Mechanisms of supraventricular tachycardia.

One of several mechanisms may be responsible for the arrhythmia in patients with attacks of SVT. When conventional therapy is ineffective in preventing the attacks and when the attacks occur frequently, then electrophysiologic studies should be performed. These studies help determine the mechanism of the arrhythmia and provide the means for testing antiarrhythmic drugs for prophylactic use.

Electrical alternation.

ST segment alternans occurs relatively infrequently. However, it is seen in 30% of patients with Prinzmetal's angina, especially when the ST segment is markedly elevated. Ventricular arrhythmias occur frequently during ST segment alternation.

Acute myocardial infarction and fascicular block.

Left anterior fascicular block may mask an IWMI. On the other hand, a narrow QS deflection in a VF is often misinterpreted as evidence of extensive inferior wall infarction. Vectorcardiographic analysis usually establishes the coexistence of LAFB and IWMI.