ST Elevation Myocardial Infarction

Complete thrombotic occlusion of an epicardial coronary artery producing transmural ischaemia, manifest as new ST elevation of 1 mm or more in two or more anatomically contiguous leads, or a new left bundle branch block with Sgarbossa criteria, or a true posterior infarction pattern.

Time is myocardium

Necrosis begins in the subendocardium within 20 to 30 minutes of occlusion and progresses transmurally as a wavefront over 6 to 12 hours. The quantity of myocardium salvaged, and therefore mortality and long term heart failure, is a direct function of the time to reperfusion. Every step in the pathway is designed to compress this interval.

Target intervals

• Electrocardiogram within 10 minutes of arrival.
• Primary percutaneous coronary intervention with the wire crossing the lesion within 90 minutes of first medical contact, and within 120 minutes if transfer to another facility is required.
• Fibrinolysis administered within 10 minutes of the diagnosis where percutaneous intervention cannot be delivered within 120 minutes.
• Transfer to the catheter laboratory within 30 minutes of arrival where primary intervention is the chosen strategy.

Choosing the strategy

• Symptom onset within 12 hours: consult an interventional cardiologist to determine whether the patient is for fibrinolysis or primary percutaneous intervention.
• Primary percutaneous coronary intervention is superior where it can be delivered within the time targets, giving lower mortality, lower reinfarction and lower intracranial haemorrhage.
• Fibrinolysis is chosen where percutaneous intervention is not available within 120 minutes, and is most effective within the first 3 hours where its benefit approaches that of intervention.
• Symptom onset beyond 12 hours: consult an interventional cardiologist. Fibrinolysis is generally not beneficial beyond 12 hours, but percutaneous intervention is indicated for ongoing ischaemic symptoms, haemodynamic or electrical instability, and evolving infarction.

As for acute coronary syndrome, with the additional recognition that ST elevation infarction is more often accompanied by haemodynamic and electrical complications.

Complications to anticipate

• Ventricular fibrillation, which is most likely in the first 4 hours and which is the reason the patient is attached to a defibrillator from the outset.
• Ventricular tachycardia and accelerated idioventricular rhythm, the latter being a common reperfusion rhythm that is usually benign and does not require treatment.
• Bradycardia and heart block, particularly in inferior infarction from atrioventricular nodal ischaemia, which is usually transient and responsive to atropine. Complete heart block in anterior infarction indicates extensive septal necrosis and carries a poor prognosis.
• Cardiogenic shock, occurring in 5 to 10 percent, with hypotension, oliguria, cool peripheries and pulmonary congestion.
• Right ventricular infarction with hypotension, raised jugular venous pressure and clear lung fields, which is preload dependent and requires fluid rather than nitrates or diuretics.
• Acute mitral regurgitation from papillary muscle rupture, with a new murmur and acute pulmonary oedema.
• Ventricular septal rupture with a new harsh pansystolic murmur and a thrill.
• Free wall rupture with tamponade and electromechanical dissociation.
• Pericarditis and, later, Dressler syndrome.
• Mural thrombus and systemic embolism.

• Twelve lead electrocardiogram within 10 minutes, with additional leads as described.
• Attach the patient to a defibrillator immediately and keep them attached.
• Establish intravenous access, preferably in the left forearm or antecubital vein, which preserves the right radial artery for arterial access at intervention and avoids sites that would complicate a femoral or radial approach.
• Send blood for urea and electrolytes and high sensitivity troponin, and for a full blood count, coagulation screen, glucose, lipids and renal function.
• Do not wait for troponin. Reperfusion is decided on the electrocardiogram and the history.
• Confirm the time of symptom onset explicitly and record it, since this determines the strategy.
• Complete the fibrinolysis checklist where fibrinolysis is contemplated.

