WOLFF-PARKINSON-WHITE
SYNDROME
The Wolff-Parkinson-White (WPW) complex is caused by the presence of an accessory pathway. About 20% of patients with an accessory pathway have organic heart disease, and 80% have the ECG abnormality only. The accessory pathway connects the atria to the ventricles, over which depolarization occurs rapidly from atria to ventricles, resulting in ventricular preexcitation. The syndrome related to preexcitation is most often seen in young subjects who have frequent attacks of supraventricular or even ventricular tachycardia. Between attacks of rapid heartbeat, the QRS complexes consist of a short P-R interval (usually <0.11 second), a QRS complex widened by a Δ wave, and, usually, a QRS complex whose duration is from 0.11 to 0.14 second. Actually, the P-R interval is decreased by the amount the QRS complex is increased, so that the P-J interval remains quite normal (see Plate 2-23). (The P-J interval is from the beginning of the P wave to the end of the QRS. “J” stands for the junction between the QRS and ST segment on the ECG.).
The upstroke of
the R wave in lead I in a patient with a right-sided accessory pathway is
usually “slurred” because of the Δ wave at the beginning of the QRS complex. If the accessory pathway
connects left atrium to left ventricle, depolarization will be from left to
right, which will produce QRS complexes in lead I that are primarily negative.
Most often, however, the acces- sory pathway is on the right, with the
accession wave going from right to left and a Δ wave appearing at the beginning of the QRS complex in
lead I. The accessory pathway could be posterior or anterior, and different ECG
configurations result. The precise location of the accessory pathway is
determined at electrophysiologic study before an ablation procedure.
Impulses from
the sinus node travel more rapidly through the accessory pathway than through
the A-V node and bundle of His. The widening of the QRS complex and the
slurring of the upstroke of the R wave in lead I are explained by the depolarization
wave entering the right ventricle early and without delay, through the abnormal
connection between the atria and the ventricles. Since the depolarization
process through the ventricles is longer than normal, because of its abnormal
direction, the QRS complex is exceptionally wide. After the early
depolarization of the ventricle from the accessory pathway has begun, the
normal atrial impulses, which were delayed at the A-V node, enter the ventricle
by the normal pathway, and the depolarization of the ventricles is completed in
a normal fashion. Thus, the terminal portions of the QRS complexes are normal.
The accessory
pathway predisposes to attacks of paroxysmal tachycardia by facilitating
retrograde conduction into the atria with the initiation of circus movements or
antegrade conduction in the pathway with retrograde conduction in the
His-Purkinje system. Therefore, all young patients complaining of attacks of
tachycardia should have an ECG during a period of normal heart rate to
determine if a WPW pattern exists.
Procainamide or
flecainide is often used successfully to block preexcitation. Digitalis is
usually ineffective and may be dangerous if given alone because of 1 : 1
conduction to the ventricle in certain atrial arrhythmias (e.g., atrial fibrillation
or flutter), which may result in ventricular fibrillation. Other drugs that
block conduction through the A-V node can enhance conduction through the
accessory pathway as well (e.g., calcium blocker adenosine).
