Myocardial Infarction

Question 1

what is the definition of myocardial infarction?

Answer 1

Necrosis of cardiac tissue (myocyte death) due to prolonged myocardial ischaemia due to COMPLETE occlusion of artery by thrombus. Can be STEMI or NSTEMI. STEMI is the most common medical emergency.

Question 2

what is the epidemiology of myocardial infarction?

Answer 2

• STEMI results in 5/1000 deaths per annum in the UK

- Worse prognosis in the elderly and those with left ventricular failure

Question 3

what is the aetiology of myocardial infarction?

Answer 3

Imbalance of oxygen supply and demand, most commonly caused by a plaque rupture with thrombus formation in a epicardial coronary artery resulting in acute reduction of blood supply to a portion of the myocardium

Question 4

what are the risk factors for myocardial infarction?

Answer 4

• Age
• Male
• History of premature coronary heart disease
• Premature menopause
• Diabetes mellitus
• Smoking
• Hypertension
• Hyperlipidaemia
• Obesity and sedentary lifestyle
• Diabetes mellitus
• Family history of Ischaemic Heart Disease (IHD) - MI in first degree relative below 55

Question 5

what is the pathophysiology of myocardial infarction?

Answer 5

• Rupture or erosion of vulnerable fibrous cap of coronary artery atheromatous plaque
• This results in platelet aggregation, adhesion, local thrombosis, vasoconstriction and DISTAL THROMBUS EMBOLISATION resulting in PROLONGED COMPLETE ARTERIAL OCCLUSION = myocardial necrosis within 15-30 minutes in a STEMI (since major artery occluded fully)
• STEMI:
  • The subendocardial myocardium is initially affected but, continued ischaemia, the infarct zone extends through the subepicardial myocardium, producing a transmural Q wave MI
  • Early reperfusion may salvage regions of the myocardium - reducing future mortality and morbidity

Question 6

what are the key presentations of myocardial infarction?

Answer 6

Any patient presenting with severe chest pain lasting more than 20 minutes may be suffering from an MI. Chest pain and breathlessness

Question 7

what are the signs of an MI?

Answer 7

Significant hypotension

Bradycardia or tachycardia

Question 8

what are the symptoms of an MI?

Answer 8

• Chest pain
  • Severe central ongoing pain, lasting more than 20 minutes
  • Pain may radiate to the left arm, jaw or neck
  • Pain DOES NOT usually respond to sublingual GTN spray - opiate analgesia is required
  • Pain described as substernal pressure, squeezing, aching, burning or even sharp pain
  • Associated with; sweating, nausea, vomiting, dyspnoea, fatigue and/or palpitations
• Breathlessness
• Fatigue
• Distress and anxiety
• Pale, clammy and marked sweating
• Significant hypotension (low BP)
• Bradycardia or tachycardia

Question 9

what are the first line and gold standard investigations for an MI?

Answer 9

ECG - STEMI = ST elevation in two consecutive leads, STEMI = ST elevation, tall T waves, L bundle branch block, T wave inversion after a few hours and pathological Q waves follow
NSTEMI - ST depression, T wave inversion
coronary angiography - acute occlusion or critical stenosis
cardiac troponin - elevated in both types of MI
glucose - normal or elevated
FBC
U and E - note eGFR
CRP - normal or elevated
serum lipids - normal or elevated
STEMI - diagnosed on presentation
NSTEMI - troponin test, troponin I or T increased

Question 10

what other tests may be done for an MI?

Answer 10

• Increased myoglobin,

- Transthoracic echocardiography may confirm MI due to wall abnormalities in early STEMI

Question 11

what are the differential diagnoses for an MI?

Answer 11

• Stable angina, unstable angina, NSTEMI, pneumonia, pneumothorax, oesophageal spasm, GORD, acute gastritis, pancreatitis and MSK chest pain

Question 12

how are MIs managed?

Answer 12

• Pre-hospital:
  • Aspirin 300mg chewable
  • GTN (sublingual)
  • Morphine

- Hospital: 
• IV morphine 
• Oxygen if their sats are below 95% or are breathless 
• Beta-blocker - Atenolol 
• P2Y12 inhibitor - Clopidogrel 

• Coronary revascularisation:
  • PCI: Presented to all patients who present with an acute STEMI who can be transferred to a primary PCI centre WITHIN 120 MINUTES of first medical contact. If not possible then give patient fibrinolysis and then transfer to PCI centre after infusion
  • CABG,
• Fibrinolysis - enhance the breakdown of occlusive thrombosis by the activation of plasminogen to form plasmin

- Risk factor modification: 
• Stop smoking 
• Lose weight and exercise daily 
• Healthy diet 
• Treat hypertension & diabetes 
• Low fat diet with statins 
- Secondary prevention: 
• Statins 
• Aspirin long term 
• Warfarin if large MI 
• o blockers 
• ACE inhibitors

Question 13

how are MIs monitored?

Answer 13

ECG constantly in hospital

Question 14

what are the complications of MIs?

Answer 14

• Sudden death - often within hours often due to ventricular fibrillation
• Arrhythmias - in the first few days due to electrical instability following infarction, pump failure and excessive sympathetic stimulation
• Persistent pain - 12 hours-few days after due to progressive myocardial necrosis
• Heart failure:
- When cardiac output is insufficient to meet the bodies metabolic demands
- Due to ventricular dysfunction following muscle necrosis also resulting in arrhythmias
• Mitral incompetence - can happen in the first few days or occur later. Due to myocardial scarring preventing valve closure
• Pericarditis - due to transmural infarct resulting in inflammation of pericardium, more common in STEMI
• Cardiac rupture:
- Early rupture - the result of shearing between mobile and immobile myocardium
- Late rupture - due to weakening of wall following muscle necrosis and acute inflammation
• Ventricular aneurysm - due to stretching of newly formed collagenous scar tissue

Question 15

what is the prognosis of an MI?

Answer 15

30% mortality rate, 50% of these prior to hospitalisation, 5-10% survivors die within a year