Posterior Myocardial Infarction

Posterior myocardial infarction is infarction of the posterior (basal inferolateral) wall of the left ventricle, usually from occlusion of the left circumflex artery or of the posterolateral branch of a dominant right coronary artery. It is a ST-elevation infarct equivalent that is easily missed because a routine 12-lead ECG shows only reciprocal changes.

Isolated posterior infarction accounts for about 3 to 11% of infarcts, and it more often accompanies inferior or lateral infarction. It is under recognised, so reperfusion is often delayed.

Complications are posteromedial papillary muscle rupture (single blood supply), mitral regurgitation, atrioventricular block (right coronary artery), heart failure, ventricular arrhythmia and, in extensive disease, cardiogenic shock. Outcome is worse when reperfusion is delayed.

Pathophysiology.

The posterior wall faces away from the anterior chest leads. Injury current from the posterior wall is recorded in V1 to V3 as a mirror image: ST depression, tall broad R waves (loss of posterior forces seen from the front), and upright T waves.

The left circumflex artery may be electrically silent on a 12-lead ECG, so posterior leads reveal the culprit. Larger posterior infarcts are often accompanied by right ventricular infarction when the right coronary artery is the culprit.

Chest pain, dyspnoea, nausea, sweating and syncope, often with no distinctive signs. A new mitral regurgitation murmur, bradycardia, heart block, hypotension or heart failure may be present. Right coronary involvement gives inferior and right ventricular features.

• ECG: ST depression of 0.5 mm or more in V1 to V3 (maximal in V1 to V3 with a horizontal pattern), tall wide R waves (duration over 40 ms, R to S ratio over 1 in V1), and upright T waves in V1 to V3.
• Record posterior leads V7 (posterior axillary line), V8 (tip of the scapula) and V9 (left paraspinal) at the same horizontal level as V6.
• ST elevation of 0.5 mm or more in any of V7 to V9 confirms the diagnosis. Inferior or lateral ST elevation may accompany it.
• Echocardiography: posterior or inferolateral hypokinesia, mitral regurgitation, papillary muscle dysfunction, right ventricular involvement.
• Troponin (high sensitivity assay, rising), but do not wait for the result to act on a diagnostic ECG.
• Emergency coronary angiography.

Differential diagnosis.

• Anterior subendocardial ischaemia and non-ST-elevation myocardial infarction.
• Right ventricular hypertrophy, right bundle branch block and Wolff-Parkinson-White syndrome (tall R in V1).
• Normal variant and old posterior infarction.
• Takotsubo syndrome, pulmonary embolism and hyperkalaemia.

Reperfusion. Primary percutaneous coronary intervention with first medical contact to wire passage within 120 minutes.

If this is not possible, give fibrinolysis within 10 minutes of diagnosis with tenecteplase, dosed by weight: 30 mg under 60 kg, 35 mg for 60 to under 70 kg, 40 mg for 70 to under 80 kg, 45 mg for 80 to under 90 kg, 50 mg for 90 kg or over. Use half dose from age 75.

Alteplase accelerated regimen: 15 mg bolus, then 0.75 mg/kg over 30 minutes (maximum 50 mg), then 0.5 mg/kg over 60 minutes (maximum 35 mg).

Absolute contraindications: previous intracranial haemorrhage, ischaemic stroke in 6 months, central nervous system tumour or vascular malformation, major trauma or surgery in 1 month, gastrointestinal bleeding in 1 month, known bleeding disorder, aortic dissection, and non-compressible puncture in 24 hours.

Rescue angioplasty if ST resolution is under 50% at 60 to 90 minutes. Routine angiography 2 to 24 hours after successful lysis. Judge ST resolution on posterior leads or the anterior ST depression.

Antithrombotic therapy.

• Aspirin 150 to 300 mg orally loading dose, then 75 to 100 mg daily.
• Ticagrelor 180 mg orally loading dose, then 90 mg twice daily for 12 months.
• Alternatively prasugrel 60 mg loading dose, then 10 mg daily (5 mg when weight is under 60 kg or age is 75 or over).
• Alternatively clopidogrel 600 mg loading dose, then 75 mg daily. With fibrinolysis, clopidogrel 300 mg (or 75 mg from age 75), then 75 mg daily.
• Unfractionated heparin 70 to 100 units/kg intravenous bolus in percutaneous coronary intervention (60 units/kg with a glycoprotein IIb/IIIa inhibitor).
• With fibrinolysis, give enoxaparin 30 mg intravenous bolus then 1 mg/kg subcutaneously every 12 hours (maximum 100 mg for the first two doses). Give it once daily when creatinine clearance is under 30 mL/min. From age 75, use 0.75 mg/kg every 12 hours without bolus.

Supportive treatment.

• Oxygen only for SpO2 under 90%.
• Glyceryl trinitrate 400 to 800 microgram sublingually, then 10 to 200 microgram/min intravenously for pain. Avoid it with systolic pressure under 90 mmHg or right ventricular involvement.
• Intravenous opioid for pain in small titrated doses (fentanyl 25 to 50 microgram), since opioids delay absorption of oral antiplatelets.
• Metoprolol 25 to 50 mg orally twice daily within 24 hours when there is no heart failure, hypotension or block.
• Bradycardia and block (right coronary lesions): atropine 0.5 mg intravenously, repeated to 3 mg, then temporary pacing if needed.
• Mitral regurgitation from papillary muscle rupture: urgent surgery with afterload reduction and balloon counterpulsation as a bridge.

Secondary prevention.

• Atorvastatin 40 to 80 mg once daily (LDL cholesterol under 1.4 mmol/L).
• Ramipril 2.5 mg daily titrated to 10 mg.
• Metoprolol succinate 25 to 200 mg daily.
• Eplerenone 25 to 50 mg daily when ejection fraction is 40% or under with heart failure or diabetes (potassium under 5.0 mmol/L).
• Omeprazole 20 mg daily while on dual antiplatelet therapy.
• Smoking cessation and cardiac rehabilitation.

Monitoring. Continuous ECG for 24 to 48 hours, troponin at presentation and 3 to 6 hours later, echocardiography at 24 to 48 hours, creatinine and potassium at 1 to 2 weeks after starting ACE inhibitor or eplerenone, lipids at 4 to 12 weeks.

Referral and escalation. Activate the catheter laboratory on the basis of the posterior lead ECG. Urgent surgical referral for new mitral regurgitation, heart failure or shock.

Special groups.

• Older adults: use half dose tenecteplase from age 75 and enoxaparin 0.75 mg/kg subcutaneously every 12 hours without a bolus; watch bleeding.
• Kidney impairment: give enoxaparin once daily when creatinine clearance is under 30 mL/min and avoid contrast excess.
• Pregnancy: spontaneous coronary artery dissection is the usual cause; avoid fibrinolysis where possible.
• Region specific: where angiography is delayed, record posterior leads early to avoid missing the diagnosis.