Clinicians often use the terms ischemia and infarction interchangeably, but physiologically they represent very different processes. Understanding the distinction is essential for interpreting ECG changes accurately (see Getting Acquainted With Ischemia and Infarction).” and for recognizing when myocardial injury is reversible versus permanent.
Ischemia: A Supply–Demand Imbalance
Ischemia occurs when myocardial oxygen demand exceeds supply (discussed in Introduction to Electrocardiography for ABSOLUTE BEGINNERS). Key features include:
preserved cell membrane integrity
reversible metabolic disturbance
subendocardial predominance due to wall stress and perfusion gradients
Injury: The Transitional State
Prolonged ischemia produces cellular injury, characterized by:
loss of membrane stability
ionic shifts
development of injury currents that alter the ST segment (explored further in The Masterclass in Advanced Electrocardiography)
Infarction: Irreversible Necrosis
Infarction represents cell death. Mechanistic hallmarks:
membrane rupture
loss of electrical activity
development of Q waves or QS complexes in some cases, as discussed in depth in the book Getting Acquainted With Ischemia and Infarction
Why the Distinction Matters on the ECG
Ischemia → T‑wave changes, minimal ST shifts
Injury → ST elevation or depression
Infarction → Q‑wave formation or persistent ST‑T abnormalities
Related: Key Mechanism‑Based Concepts in Ischemia and Infarction