Myocardial Infarction(MI)
Ischaemia vs infarction:
Myocardial ischaemia = myocardial O2 supply < myocardial O2 demand
Myocardial infarction = ischaemic necrosis
Rare causes of MI:
Coronary Artery Disease (CAD):
- vasospasm (drug-induced (cocaine, triptans))
- in-situ-thromobosis (prothrombotic states)
- thromboemolism (I.E., cardiac catheterisation)
- vasculitis (SLE, PAN, Takayasu's)
- coronary wall thickening (amyloidosis, Hurler’s, DXR)
- coronary dissection (aortic dissection)
- anatomical anomaly (aneurysm (Kawasaki’s), congenital)
Haemodynamic:
- decreased ABP (shock, aortic stenosis, tricuspid regurgutation)
- HR affected (anaemia, thyroid disease)
Diagnosis of MI:
Triad of:
1. Typical clinical features
2. ECG changes (80 % STEMI / 20 % NSTEMI)
3. Cardiac enzymes:
- Troponin I (<0.2 = negative / >1.0 = MI)
- Troponin T (similar)
- CK-MB
- AST
- LDH
Clinical Features of MI:
History:
- chest pain (severe - but can be 'silent' in elderly diabetics, onset over mins, duration > 20 mins (and usually > 60 mins), NOT totally relieved by nitrates, Levine’s sign: patient illustrates pain with a 'clenched fist')
- associated syptoms (sweating, nausea & vomiting, shortness of breath, syncope)
- risk factors (atherosclerosis, ?oral contraceptive pill in young females)
Examination:
- Cardiogenic shock (cold, clammy extremities, decreased ABP, oliguria)
- Complications (arrhythmias: bradycardias / tachycardias, acute LVF: S3 / bibasal creps, acute RVF: raised JVP, acute mitral regurgitation: murmur)
- CVS risk factors (peripheral signs: cigarette (nicotine) stains, xanthelasma, arcus senilis)
Complications of MI:
All times:
- re-infarction
Immediate:
- arrhythmias (VT, VF --> death)
- complete AV block (bradycardia)
- acute LVF pulmonary oedema (cardiogenic shock)
- pericarditis
Early (1-10days):
- arrhythmias (VT)
- rupture of papillary muscle (MR - acute LVF)
- rupture of septum (VSD - acute LVF)
- rupture of LV (tamponade - cardiac arrest (PEA))
- thromboembolism (CVA, ALI, acute gut ischaemia)
Late (>6 weeks):
- Dressler’s syndrome: autoimmune reaction to necrosed myocardium, symptoms include pleuropericarditis & persistent pyrexia, treat with NSAIDs
- LV aneurysm (rupture, arrhythmias, thromboembolism, CCF)
- CCF
Notes:
- inferior MIs: 3° HB is common (damage to AV node)
- anterior MIs: 3° HB has poor prognosis (extensive damage involving conducting system)
- consider pacing if ABP unstable (but ?perforation / arrhythmias)
