Acute Myocardial Infarction

Myocardial infarction is now considered part of a spectrum referred to as acute coronary syndromes, which refers to a range of acute myocardial ischaemia, that also includes unstable angina and non-ST segment elevation myocardial infarction. The new criteria for diagnosing myocardial infarction are an increase in biochemical markers of myocardial necrosis, with at least one of the following:

• Ischaemic symptoms
• Development of pathological Q waves on the ECG
• ECG changes indicative of ischaemia (ST segment elevation or depression)
• Coronary artery intervention (e.g. angioplasty)

• Coronary heart disease is the most common cause of death in the UK. CHD is responsible for the deaths of approximately one in five men, and one in six women.¹
• The average incidence of myocardial infarction for those aged between 30 and 69 years is about 600 per 100,000 for men, and 200 per 100,000 for women.¹
• A male predominance in incidence exists up to approximately age 70 years, when the sexes converge to equal incidence.
• Pre-menopausal women appear to be protected from atherosclerosis. Incidence increases with age and elderly people also tend to have higher rates of morbidity and mortality from their infarcts.

Risk Factors

• Non-modifiable risk factors for atherosclerosis: increasing age, male, family history of premature coronary heart disease, premature menopause
• Ethnic Group: In the UK, the highest recorded rates of coronary artery disease mortality are in people born in India, Pakistan and Bangladesh²
• Modifiable risk factors for atherosclerosis: smoking, diabetes mellitus (and impaired glucose tolerance), hypertension, raised LDL cholesterol, reduced HDL cholesterol, obesity, inactivity

• Chest pain: three quarters of patients present with characteristic central or epigastric chest pain radiating to the arms, shoulders, neck, or jaw. The pain is described as sub-sternal pressure, squeezing, aching, burning, or even sharp pain. Radiation to the left arm or neck is common. Chest pain may be associated with sweating, nausea, vomiting, dyspnoea, fatigue, or palpitations.
• Shortness of breath: may be the patient's anginal equivalent or a symptom of heart failure.
• Atypical presentations are common (especially women, older men, people with diabetes, and people from ethnic minorities) e.g. abdominal discomfort or jaw pain; elderly patients may present with altered mental state.

Signs

Examination findings can vary enormously:

• Low-grade fever, pale and with cool, clammy skin
• Hypotension or hypertension can be observed depending on the extent of the MI
• Dyskinetic cardiac impulse (in anterior wall MI) occasionally can be palpated
• Third and fourth heart sound, systolic murmur if mitral regurgitation or ventricular septal defect develop, pericardial rub
• May be signs of congestive heart failure, including pulmonary rales, peripheral oedema, elevated jugular venous pressure

Consider non-atherosclerotic causes in younger patients or if there is no evidence of atherosclerosis: coronary emboli from sources such as an infected cardiac valve, coronary occlusion secondary to vasculitis, coronary artery spasm, cocaine use, congenital coronary anomalies, coronary trauma, increased oxygen requirement (e.g. hyperthyroidism) or decreased oxygen delivery (e.g. severe anaemia)

• Cardiovascular: acute pericarditis, myocarditis, aortic stenosis, aortic dissection, pulmonary embolism
• Respiratory: pneumonia, pneumothorax
• Gastrointestinal: oesophageal spasm, oesophagitis, gastro-oesophageal reflux, acute gastritis, cholecystitis, pancreatitis
• Musculoskeletal chest pain

• If diagnosis is suspected arrange urgent admission immediately (999 Ambulance).
• ECG: may be helpful pre-hospital setting if the diagnosis is uncertain or in a remote area in the assessment for pre-hospital thrombolysis, but otherwise should not delay getting the patient to hospital.
  Features that increase the likelihood of infarction: new ST segment elevation; new Q waves; any ST segment elevation; new conduction defect. Other features of ischaemia are ST segment depression and T wave inversion.

In hospital

• Full blood count to rule out anaemia; leucocytosis is common; monitor potassium levels (electrolyte disturbances may cause arrhythmias, especially potassium and magnesium); renal function (eGFR) should be measured prior to starting an ACE inhibitor. Lipid profile needs to be obtained at presentation because levels can change after 12-24 hours of an acute illness. Measure CRP and other markers of inflammation.
• Cardiac enzymes: cardiac troponins T and I are highly sensitive and specific for cardiac damage. The risk of death from an acute coronary syndrome is directly related to troponin level and patients with no detectable troponins have a good short-term prognosis. Serum levels increase within 3-12 hours from the onset of chest pain, peak at 24-48 hours, and return to baseline over 5-14 days. Myocardial muscle creatine kinase (CK-MB) is found mainly in the heart. CK-MB levels increase within 3-12 hours of onset of chest pain, reach peak values within 24 hours, and return to baseline after 48-72 hours. Sensitivity and specificity are not as high as for troponin levels.
• Serial ECGs and continuous ECG monitoring in CCU.
• Chest x-ray: to assess patient's heart size and the presence or absence of heart failure and pulmonary oedema.
• Cardiac catheterisation and angiography: cardiac angiography defines the patient's coronary anatomy and the extent of the disease. Whether all patients with acute myocardial infarction should ideally undergo cardiac catheterization is controversial and present consensus is for angiography only if indicated by recurrent chest pain or significant ischaemia shown by exercise ECG or perfusion imaging. Patients with cardiogenic shock, intractable angina despite medications or severe pulmonary congestion should undergo cardiac catheterization and revascularization immediately.
• Echocardiography can define the extent of the infarction and assess overall ventricular function and can identify complications, such as acute mitral regurgitation, left ventricular rupture or pericardial effusion.
• Myocardial perfusion scintigraphy using SPECT: NICE recommends that myocardial perfusion scintigraphy using SPECT should be the first test used for:³
  • People where stress ECG may not give accurate or clear results, e.g. women, people who have certain unusual patterns in the electrical activity of their heart, people with diabetes or people for whom exercise is difficult or impossible.
  • The diagnosis of people who are less likely to have coronary artery disease and who are at lower risk of having heart problems in the future. The likelihood of a person having coronary artery disease can be assessed by considering a number of factors, e.g. age, sex, ethnic background and family history as well as the results of physical examination and investigations.
  • As an investigation in people who still have symptoms following a myocardial infarction or despite having had treatment to improve coronary artery blood flow.

See Acute Myocardial Infarction Management and Cardiac Rehabilitation

See Complications of Acute Myocardial Infarction.

• After a first myocardial infarction:¹
  • 23% of people die before reaching hospital
  • 13% die during hospital admission
  • 10% die within the first year following hospital discharge
  • 5% die each year thereafter (this persists indefinitely)
• Prognosis correlates with the degree of myocardial necrosis. Greater degrees of myocardial necrosis are associated with a worse prognosis. The degree of myocardial necrosis can be estimated by various factors, e.g.¹
  • The rise in serum troponin T
  • Degree and extent of ECG changes
  • Degree of left ventricular dysfunction on echocardiography
• Prognosis is worse in women, increasing age, increasing ventricular dysfunction, ventricular dysrhythmias and recurrent infarction. Other indicators of poorer prognosis are delay in reperfusion or unsuccessful reperfusion, anterior infarction, number of leads showing ST elevation, bundle branch block and systolic blood pressure less than 100 mm with tachycardia greater than 100 per minute.
• Better prognosis is associated with early reperfusion, inferior wall infarct, preserved LV function, short-term and long-term treatment with beta-blockers, aspirin, statins and ACE inhibitors.
• Elderly patients with acute MI are at increased risk of developing complications and should be treated aggressively.

See records on Primary cardiovascular disease prevention and secondary cardiovascular disease prevention.⁴