Evaluation of Asymptomatic Wolff-Parkinson-White EKG Morphology

January 3, 2008

800px-wolff-parkinson-white_syndrome_12_lead_ekg.pngCommentary by David Steckman MD, PGY-2 and William Slater MD, Associate Professor of Medicine, Division of Cardiology

Case: A 42 year-old man presents to clinic for routine follow-up. He is found to be hypertensive for the second consecutive clinic appointment. On routine EKG, you find a shortened PR interval and what looks to be a delta wave in V1-V3. The patient does not report any history of syncope, chest pain, palpitations or shortness of breath. In addition to treating his hypertension, what further work-up is required for his asymptomatic pre-excitation pathway?

The Wolff-Parkinson-White (WPW) EKG morphology was first described in 1930 in 11 patients that had recurrent episodes of tachycardia and a characteristic EKG pattern of a shortened PR interval and a delta wave. The current prevalence of WPW on surface EKG is thought to be 0.15 to 0.25% in the general population. There is a 4-fold increase among family members of an affected patient (1). WPW syndrome is reserved for those patients with both EKG evidence of pre-excitation and tachyarrhythmia, most commonly A-V reentrant tachycardia (AVRT) accounting for 95% of associated tachycardia. Atrial fibrillation (AF) is a potentially life-threatening arrhythmia in patients with WPW syndrome. If an accessory pathway has a short anterograde refractory period, then rapid repetitive conduction to the ventricles during AF can result in a rapid ventricular response with subsequent degeneration to ventricular fibrillation (VF). The incidence of sudden cardiac death (SCD) is estimated to range from 0.15 to 0.38% over 3 and 10 years of follow-up (1). Although cardiac arrest is rare in patients with known WPW syndrome, in approximately half of patients with an unknown accessory tract, the first presenting symptom is cardiac arrest. Factors associated with increased risk of SCD include younger age, male gender, high adrenergic state, multiple accessory pathways, ease of inducing AVRT with electrophysiologic (EP) testing, and short R-R periods during atrial fibrillation (1-3).

Patients with a WPW pattern on ECG can be divided into 3 groups, with different considerations for management: asymptomatic patients, those with spontaneous SVT, and those presenting with AF. We will consider these groups separately.

It is controversial whether patients presenting with asymptomatic EKG findings suggesting pre-excitation (electric stimulation of the ventricles via an accessory pathway) require further testing and subsequent therapy. Current guidelines suggest risk stratifying patients into groups requiring further testing and those requiring simple close observation. The best indicator of low risk is the abrupt disappearance of pre-excitation during exercise, indicating a long refractory period of the accessory pathway (3). This demonstrates that the accessory pathway would be less capable of transmitting a rapid atrial rate to the ventricle in AF. In addition, an intermittent delta wave on a routine EKG indicates longer refractory times and thus implies very low risk. One must be careful to distinguish intermittent pre-excitation from ventricular bigeminy with loss of delta waves on the PVC beat.

Patients with asymptomatic WPW may undergo non-invasive testing including stress testing or Holter monitoring (i.e., to look for intermittency of pre-excitation). If these studies do show EKG findings suggestive of longer refractory periods, such patients may be discharged safely with an information packet, a copy of the diagnostic EKG and strict counseling to return to the ER if symptomatic arrhythmias occur. Exceptions are made for those patients with high risk occupations (bus driver, scuba diver, pilot). Many clinicians may start low dose beta-blockers if the accessory pathway is proven during EP testing to be incapable of rapid conduction. There have been no randomized-controlled trials of drug prophylaxis for AVRT.

While the standard of care has been conservative follow-up of asymptomatic patients with WPW on their routine ECG, recent data from Pappone’s group in Italy has raised questions about this approach (2). In this study, 224 asymptomatic WPW patients were investigated and 3 suffered VF over an 8-year follow-up. While this is a higher incidence of VF than previously recognized, it is the largest consecutive follow-up of this group. In fact, all 3 patients had become symptomatic with SVT prior to their cardiac arrest, but many feel that in young patients this is too high a risk to accept. The field is now in flux, with many cardiologists recommending EP study (and ablation if a short refractory period is found) in asymptomatic patients, particularly if loss of delta wave is not observed with Holter and stress testing. The problem with this approach is that approximately 1/3 of such patients will have sufficiently short refractory periods to warrant ablation, yet the sudden death risk is a tiny proportion of these, and ablation is not risk-free. So, in aggregate, a strategy of routine EP study in asymptomatic patients may hurt an equally small number as it will save from VF; hence, the controversy in management in 2007.

The second group to consider is the group with SVT, usually narrow complex SVT. This looks like routine AV nodal re-entry on ECG except that there are usually retrograde p waves seen immediately after the QRS. This occurs because the impulse takes longer to get back from ventricle to atrium since it uses an accessory pathway as the backwards limb. Since the narrow complex SVT in WPW uses the AV node as it’s forward limb, treatment of SVT is identical to the routine treatment of SVT (CSP, adenosine, beta blocker, Ca-blocker). If retrograde p waves are seen during the SVT, it alerts us to look very carefully for a delta wave in NSR. A crucial point in WPW is that the patient with SVT needs very careful evaluation, usually including EP study. THE PATIENT WITH SVT IS AT RISK FOR EVENTUAL DEGENERATION TO AF WITH POTENTIALLY RAPID VENTRICULAR RATES. This occurs because the bypass tract acts like a web, with the impulses traveling backwards in the SVT activating the atrium at different times, creating electrical heterogeneity and risk for AF. Approximately 20% of patients with SVT will develop AF from it, and when AF occurs the risk is entirely governed by the antegrade refractory period of the bypass tract. This cannot be predicted by the rate in SVT, since the pre-excited ventricular rate in AF has nothing to do with the AV node. Therefore, most clinicians will send a patient with WPW and SVT for EP study; if the bypass tract has a short refractory period, it should be ablated to prevent rapid conduction in AF.
The third group to consider is the patient who presents with AF; in WPW, the AF will be wide complex since the impulses go down the accessory pathway. In studies of patients who have had VF due to rapid conduction of AF, the shortest pre-excited R-R interval in AF is almost always under 240 msec (6 small boxes on an EKG tracing). Patients with rapid VF or R-R’s shorter than 240 msec should undergo ablation of the AF.

In conclusion, the management of the patient presented here remains controversial. Noninvasive testing to look for intermittency of pre-excitation, an EP study to examine the antegrade refractory period of the accessory pathway, or conservative follow-up with clear instructions to report symptoms of SVT promptly would all be appropriate management options for this patient.

References:

1. ACC/AHA/ESC Guidelines for the Management of Patients With: Supraventricular Arrhythmias: AV Reciprocating Tachycardia
2. Pappone et al. A Randomized Study of Prophylactic Catheter Ablation in Asymptomatic Patients with the Wolff-Parkinson-White Syndrome. NEJM 2003;349:1803-1811
3. Wellens et al. When to Perform Catheter Ablation in Asymptomatic Patients with a Wolff-Parkinson-White Electrocardiogram. Circulation 2005;112:2201-2216.

Image courtesy of Wikimedia Commons

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