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Define myocardial infarction according to Harrison's.

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Myocardial Infarction - Definition According to Harrison's (22nd Edition, 2025)

Myocardial Injury (prerequisite concept)

Before defining MI, Harrison's distinguishes myocardial injury: the term is used when there is evidence of elevated cardiac troponin (cTn) levels with at least one value above the 99th percentile upper reference limit (URL). The injury is considered acute if there is a rise and/or fall of cTn values.

Acute Myocardial Infarction - Core Definition

"The term acute myocardial infarction (MI) should be used when there is acute myocardial injury with clinical evidence of acute myocardial ischemia and with detection of a rise and/or fall of cTn values with at least one value above the 99th percentile URL and at least one of the following:"
  1. Symptoms of myocardial ischemia
  2. New ischemic ECG changes
  3. Development of pathologic Q waves
  4. Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality in a pattern consistent with an ischemic etiology
  5. Identification of a coronary thrombus by angiography or autopsy (not applicable for types 2 or 3 MI)
  • Harrison's Principles of Internal Medicine, 22E, Chapter 286, p. 2161

MI Types (1-5)

TypeDefinition
Type 1Acute atherothrombosis in the artery supplying the infarcted myocardium (spontaneous MI from plaque rupture/erosion)
Type 2Imbalance between myocardial oxygen supply and demand unrelated to acute atherothrombosis
Type 3Cardiac death with symptoms suggestive of ischemia and presumed new ischemic ECG changes, before cTn values became available or abnormal
Type 4aPCI-related MI: cTn elevation >5x the 99th percentile URL within 48 h of PCI
Type 5CABG-related MI: cTn elevation >10x the 99th percentile URL within 48 h of CABG
For types 4a and 5, at least one of the following must also be present: new ischemic ECG changes (type 4a), new pathologic Q waves, or imaging evidence of new ischemic myocardial loss.

ECG Basis (Pathophysiology)

Harrison's also explains the ECG correlate: severe acute ischemia lowers the resting membrane potential and shortens action potential duration, generating currents of injury reflected as ST-segment deviations. Transmural ischemia shifts the ST vector outward (epicardial), producing ST elevation, while subendocardial ischemia shifts it inward, producing ST depression. This underpins the clinically important division of MI into STEMI and non-STEMI types, since emergency reperfusion therapy has consistent efficacy primarily in STEMI.
  • Harrison's Principles of Internal Medicine, 22E, Chapter 247 (ECG chapter), p. 1916
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