Subendocardial ischemia during exercise testing as viewed from a V5 electrode

Panel A: At rest, a normal ECG results from the firing of hypothetical action potentials from subendocardial (Endo) and subepicardial (Epi) muscle cells. Panel B: A diastolic injury current from injured subendocardial cells causes TQ segment elevation (or "pseudo" ST depression). Panel C: Systolic and diastolic injury currents from injured subendocardial cells (panel C) causes TQ elevation and "true" ST depression.