OVERVIEW
• Myocardial infarction (MI) refers to death of a portion of the heart tissue. In many cases, it is an acute event usually due to sudden rupture of an atherosclerotic plaque in the wall of a coronary vessel.

• This disruption of a pre-existent plaque with subsequent thrombus formation is termed acute coronary syndrome, or ACS. Disruption of the plaque results in narrowing/block of a coronary vessel leading to sudden imbalance in myocardial oxygen supply versus demand

• Acute myocardial infarction (AMI) (heart attack) is one of the leading causes of death in the developed nations. The global prevalence of the disease is nearly three million cases annually.

• Acute myocardial infarction can be divided into two categories namely non-ST-segment elevation MI (NSTEMI) and ST-segment elevation MI (STEMI). Unstable angina (UA) is similar to NSTEMI. However, cardiac markers are not elevated.

• In NSTEMI, there is partial occlusion of the coronary vessel with subendocardial (innermost heart layer) ischemia. In STEMI, there is total occlusion and ischemia extends across all the layers of the heart i.e. transmural. These infarctions are thus larger in size

PATHOPHYSIOLOGY OF AMI
Mechanisms leading to the development of an acute myocardial infarction include the following –
• Ruptured atherosclerotic plaque with thrombus formation
• Dynamic obstruction (spasm or constriction of coronary vessels) and mechanical blockage of vessel leading to reduced blood flow to the myocardium
• Associated coronary artery inflammation

DIAGNOSIS OF ACUTE MYOCARDIAL INFARCTION
Diagnosis of AMI is based on a combination of detailed history, physical examination, ECG changes and cardiac biomarkers measurement

HISTORY & EXAMINATION
• Severe chest pain or epigastric pain
• Central crushing type of chest pain
• Chest pain radiating to neck, shoulder, jaw or arms
• Feeling of tightness or pressure over chest with cramping and burning sensation
• Feeling of indigestion and epigastric pain
• Breathlessness
• Syncope
• Hypotension (low blood pressure)
• Excessive sweating (diaphoresis)

PAST MEDICAL HISTORY
• Previous history of CABG (coronary artery bypass graft), PCI (percutaneous coronary intervention), history of angina or MI
• History of taking GTN (glyceryl trinitrate) for pain relief
• Presence of risk factors such as smoking, hypertension, diabetes, hypercholesterolemia, family history, use of cocaine or methamphetamine

ECG CHANGES
ECG should be performed at presentation. For patients presenting to the emergency department with chest pain raising suspicion of acute coronary syndrome, the diagnosis of STEMI can be confirmed by the electrocardiogram. If initial ECG does not show diagnostic changes and patient remains symptomatic, repeat every 15 -30 minutes. Characteristic changes include

• Peaked T waves or hyperacute T waves
• ST elevation > 2 mm in two contiguous inferior leads indicate ST elevation MI (STEMI)
• Finding of pathological Q waves in leads with ST elevation
• ST depression and/or T wave inversion in NSTEMI

CARDIAC BIOMARKERS
• Cardiac biomarkers are particularly useful in diagnosing NSTEMI
• Elevated serum troponins (peak at 12 hours) and persist for 7 days
• Elevated CK-MB (peaks at 10 hours) and persists for 2-3 days (not usually used routinely clinically)

MANAGEMENT OF ACUTE MI – FIRST 12 HOURS
The patient should be rushed to the nearest well equipped secondary/tertiary care hospital by emergency ambulance
• For all AMIs (whether STEMI and NSTEMI) – Assess airway breathing and circulation
• Cardiopulmonary resuscitation and defibrillation if cardiac arrest
• Immediate chewable aspirin 160-325 mg
• Establish intravenous access
• Blood tests for full blood count, renal function and electrolytes, blood sugar, lipids, clotting screen, C-reactive protein (CRP)
• Oxygen supplementation if hypoxic (less than 91% saturation)
• Sublingual nitroglycerine along with opioids for pain relief if blood pressure is adequate
• Concurrent management of left heart failure with airway stabilization, diuretics and afterload reduction
• Management of life threatening ventricular arrhythmias
• Serial troponin I or T levels at presentation and every 3-6 hours
• Immediate reperfusion in STEMI – The first step in treating an acute ST-elevation myocardial infarction (STEMI) is prompt recognition, since the benefits of reperfusion therapy are greatest when performed as soon as possible (preferably within 90 minutes).
Preferred treatment is PCI if available. Before PCI, patients should receive dual antiplatelet agents including intravenous heparin infusion and ADP (adenosine diphosphate) inhibitor receptor (P2Y2 inhibitor) such as ticagrelor. Additionally, Glycoprotein IIb/IIIa inhibitor or direct thrombin inhibitor can be administered during percutaneous intervention.
• If PCI is unavailable within 90 minutes of STEMI diagnosis, reperfusion should be done with an intravenous thrombolytic agent
• NSTEMI in stable asymptomatic patients may not benefit from immediate PCI and should be medically managed with antiplatelet agents and PCI can be done in 48 hours
• In NSTEMI patients with refractory ischemia, hemodynamic instability and arrhythmias, PCI should be performed urgently

DIFFERENTIAL DIAGNOSIS
• Aortic dissection
• Pulmonary embolism
• Acute cholecystitis
• Acute perforated gastric ulcer
• Pneumothorax
• Esophagitis
• Myocarditis
• Severe asthma

COMPLICATIONS OF AMI
Complications of AMI can be classified into the following types
• Ischemic (including failure of reperfusion) – Angina, re-infarction, extension of infarct
• Mechanical – Heart failure, LV aneurysm, cardiogenic shock, mitral valve dysfunction, cardiac rupture.
• Arrhythmias – Atrial or ventricular arrhythmias, sinus or atrioventricular (AV) node dysfunction.
• Thromboembolic – Stroke or peripheral embolisation.
• Inflammatory – Pericarditis.

RISK ASSESSMENT FOR FUTURE CARDIAC EVENTS (NSTEMI and UA)
• Presence and extent of ST depression
• Elevated cardiac biomarkers
• Presence of hemodynamic instability
• Persistent chest pain despite appropriate medical treatment
• Ideally do coronary angiography for all patients prior to discharge

DISCHARGE ADVICE
Before discharge acute MI, patients may routinely be given aspirin, high-dose statin, beta-blocker, and/or ACE-inhibitor
Lifestyle changes such as stress management, quitting of smoking, regular exercise, weight management and dietary changes