High-risk patients with ventricular preexcitation--a pendulum in motion.

Since they were first described, preexcitation syndromes have intrigued physicians. The interest derives in part from the myriad bizarre electrocardiographic abnormalities associated with these syndromes and from the variety of complex arrhythmias that may be manifested in affected patients. Symptomatic tachycardias develop in patients with the Wolff–Parkinson–White syndrome because there is conduction over an accessory (extranodal) atrioventricular pathway consisting of working myocardium. Since an impulse arising from the sinus node can activate the atrioventricular node and the more rapidly conducting accessory pathway in parallel, the ventricle ipsilateral to the site where the accessory pathway enters the myocardium is excited prematurely. The consequent classic electrocardiographic triad is a short PR interval, a slurred QRS upstroke (“delta” wave), and a prolonged QRS complex. The estimated prevalence of the Wolff–Parkinson–White pattern is 0.1 to 0.3 percent. Continued fascination with the Wolff–Parkinson–White syndrome rests on two compelling findings. First, the most common arrhythmia associated with this syndrome, atrioventricular reciprocating tachycardia, represents the clearest clinical paradigm of functional and anatomical reentry, and second, sudden death from ventricular fibrillation may occur in young, previously asymptomatic persons. Orthodromic atrioventricular reciprocating tachycardia is composed of a circuit in which the normal atrioventricular conduction system serves as the anterograde limb and the accessory pathway serves as the retrograde limb. The electrocardiogram shows a narrow QRS complex and a retrograde P wave inscribed shortly after the QRS complex. The reversal of this circuit, antidromic atrioventricular reciprocating tachycardia, is far less common and presents as tachycardia with a wide QRS complex. Either form of atrioventricular reciprocating tachycardia can degenerate into atrial fibrillation, which can be particularly ominous. In the normal heart, the atrioventricular node protects the ventricles from extremely rapid and potentially destabilizing atrial conduction because there is an inverse relation between the atrial rate and the conduction velocity in the atrioventricular node. However, no such safety net exists with regard to the accessory pathway. As a result, the capacity to conduct rapidly over the accessory pathway allows atrial fibrillation to precipitate ventricular fibrillation and sudden death. A patient’s arrhythmia may be reproduced by programmed stimulation in the invasive-electrophysiology laboratory, and the properties of the reentrant arrhythmia can be characterized. The electrophysiologist can introduce a premature atrial beat by means of a catheter placed in the right atrium or coronary sinus. If the refractory period of the accessory pathway is longer than that of the atrioventricular node, the impulse will proceed only over the slowly conducting atrioventricular node. However, if excitability in the accessory pathway recovers, the impulse can engage the pathway from the ventricle in a retrograde fashion and then reenter the atria to repeat the sequence continuously. Several measurements are obtained during electrophysiological evaluation to assess the risk of a potentially lethal arrhythmia. These include the anterograde refractory period of the accessory pathway, a surrogate marker for the rate of conduction over the pathway during atrial fibrillation, as well as the mean and shortest RR intervals during preexcited atrial fibrillation. Studies can be performed during the infusion of a catecholamine, such as isoproterenol, to simulate the conditions observed during exercise, and the results can be compared with base-line measurements. The rate at which 1:1 conduction proceeds over the accessory pathway during atrial pacing is also assessed. Septal locations of accessory pathways and the presence of multiple pathways have both been thought to increase the