Preexcitation Arrhythmias

The phenomenon of preexcitation , known as Wolff-Parkinson-White syndrome , is related to a congenital abnormality of the heart in which an accessory tract conducts impulses from the atria to the

ventricles. This tract is located in a different part of the heart than the normal conduction system. It can short-circuit the conduction between the two chambers (the normal impulse is delayed within the A-V node) and stimulate prematurely certain portions of the ventricular muscle. As a result, the electrocardiogram shows a pronounced distortion of the QRS complexes, which could be mistaken for serious heart disease. Yet in most persons with this abnormality the heart is structurally and functionally normal in all other respects, and they remain unaware of the existence of this anomaly until they undergo an electrocardiogram for some unrelated reason. In a small number of such persons, however, the abnormal bypass tract is a source of disabling or even life-threatening arrhythmias. The disparity between the conduction time through the normal channels and the bypass tract may under certain circumstances cause reentry tachycardias, which permit an impulse reaching the ventricle by way of either tract to return to the atrium via the other pathway. If atrial fibrillation or flutter develops, there may be an unusually rapid ventricular rate—up to 300 beats a minute—which normally would be averted by the A-V node. Some patients can be treated medically by antiarrhythmic drugs. If no effective and safe drugs for controlling the arrhythmias can be found, catheter ablation can be performed to sever the abnormal tracts. This procedure has been found more consistently successful than other catheter ablation used in controlling intractable tachycardias. If catheter ablation fails, a surgical ablation may be used.