The ECG changes of ischemia are not arbitrary patterns; they are the surface expression of underlying electrical gradients created by injured myocardium. Understanding these mechanisms allows clinicians to interpret ischemic changes with greater precision (expanded in The Masterclass in Advanced Electrocardiography).
The Injury Current
Ischemic cells maintain a different resting membrane potential than healthy cells (as discussed in the book, Getting Acquainted With Ischemia and Infarction). This creates:
- diastolic injury currents
- systolic injury currents
- shifts in the ST segment relative to the TP baseline
Subendocardial vs Transmural Effects
- Subendocardial ischemia → ST depression, upright T waves
- Transmural ischemia → ST elevation, reciprocal depression
T‑Wave Changes
Ischemia alters repolarization, producing:
- hyperacute T waves
- symmetric T‑wave inversion
- pseudonormalization in evolving ischemia
Reciprocal Changes
Reciprocal ST depression is not a “mirror image” but a vector phenomenon reflecting the direction of the injury current.
Related: Key Mechanism‑Based Concepts in Ischemia and Infarction