Myocardial infarction (MI)

Epidemiology:

• Commonest cause of death in the UK
• Incidence of 5/1000 per year
• 50% survive the acute event. Of these, a further 10% die in hospital and up to 20% more die in the first 2 years
• 50% of initial survivors are alive at 10 years

Clinical features:

• Severe central chest pain, often described as crushing in nature
• Pain may radiate to the jaw and left arm
• Onset is usually sudden and persists fairly constantly for several hours (often until diamorphine is given)
• Up to 20% of patients experience no pain – so called ‘silent MI’. This is commonly seen in diabetics and the elderly

Other symptoms/signs:

• Sweating
• Breathlessness
• Nausea/vomiting
• Pallor

Diagnosis:

Requires at least 2 of the following:

• A history of ischaemic-type chest pain
• Evolving ECG changes
• A rise and fall in cardiac enzymes

Investigations – ECG:

• Abnormal Q waves (usually permanent)
• Inverted T waves
• Elevated ST segments

Investigations – cardiac enzymes:

Creatinine kinase (CK):

• Peaks within 24 hours and is usually back to normal within 48 hours
• Produced by damaged:
• Cardiac muscle
• Skeletal muscle
• Brain
• Cardiac-specific isoforms (CK-MB) can be measured, allowing greater diagnostic accuracy
• The size of the enzyme rise is broadly proportional to the infarct size

Cardiac-specific troponins:

• Troponin T and I have a very high specificity for cardiac injury
• Released early (2-4 hours) and can persist for up to 7 days

Lactate dehydrogenase (LDH):

• Non-specific
• Rarely used

Acute management of an MI:

• Immediate ECG
• If diagnostic ST elevation is present, thrombolysis should be commenced without delay
• Aspirin given (150mg chewed)
• Adequate analgesia (e.g. diamorphine 5-10mg IV)
• Antiemetics (e.g. cyclizine 50mg IV or metoclopramide 10mg IV) should be given if required
• Thrombolytic therapy can achieve reperfusion in 50-70% of patients
• Following initiation of thrombolysis, the patient should be transferred to the CCU
• An IV β-blocker (e.g. metoprolol 5-10mg) should be given, especially if the HR is greater than 100bpm
• 60% oxygen is given routinely by a face mask or nasal cannula

Criteria for thrombosis in acute MI:

Indications:

• Chest pain consistent with MI, within 12 hours

AND

• ST segment elevation (>1mm in 2 or more contiguous leads) or new LBBB

Contraindications:

• Stroke or active bleeding in last 2 months
• Systolic BP >200mmHg
• Proliferative diabetic retinopathy
• Pregnancy

Relative contraindications:

• Prolonged or traumatic resuscitation
• Recent (>2 weeks) surgery or trauma
• Current use of anticoagulants

Thrombolysis:

• Streptokinase (1.5 million U over 1 hour) is the agent used most commonly
• Tissue plasminogen activator (t-PA) achieves higher reperfusion rates but is more expensive and is associated with a higher risk of stroke
• t-PA tends to be given in preference to patients:
• under 50 years of age with anterior MIs
• with a low BP (systolic <100mmHg)
• who have previously received streptokinase
• t-PA must be followed by IV heparin

Risks of thrombolysis:

• 1% risk of stroke
• 0.7% risk of a major haemorrhage
• 2% risk of an allergic reaction with streptokinase

Subsequent management in hospital:

• Monitor the patient in the CCU for 48 hours
• Aspirin 150mg daily is given unless contraindicated
• Β-blockers unless contraindicated (e.g. COPD)
• Patients with clinical evidence of pulmonary oedema or reduced ejection fraction on the echocardiogram should be put on an ACEI
• Gradual mobilization is commenced on the second day
• The patient should be pain-free and fully ambulant before discharge (6 days in an uncomplicated case)
• Patients should be advised that they cannot drive for 1 month

Follow-up:

• Most will be fully recovered at 2 months and will be able to return to work
• Most patients should be reviewed as outpatients at 6-8 weeks
• Aspirin should be continued indefinitely
• Lipid-lowering therapy should be considered if the fasting lipid profile is unfavourable
• ACEIs should be continued indefinitely in those with persistent impairment of LV function (i.e. an ejection fraction of <40%)

Complications:

Acute (2-3 days post-infarct):

• Cardiac arrhythmias
• Cardiac failure
• Pericarditis

Late:

• Recurrent infarction
• Angina
• Thromboembolism
• Ventricular aneurysm
• Mitral valve regurgitation

Ventricular extrasystoles:

• Commonly occur after MI
• May precede the development of VF, particularly if they are:
• Frequent (>5 per minute)
• Multiform (different shapes)
• R-on-T

Ventricular tachycardia:

• May degenerate into VF
• May itself produce serious haemodynamic consequences
• Can be treated with:
• IV lignocaine
• Synchronised cardioversion (initially 200J)

Ventricular fibrillation:

• May occur in the first few hours or days following an MI
• Treated with prompt defibrillation (200-360J)
• Recurrences of VF can be treated with lignocaine infusion or, in cases of poor LV function, amiodarone
• When VF occurs in the setting of HF, shock or aneurysm (so-called ‘secondary VF’), the prognosis is very poor unless the underlying haemodynamic or mechanical cause can be corrected
• It is prudent to ensure the serum K⁺ is above 4.5mmol/L

Atrial fibrillation:

• Occurs in ~10% of patients with MI
• May be managed with IV digoxin or IV amiodarone and by treatment of the underlying pathology
• It is not usually a long-standing problem

Sinus bradycardia:

• Especially associated with acute inferior wall MI
• When symptomatic, treatment consists of:
• Elevating the foot of the bed
• IV atropine 600μg

Prognosis:

• Variable
• Approximately 25% of patients surviving the initial heart attack die in the first 2 years
• 5-year mortality approaches 30%