweat, and a feeling of tiredness. Women often present atypically with neck pain, arm pain, or fatigue rather than chest pain. Approximately 30% of people exhibit atypical symptoms, and about 5% of those over 75 years old may have had an MI with little or no history of symptoms. Silent MIs, occurring without any symptoms, are more common in the elderly, diabetics, and post-heart transplant patients.

Areas where pain is experienced in myocardial infarction, showing common (dark red) and less common (light red) areas on the chest.

Areas where pain is experienced in myocardial infarction, showing common (dark red) and less common (light red) areas on the back.

Diagnosis

Diagnosis involves a combination of clinical evaluation, ECG, and biomarkers. An ECG is central in diagnosing MI, showing abnormalities like ST elevation, T wave inversion, or new Q waves. Elevated cardiac biomarkers, particularly troponins, are highly specific and indicate myocardial damage. Echocardiography and other imaging techniques like coronary angiography can further assess and confirm the diagnosis.

Diagram showing the blood supply to the heart by the two major blood vessels, the left and right coronary arteries.

Risk Factors

Major risk factors for MI include high blood pressure, smoking, diabetes, lack of exercise, obesity, and high blood cholesterol. Genetics also play a role, with a family history of ischaemic heart disease increasing risk. Other factors include diet, stress, and certain medical conditions such as endometriosis in women under 40.

Mechanism

The primary mechanism is the rupture of an atherosclerotic plaque in a coronary artery, leading to thrombus formation and arterial occlusion. This blockage prevents oxygen-rich blood from reaching the heart muscle, resulting in tissue death. Other causes include coronary artery spasms and increased oxygen demand in a heart with limited blood supply.

Cross section showing anterior left ventricle wall infarction.

Treatment

Immediate treatment aims to restore blood flow and minimise heart damage. Aspirin is administered to reduce clot size, and nitroglycerin may be used for pain relief. In STEMI, primary percutaneous coronary intervention (PCI) is preferred if performed within 90-120 minutes; otherwise, thrombolysis is recommended. For NSTEMI, early PCI is considered within 1-3 days. Long-term management includes lifestyle modifications and medications like aspirin, beta blockers, ACE inhibitors, and statins to prevent recurrence.

Inserting a stent to widen the artery.

Prognosis

The prognosis varies depending on the extent of heart damage and the development of complications. Factors such as age, social isolation, and infarct location affect outcomes. Without treatment, the mortality rate is high, but early and effective treatment has significantly improved survival rates.

A 12-lead ECG showing an inferior STEMI due to reduced perfusion through the right coronary artery. Elevation of the ST segment can be seen in leads II, III and aVF.

Prevention

Prevention strategies include lifestyle modifications such as regular exercise, maintaining a healthy weight, a balanced diet, quitting smoking, and moderating alcohol intake. Medications like statins are used to manage cholesterol levels, and aspirin may be prescribed for those at high risk. Public health measures and patient education are essential in reducing the incidence of MI.

Self-assessment MCQs (single best answer)

What is the primary cause of a myocardial infarction (MI)?

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Which symptom is most commonly associated with myocardial infarction?

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In an ECG, which abnormality is indicative of a myocardial infarction?

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Which cardiac biomarker is highly specific for indicating myocardial damage?

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What is the preferred immediate treatment for STEMI if performed within 90-120 minutes?

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Which of the following is considered a major risk factor for myocardial infarction?

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Which medication is commonly administered to reduce clot size during a myocardial infarction?

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What is the primary goal of immediate treatment for myocardial infarction?

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Which demographic is more likely to experience a silent myocardial infarction?

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What lifestyle modification is NOT a recommended strategy for preventing myocardial infarction?

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