Immediate measures

• Attach to a defibrillator.
• Intravenous access in the left forearm or antecubital fossa, with bloods sent.
• Aspirin 300 mg to chew.
• Nitroglycerin sublingual spray 0.4 mg every 5 minutes to a maximum of three doses, with the contraindications above, particularly right ventricular infarction.
• Fentanyl 25 to 50 micrograms intravenously if pain persists after three doses of nitroglycerin, repeated once after 5 minutes. Consult a cardiologist or physician for persistent pain and consider an intravenous nitroglycerin infusion.
• Continuous monitoring and preparation for defibrillation.

Primary percutaneous coronary intervention pathway

• Obtain informed consent.
• Give a P2Y12 inhibitor:
• Ticagrelor 180 mg orally, or
• Prasugrel 60 mg orally, which is contraindicated in previous stroke or transient ischaemic attack, and used with caution in patients aged 75 or above and those weighing under 60 kg, or
• Clopidogrel 600 mg orally where the newer agents are unavailable or contraindicated.
• Give enoxaparin 0.5 mg/kg as an intravenous bolus, or unfractionated heparin according to local protocol.
• Transfer to the catheter laboratory, connected to a defibrillator, within 30 minutes of arrival in the emergency department.
• Continue monitoring throughout transfer with a clinician capable of managing arrest.

Fibrinolysis pathway

Review the checklist before giving any agent.

Absolute contraindications:

• Any prior intracranial haemorrhage.
• Known structural cerebral vascular lesion such as an arteriovenous malformation.
• Known malignant intracranial neoplasm, primary or metastatic.
• Ischaemic stroke within 6 months, except an acute ischaemic stroke within the preceding 4.5 hours.
• Suspected aortic dissection.
• Active bleeding or a bleeding diathesis, excluding menstruation.
• Significant closed head or facial trauma within 1 month.
• Intracranial or intraspinal surgery within 1 month.
• Severe uncontrolled hypertension unresponsive to emergency treatment.
• Previous treatment with streptokinase within the preceding 6 months, where streptokinase is the intended agent.
• Non compressible vascular puncture within the preceding 24 hours.

Relative contraindications:

• Severe hypertension on presentation with a systolic pressure above 180 mmHg or a diastolic above 110 mmHg, which should be lowered before fibrinolysis.
• History of transient ischaemic attack within the preceding 6 months.
• Traumatic or prolonged cardiopulmonary resuscitation exceeding 10 minutes.
• Major surgery within 3 weeks.
• Recent internal bleeding within 2 to 4 weeks.
• Pregnancy or within 1 week postpartum.
• Active peptic ulcer.
• Oral anticoagulant therapy.

Adjunctive therapy with fibrinolysis

• Clopidogrel 300 mg orally, reduced to 75 mg in patients over 75 years, since a loading dose in the elderly increases intracranial haemorrhage. Ticagrelor and prasugrel are not used with fibrinolysis.
• Enoxaparin:
• Age under 75 years: 30 mg intravenous bolus, followed 15 minutes later by 1 mg/kg subcutaneously twice daily, with a maximum of 100 mg for each of the first two doses.
• Age 75 years or above: no bolus, then 0.75 mg/kg subcutaneously twice daily, with a maximum of 75 mg for each of the first two doses.
• Creatinine clearance below 30 mL/min at any age: 1 mg/kg subcutaneously once daily.

Administering fibrinolysis

• Obtain informed consent, explaining the benefit and the approximately 1 percent risk of intracranial haemorrhage.
• Ensure the patient is connected to a defibrillator with electrocardiographic, saturation and blood pressure monitoring, and repeat baseline observations.
• Administer within 10 minutes of the diagnosis.

Agents:

