Answer :USMLE step 2 Mcq 362:A patient with symptomatic WPW syndrome undergoes an intracardiac electrophysiological Study




Correct Answer: A

The exact nature of the prexcitation syndrome is assessed. Most of the WPW syndrome are related to a atrioventricular accessory connection or Kent bundle : the degree of prexcitation increases during premature atrial stimulation until the refractory period of accessory pathway is reached, because the conduction time does not change in accessory pathway with the shortening of atrial cycle length while it increases in the AV node.   Rarely the WPW syndrome is related to a nodoventricular accessory pathway or Mahaim bundle and the degree of preexcitation remains unchanged during premature atrial stimulation.   The accessory pathway refractory period depends on the driven cycle length. Refractory period of the accessory pathway decreases as the driven cycle length shortens..   Beta adrenergic stimulation results in shortening of the anterograde refractory period of the accessory pathway and an increase in ventricular rates during atrial pacing and atrial fibrillation.   Isoproterenol test was also previously used to verify the efficacy of antiarrhythmic drug before the era of catheter ablation of accessory pathway. The loss of efficacy of some antiarrhythmic drugs was demonstrated after isoproterenol administration.   Atrial fibrillation is easily induced during intracardiac studies by salvos of rapid atrial stimulation and is not specific. The induction of an atrial fibrillation by intracardiac programmed stimulation is obtainedin 45% in asymptomatic patients and in 75 % of patients with only documented reentrant tachycardia, atrial fibrillation is induced in 95 % of those with documented atrial fibrillation. The important variations of the incidence of induced atrial fibrillation depends on the technique of programmed stimulation, on the interpretation of the duration of induced arrhythmia and on the use of isoproterenol infusion or other means to reproduce the effects of adrenergic stimulation. <br> The induction of an atrial fibrillation during transesophageal pacing has a best clinical significance :The incidence of induction of atrial fibrillation also depends on the presence of an associated heart disease and the age of the patient : the induction of atrial fibrillation is rarely noted in children younger than 10 years, is induced in 20 % of teenagers and adults without heart disease and becomes relatively frequent in elderly   - Ventricular tachyarrhythmias also are easily induced in asymptomatic or symptomatic patients by programmed ventricular stimulation and are not specific in patients with WPW syndrome : the induction of a ventricular fibrillation is noted in 4 % of WPW syndrome and the induction of nonsustained multiform ventricular tachycardia in 37 % of them.   - Antidromic tachycardia which is a reciprocating tachycardia using the accessory pathway for the anterograde conduction and the normal AV conduction system for retrograde conduction, is a rare finding (5%), more frequently noted in young patients with a good retrograde normal VA conduction or in patients with several accessory pathways and seems more frequent in patients at risk of rapid arrhythmias. <br> - Orthodromic tachycardia which is a reciprocating tachycardia using the normal AV conduction system for the anterograde conduction and the accessory pathway for the retrograde conduction, is rarely induced in asymptomatic patients (< 10%) , but represents the most frequent tachycardia of symptomatic patients complaining tachycardia and palpitations (90 %).


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