Using the twelve-lead electrocardiogram to localize the site of origin of ventricular tachycardia

The basis of this review is the underlying hypothesis that the QRS morphology on 12-lead ECG is, to a great extent, determined by the site from which a focal ventricular tachycardia (VT) arises or from which a reentrant circuit exits the central isthmus to activate the “normal” myocardium. The ability to localize or, at the very least, regionalize “the sites of origin” of VTs enables the electrophysiologist to concentrate mapping to a specific region. Several factors limit the ability of the QRS patterns to localize VT origin, including (1) presence and size of infarction, (2) degree of intramyocardial fibrosis, (3) shape of the heart (e.g., aneurysm) and its position within the chest cavity, (4) site and mechanism of VT within an infarct or scarred area, (5) influence of nonuniform anisotropy in affecting propagation from the site of the tachycardia, (6) effects of acute ischemia, antiarrhythmic drugs, or metabolic abnormalities on conduction, (7) integrity of the His-Purkinje system, (8) presence of increased myocardial mass, and (9) presence of structural abnormalities unrelated to tachycardia origin or mechanisms. Despite these limitations, the ECG remains useful at least in regionalizing the origin of VT and provides more accurate localizing information in the structurally normal heart. Some features on the ECG can give clues to the underlying substrate. The more rapid the initial forces, the more likely VT is arising from normal myocardium. Slurring of the initial forces is frequently seen when the tachycardia arises from an area of scar or from the epicardium. VTs originating in very diseased hearts have lower-amplitude complexes than those arising in normal hearts. The presence of notching of the QRS is a sign of scar tissue. Whereas QS complexes may be seen in a variety of disorders, the presence of qR or QR or Qr complexes in related leads is highly suggestive of the presence of an infarct. Occasionally it is easier to recognize the presence of infarct during VT than during sinus rhythm.