Myocardial Infarction (MI)  

Definition 

A myocardial infarction is the necrosis of cardiac myocytes due to prolonged ischaemia, usually caused by occlusion of a coronary artery. It is a type of severe acute coronary syndrome (ACS). 

Aetiology / Risk Factors 

Most common cause:  
Atherosclerotic plaque rupture → thrombus formation → coronary artery occlusion. 

Other causes: Coronary artery spasm, embolism, dissection, cocaine/stimulant use. 

Risk factors:  

– Modifiable:  Smoking, hypertension, hyperlipidemia, diabetes, obesity, sedentary lifestyle.  
– Non-modifiable: Age, male sex, family history. 

Pathophysiology 

Plaque rupture → platelet aggregation → thrombus formation. 
Coronary occlusion → ischaemia. 
Cellular changes: reversible injury → necrosis → inflammation → fibrosis. 

Most commonly affected arteries:  

– LAD: anterior wall, septum  
– RCA: inferior wall, right ventricle  
– LCX: lateral wall 

Clinical Features 

Typical symptoms:  

Central crushing chest pain, radiating to left arm/neck/jaw/back, diaphoresis, nausea, dyspnea. 

Atypical: Fatigue, syncope, epigastric pain (elderly, diabetics, women). 

Signs: Tachy/bradycardia, hypotension, S4, pulmonary rales. 

Types of MI 

STEMI: Full-thickness (transmural) 
NSTEMI: Partial-thickness (subendocardial) 

Investigations 

ECG:  
– STEMI: ST elevation  
– NSTEMI: ST depression/T-wave inversion  
– Q waves may develop later (sign of late recognition of MI if seen on ECG) 

Cardiac biomarkers: 

 – Troponin I/T: sensitive, specific  
– CK-MB: rises 3–6h, returns 48–72h  
– (Myoglobin: early, not specific) 

Imaging: Echocardiography, coronary angiography. 

Complications 

Early: Arrhythmias, cardiogenic shock, acute heart failure, pericarditis 
Late: Ventricular aneurysm/rupture, mitral regurgitation, heart failure 

Management 
Acute (MONA-B): Morphine, Oxygen, Nitrates, Aspirin, Beta-blockers 
Reperfusion: Percutaneous Coronary Intervention (p-PCI) (peform early in acute MI or non refractory unstable Angina)) or fibrinolysis (now less common)
Adjunctive: Anticoagulation, dual antiplatelets, statins, ACE inhibitors/ARBs 
Long-term: Lifestyle modification, cardiac rehab, comorbidity control