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