Myocardial Infarction: Definitions & Types
Injury vs. Infarction
Myocardial injury: Cardiac troponin values are elevated above the 99th percentile upper reference limit.
Myocardial infarction: Myocardial injury (that is, elevated troponin levels) coupled with ischemia.
Review
ischemic heart disease.
Timing
Incident infarction is the patient's first myocardial infarction.
Recurrent infarctions occur more than 28 days after the incident infarction.
Reinfarctions occur within 28 days of an incident or recurrent infarction.
Silent Infarctions
Most myocardial infarctions present with symptoms such as chest pain, dyspnea, and weakness; however, silent myocardial infarctions, which are asymptomatic, account for approximately 1 in 5 cases.
MINOCA
Although many myocardial infarctions occur in the presence of obstructive coronary artery disease, a significant number of heart attacks are classified as MINOCA: myocardial infarction with
non-obstructive coronary artery disease.
Takotsubo cardiomyopathy
Similar symptoms as heart attack: Takotsubo cardiomyopathy (sometimes called "broken heart syndrome").
"Takotsubu" refers to the Japanese octopus trap, which resembles the extended shape of the left ventricle in patients with this condition.
Patients have left ventricular dysfunction and damage due to increased catecholamines, which can be induced by physical or emotional triggers.
ECG often shows ST-segment elevation, mild to severe biomarker elevation, and normal or non-obstructive coronary arteries.
Formerly believed to be benign, we now know that this condition is associated with increased mortality.
Myocardial Infarction Types
Based on ECG changes
- ST-elevation myocardial infarction (STEMI): ECG shows ST-segment Elevation.
— Often associated with totally occluded coronary arteries.
- Non-ST-Elevated Myocardial Infarctions (NSTEMI): No ST-segment Elevation, but can have ST-depression or inverted T-waves.
— Often, though not always, associated with partial occlusions.
- Some patients also develop pathological Q waves.
Mural Thickness
- Transmural: the ischemia traverses the width of the myocardial tissue.
- Subendocardial: only the inner one-third of the myocardium.
Be aware that earlier publications often equated transmural MI with Q-wave MIs; however, autopsy reports failed to confirm this association, and these classification schemes have largely been replaced by the labels STEMI and NSTEMI.
Etiology
Based on 2018 publication by the Joint European Society of Cardiology/American Heart Association/World Heart Federation Task Force for the Universal Definition of Myocardial Infarction.
MI Type 1:
— Spontaneous myocardial infarctions caused by
thrombi, and are usually the result of plaque rupture or erosion.
— Remind ourselves that plaque rupture occurs when thin-capped plaques with lipid cores produce red thrombi, and, that erosion occurs when thick-capped plaques produce white thrombi.
— MI Type 1 is often, though not always, associated with obstructive coronary artery disease.
MI Type 2:
— Occur secondary to other conditions; they are the result of conditions that produce non-thrombotic mismatches between oxygen supply and demand, including microvascular dysfunction, hypertension, atherosclerosis, and vasospasm.
— Thus, treatment for and prevention of MI Type 2 involves discovery of the underlying causes.
MI Type 3:
— There is evidence of ischemia, but biomarkers are unobtainable due to death or other events that preclude collection.
MI Types 4 and 5 are associated with revascularization procedures:
— Type 4a is associated with Percutaneous Coronary Intervention (PCIs).
— Type 4b is specifically associated with stent and scaffold PCIs.
— Type 4c is characterized by restenosis associated with PCI.
— Type 5 is associated with coronary artery bypass grafting.
Myocardial Infarction vs Injury
We start this investigation when a patient has elevated cardiac troponin values.
If elevated levels are stable, the patient likely has chronic myocardial injury (which could be caused by structural heart disease, for example).
However, if the troponin levels rise and fall, we'll need to determine whether ischemia is present.
If no ischemia is present, then the patient has myocardial injury, which could be caused by
myocarditis or other conditions.
If ischemia is present, then the patient has an acute myocardial infarction, and we need to establish the presence or absence of thrombus.
If thrombus is present, it's a myocardial infarction Type 1.
If there is no thrombus, then we conclude that an underlying condition has produced a mismatch between oxygen supply and demand, and we classify the event as Type II MI.
