Myocardial Infarction (MI)
MI is myocardial injury due to ischaemia. It is defined pathologically as myocardial cell death due to prolonged ischaemia. MI is diagnosed when there is a rise and/or fall in cardiac troponins and accompanied with at least one of the following:
Clinical history of chest pain from ischaemic origin lasts more than 30 minutes.
ECG changes of ischaemia/infarction and/or the
Development of pathological Q waves.
New or presumed new significant ST-T wave changes or left bundle branch block
Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality.
Identification of an intracoronary (IC) thrombus by angiography or autopsy.
MI can be divided into 5 types
Type 1: Spontaneous MI due to coronary athero-thrombosis – atherosclerotic plaque rupture, ulceration, fissuring, erosion, or dissection with resulting intraluminal thrombus in coronary artery leading to decreased myocardial blood flow or distal platelet emboli with ensuing myocyte necrosis.
Type 2: MI secondary to an imbalance between myocardial oxygen demand and supply – it may occur in the presence of coronary atherosclerosis without plaque rupture or in the absence of atherosclerosis eg coronary endothelial dysfunction, coronary artery spasm, coronary embolism, coronary artery dissection, tachy / bradyarrhythmias, anaemia, respiratory failure, sepsis, hypotension, and hypertension with or without left ventricular hypertrophy (LVH).
Type 3: MI resulting in death when biomarker values are unavailable unrelated to acute coronary athero-thrombosis
Type 4: MI related to PCI
Type 5: MI related to coronary artery bypass surgery (CABG)
Presentations of MI
Typical symptoms
Chest pain located in the centre of the chest and may radiate to the jaw or down the left arm. It may occur at rest or with activity. The pain can just be a tightness or heaviness in nature, or more often a squeezing, severe type of crushing pain. They are usually accompanied with sweating, nausea, vomiting and shortness of breath.
Atypical symptoms
Elderly, females and diabetic patients may present with unexplained fatigue, shortness of breath, dizziness, light-headedness, unexplained sweating and syncope. They may not have chest pain.
The clinical spectrum may range from patients free of symptoms at presentation to individuals with ongoing ischaemia, electrical or haemodynamic instability or even cardiac arrest. Some patients may present with ongoing myocardial ischaemia, characterized by one or more of the following:
Recurrent or ongoing chest pain,
Marked ST depression on 12-lead ECG,
Heart failure
Haemodynamic or electrical instability.
Upon clinical suspicion of ACS, a 12-lead ECG should be performed and interpreted immediately within 10 minutes of presentation. If the initial ECG is non diagnostic and the index of suspicion of STEMI is high: the ECG should be repeated at close intervals of at least 15 minutes.
To look for progressive ST changes.
Compared with previous ECG’s.
Additional chest leads (V 7-9) and right ventricular leads should be done to identify posterior and right
ventricular infarcts.