There is a high incidence of tachyarrhythmias in patients with WPW. Atrial flutter occurs in about 5 percent, atrial fibrillation in 10 to 20 percent, and paroxysmal reentrant SVT in 40 to 80 percent.
Reentrant SVT occurs when an impulse is sustained around a loop composed of the bypass tract and the AV conducting system, the impulse traveling down one and up the other. Whether the QRS complex is wide or narrow depends on which limb of the circuit is used as the downward pathway to activate the ventricles. About 80 to 95 percent of the time, reentrant SVT occurs with the impulse being conducted down the normal AV conducting system and up the bypass tract [orthodromic AV reciprocating tachycardia (OAVRT)]. In this situation, ventricular activation occurs entirely over the normal system, the QRS complex is normal, and no delta wave is seen. Because the entire heart is used as the reentrant pathway, these arrythmias are easily converted. Conversely, 5 to 10 percent of the time, the impulse is conducted down the bypass tract and retrograde up the AV node [antidromic AV reciprocating tachycardia (AAVRT)]. In this case, the QRS complex is wide, and in the ED setting, this arrythmia is treated as ventricular tachycardia. Reentry is usually initiated by a premature atrial contraction that encounters a bypass tract which still is refractory from the previous sinus beat, but the AV node has recovered partially and conducts the impulse more slowly than normal ( Fig...24-41). In some patients, the bypass tract does not conduct antegrade during sinus rhythm and so no delta wave is seen, but it does conduct retrograde so reentrant SVT occurs. Patients with concealed bypass tracts account for about 20 percent of all patients with reentrant SVT.
FIG. 24-41. Onset of reentrant SVT in Wolff-Parkinson-White syndrome.
If patients with WPW develop atrial flutter or fibrillation, impulses can reach the ventricles via the accessory tract, the normal conducting system, or both. Which pathway is used depends on the refractory period of each. Most patients with WPW have longer refractory periods in their accessory tracts than in the AV node, but a minority have the opposite. In patients with short refractory periods in their accessory tracts, more atrial impulses can be conducted through the accessory tract than the AV node, so most of the QRS complexes will be wide. In atrial flutter, 1:1 AV conduction is possible with ventricular rates of 300 ( Fig, 24-42). In atrial fibrillation, very rapid and irregular ventricular rates are possible. These rapid rhythms may resemble ventricular tachycardia, and excessive stimulation of the ventricles may precipitate ventricular fibrillation. Any patient with a ventricular rate of over 300 should raise the suspicion of preexcitation syndrome.
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