• Tenecteplase, which is fibrin specific, non antigenic and given as a single weight based bolus, making it the most practical agent. Reconstitute the 50 mg vial in 10 mL of sterile water to give 5 mg/mL, and give as an intravenous bolus: under 60 kg, 30 mg or 6 mL; 60 to 69 kg, 35 mg or 7 mL; 70 to 79 kg, 40 mg or 8 mL; 80 to 89 kg, 45 mg or 9 mL; 90 kg or above, 50 mg or 10 mL. Halve the dose in patients aged 75 or above, which substantially reduces intracranial haemorrhage without loss of efficacy. Ninety minute patency of around 85 percent.
• Reteplase, fibrin specific, given as two boluses of 10 units each, 30 minutes apart. Patency around 84 percent.
• Alteplase, fibrin specific, given as a 15 mg intravenous bolus, then 0.75 mg/kg over 30 minutes to a maximum of 50 mg, then 0.5 mg/kg over the next 60 minutes to a maximum of 35 mg, with a total not exceeding 100 mg. Patency 73 to 84 percent.
• Streptokinase, which is not fibrin specific and is antigenic, given as 1.5 million units in 50 mL of 5 percent dextrose intravenously over 30 to 60 minutes through a dedicated second line, with Ringer lactate running in another line. Patency 60 to 68 percent. It causes hypotension during infusion, which is managed by slowing or pausing the infusion and giving fluid. Allergic reactions frequently force termination before a therapeutic dose is delivered. It is absolutely contraindicated within 6 months of previous exposure because of antibody mediated inactivation and the risk of serious allergic reaction.

Monitoring during and after fibrinolysis

• Vital signs including blood pressure, pulse, respiratory rate and oxygen saturation every 15 minutes during the infusion.
• Continue monitoring for at least 30 minutes after the end of the infusion.
• Repeat the electrocardiogram at 60 to 90 minutes.
• Signs of successful reperfusion: resolution of chest pain, resolution of at least 50 percent of the ST elevation in the lead with maximal elevation, and reperfusion arrhythmias, particularly accelerated idioventricular rhythm.
• Failed reperfusion, indicated by persistent pain and less than 50 percent ST resolution at 60 to 90 minutes, requires immediate rescue percutaneous coronary intervention and transfer.
• All patients receiving fibrinolysis should undergo coronary angiography within 3 to 24 hours regardless of apparent success.
• Watch for intracranial haemorrhage, indicated by headache, altered consciousness, seizure, vomiting or a new neurological deficit. Stop all antithrombotic therapy immediately, obtain urgent computed tomography of the head, reverse with cryoprecipitate, fresh frozen plasma, platelets and tranexamic acid, and involve neurosurgery.

Transfer and ongoing care

Transfer to a coronary care or intensive care unit connected to a defibrillator, with a clinician capable of managing arrhythmia and arrest during transfer.

Additional therapy in the first 24 hours

• Beta blockade orally within 24 hours in patients without heart failure, hypotension, bradycardia, heart block, asthma or cocaine associated infarction. Avoid intravenous beta blockade in the acutely unwell, since it increases cardiogenic shock.
• High intensity statin, such as atorvastatin 80 mg daily, started before discharge regardless of the baseline lipid level.
• Angiotensin converting enzyme inhibitor within 24 hours in anterior infarction, heart failure, left ventricular ejection fraction below 40 percent and diabetes, once the blood pressure permits.
• Mineralocorticoid receptor antagonist in patients with an ejection fraction below 40 percent with heart failure or diabetes, provided renal function and potassium allow.
• Echocardiography to assess ventricular function and to identify mechanical complications.
• Glycaemic control, potassium and magnesium correction, and management of heart failure.

Management of specific complications

• Right ventricular infarction: give fluid boluses of 250 to 500 mL to increase preload, avoid nitrates, diuretics and morphine, and consider inotropic support with dobutamine if fluid fails.
• Cardiogenic shock: urgent revascularisation, which is the only intervention shown to improve survival, with inotropic and mechanical circulatory support as a bridge.
• Bradycardia and heart block: atropine 0.5 mg intravenously repeated to a maximum of 3 mg, then transcutaneous or transvenous pacing.
• Ventricular fibrillation: immediate defibrillation.
• Mechanical complications: urgent echocardiography and immediate cardiothoracic surgical referral.

Secondary prevention before discharge

Dual antiplatelet therapy for 12 months, high intensity statin, beta blocker, angiotensin converting enzyme inhibitor, smoking cessation with pharmacological support, cardiac rehabilitation referral, dietary and exercise advice, glycaemic and blood pressure control, and screening for depression, which is common after infarction and independently worsens outcome.