Non ST Elevation Myocardial Infarction

Acute coronary syndrome with myocardial necrosis (troponin rise and fall, with at least one value greater than the 99th percentile) without persistent ST elevation.

• Mechanism: plaque rupture or erosion with a non-occlusive or transient thrombus and distal microembolisation. Type 2 infarction results from supply and demand mismatch (anaemia, tachyarrhythmia, sepsis, hypotension, hypoxia) without plaque rupture.
• Accounts for about half to two thirds of acute coronary syndrome admissions. In hospital mortality is about 3 to 5%, and 1-year mortality about 10 to 15%, higher in the elderly, diabetic and renally impaired.

• Rest chest pain lasting more than 20 minutes, or crescendo pain, with radiation to arm, jaw or back, sweating, nausea and dyspnoea.
• Atypical presentations (isolated dyspnoea, epigastric pain, fatigue, syncope) are common in the elderly, women and diabetics.
• Examination is often normal. High risk signs: hypotension, new mitral regurgitation murmur, S3, basal crackles, tachyarrhythmia, cool peripheries.
• Differential diagnosis: unstable angina, pericarditis, myocarditis, pulmonary embolism, aortic dissection, Takotsubo syndrome, pneumothorax, oesophageal spasm, type 2 infarction, sepsis, acute heart failure.
• Complications: recurrent ischaemia, heart failure, cardiogenic shock, ventricular arrhythmia, mechanical complications, bleeding.

• ECG within 10 minutes, repeated at 15 to 30 minutes and with every recurrence of pain. Findings: ST depression of 0.5 mm or more in two contiguous leads, T-wave inversion, transient ST elevation. Record V7 to V9 when V1 to V3 show ST depression with a dominant R wave (posterior infarction). A normal ECG does not exclude infarction.
• High sensitivity troponin at 0 hours and 1 to 3 hours; conventional troponin at 0 and 3 to 6 hours. A rise or fall confirms myocardial injury. Persistent elevation without change suggests chronic injury (renal failure, heart failure).
• Also: full blood count, urea and electrolytes, glucose, HbA1c, lipids, chest X-ray, echocardiography (wall motion, ejection fraction, mechanical complications).
• Risk stratification: high risk is defined by a rising troponin, dynamic ST-T change, haemodynamic or electrical instability, refractory pain, or a high score on a validated risk tool.

• Aspirin 300 mg chewed once, then 75 to 100 mg daily for life.
• Second antiplatelet:
  • Ticagrelor 180 mg load then 90 mg twice daily for 12 months (preferred when an invasive strategy is planned).
  • Clopidogrel 300 to 600 mg load then 75 mg daily (patients on oral anticoagulants, high bleeding risk, or ticagrelor unavailable).
  • Prasugrel 60 mg load then 10 mg daily, only once anatomy is known (5 mg daily if under 60 kg or older than 75 years). Contraindicated after prior stroke or transient ischaemic attack.
• Anticoagulation:
  • Fondaparinux 2.5 mg SC daily (creatinine clearance greater than 20 mL/min); give an additional unfractionated heparin bolus at PCI, or
  • Enoxaparin 1 mg/kg SC 12-hourly (1 mg/kg once daily if creatinine clearance is less than 30 mL/min), or
  • Unfractionated heparin 60 units/kg IV bolus (maximum 4000 units) then 12 units/kg/h (maximum 1000 units/h), aPTT 1.5 to 2.5 times control. At PCI give 70 to 100 units/kg.
• Anti-ischaemic:
  • Glyceryl trinitrate 0.4 mg sublingual every 5 minutes up to 3 doses, then IV 5 to 10 micrograms/min titrated to 200 micrograms/min for ongoing pain, hypertension or pulmonary oedema. Avoid with systolic BP less than 90 mmHg, right ventricular infarction, or phosphodiesterase-5 inhibitor use within 24 to 48 hours.
  • Metoprolol 25 to 50 mg orally 6 to 12-hourly, or bisoprolol 2.5 to 10 mg daily, started within 24 hours when there is no acute heart failure, bradycardia, heart block, shock or severe asthma. Avoid early IV beta blockade.
  • Morphine 2 to 5 mg IV only for refractory pain (it delays oral antiplatelet absorption).
  • Oxygen only if SpO2 is less than 90%.
• Atorvastatin 80 mg daily from admission. Target LDL cholesterol less than 1.4 mmol/L and a fall of at least 50%; add ezetimibe 10 mg daily, then a PCSK9 inhibitor if not met.
• Ramipril 2.5 mg daily titrated to 10 mg for ejection fraction of 40% or less, diabetes, hypertension or chronic kidney disease. Use an ARB if cough develops. Check creatinine and potassium at 1 to 2 weeks.
• Eplerenone 25 mg daily titrated to 50 mg for ejection fraction of 40% or less with heart failure or diabetes, if potassium is less than 5.0 mmol/L and eGFR is greater than 30.
• Timing of angiography:
  • Immediate (within 2 hours): refractory pain, haemodynamic or electrical instability, acute heart failure, mechanical complication.
  • Within 24 hours: a high risk score, dynamic ST-T change, troponin rise consistent with infarction.
  • Within 72 hours: diabetes, eGFR less than 60, ejection fraction less than 40%, early post-infarct angina, prior PCI or bypass.
  • Low risk patients: non-invasive ischaemia testing.
• Revascularisation: PCI (radial access preferred) for single or two vessel disease; bypass surgery for left main or three vessel disease, especially with diabetes or reduced ejection fraction.
• Where no catheter laboratory is available: aspirin, a P2Y12 inhibitor, fondaparinux or enoxaparin, beta blocker and statin, with transfer within 24 to 72 hours, or immediately if unstable.
• Dual antiplatelet therapy for 12 months; 1 to 3 months followed by a single P2Y12 inhibitor in high bleeding risk. Add omeprazole 20 mg daily for gastric protection. With an oral anticoagulant, use clopidogrel plus the anticoagulant, with aspirin limited to the peri-PCI period.
• Special groups: elderly (reduce heparin and prasugrel doses, favour radial access); renal impairment (avoid fondaparinux if clearance is less than 20 mL/min, hydrate and limit contrast); pregnancy (consider spontaneous coronary artery dissection; avoid statins and ACE inhibitors).
• Red flags: refractory pain, hypotension, sustained ventricular arrhythmia, new murmur, pulmonary oedema. Transfer urgently to a catheter capable centre.
• Follow up: cardiac rehabilitation, review at 2 to 4 weeks and 3 months, echocardiography at 6 to 12 weeks. Consider a defibrillator if ejection fraction remains 35% or less at 40 days or more after infarction on optimal therapy.