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Diagnosis: Atrial Fibrillation in the Wolff-Parkinson-White Syndrome

Brady, William MD; Chan, Theodore MD; Harrigan, Richard MD

Cases in Electrocardiography

Dr. Harrigan is an associate professor of emergency medicine and the associate research director in the department of emergency medicine at Temple University Hospital and School of Medicine in Philadelphia; Dr. Chan is an associate professor of clinical medicine, emergency medicine, the director of CQI, and the associate medical director of the department of emergency medicine at the University of California, San Diego; and Dr. Brady is an associate professor and the program director in the department of emergency medicine at the University of Virginia School of Medicine in Charlottesville.

Soon after the verapamil was administered, the patient's ventricular response was noted to increase to approximately 240/min (Figure 3), and was accompanied by loss of consciousness. A pulse was present yet no blood pressure was found.

Figure 3

Figure 3

Figure 4 demonstrates a wide complex tachycardia with continued irregularity and a ventricular response in excess of 260/min. The patient was urgently cardioverted to normal sinus rhythm. Consciousness returned, accompanied by a blood pressure of 179/88 mmHg. A subsequent ECG revealed evidence of ventricular pre-excitation consistent with Wolff-Parkinson-White syndrome. The patient was admitted to the hospital. He underwent electrophysiologic testing that demonstrated an accessory pathway, which was successfully ablated. The patient was then discharged without recurrence of symptoms.

Figure 4

Figure 4

This patient presented with atrial fibrillation in the setting of the Wolff-Parkinson-White syndrome. The patient's already dangerous situation was worsened by administration of an atrioventricular node blocking agent (verapamil), as seen by an acceleration of the ventricular response associated with cardiovascular collapse. Urgent electrical therapy restored sinus rhythm with adequate perfusion.

DISCUSSION: The Wolff-Parkinson-White syndrome (WPW) is a form of ventricular pre-excitation involving an accessory conduction pathway between the atria and ventricles. In patients with normal conducting systems, the atrioventricular (AV) node acts to slow conduction and prevent transmission of the rapid atrial beats to the ventricles, in essence, protecting the ventricles from excessive rates.

In the WPW patient, the accessory pathway bypasses the AV node, creating a direct electrical connection between the atria and ventricles, removing this protection against excessively rapid rates. Accessory pathways are characterized by very rapid, nondecremental bidirectional, atrioventricular conduction. “Nondecremental” indicates that the accessory pathway itself does not have the ability to reduce the number of impulses transmitted onto the ventricles in marked contradistinction to the AV node, which is only able to conduct a fixed number of electrical discharges to the ventricles per unit time.

WPW patients are prone to develop a variety of supraventricular tachyarrhythmias. The majority of patients with pre-excitation syndromes remain asymptomatic throughout their lives. When symptoms do occur, they are usually secondary to tachyarrhythmias, including reentrant tachyarrhythmias (paroxysmal supraventricular tachycardia, 70%), atrial fibrillation (25%), atrial flutter (< 5%), and ventricular fibrillation (rare).1 Ventricular fibrillation is an extremely rare complication in the properly managed WPW patient.1

Atrial fibrillation occurs more frequently in patients with WPW compared with individuals without ventricular pre-excitation. It is found in up to one-quarter of patients with symptomatic arrhythmia.2 As noted, accessory pathways lack the feature of slow, decremental conduction, and the pathway then can conduct atrial beats at a rate that can approach or exceed 300 beats per minute, subjecting the ventricle to very rapid rates.

With ventricular responses at or above 300 beats per minute, the risk of ventricular fibrillation is greatly increased for two reasons.3 First, subsequent depolarization may occur during the electrically vulnerable period of ventricular repolarization, in essence, the R-on-T phenomenon which can initiate ventricular tachycardia or fibrillation. Secondly, such rapid rates markedly reduce left ventricular filling times, culminating in systemic hypoperfusion.

The diagnosis of WPW is relatively straightforward if the patient presents in normal sinus rhythm with recent symptomatic dysrhythmia. The electrocardiographic definition of WPW relies on the following electrocardiographic features:

  • ▪ A PR interval less than 0.12 seconds.
  • ▪ A slurring and slow rising of the initial segment of the QRS complex, known as a delta wave.
  • ▪ A widened QRS complex with a total duration greater than 0.12 seconds in duration.
  • ▪ Secondary repolarization changes reflected in ST segment-T wave changes that are generally directed opposite (discordant) to the major delta wave and QRS complex.4

The PR interval is shortened because the impulse progressing down the accessory pathway is not subjected to the physiological slowing that occurs in the AV node. Thus, the ventricular myocardium is activated by two separate pathways, resulting in a fused, or widened, QRS complex. The initial part of the complex, the delta wave, represents aberrant activation through the accessory pathway, while the terminal portion of the QRS represents normal activation through the His-Purkinje system from impulses having traveled through both the AV node and the accessory pathway.

When the patient presents with active arrhythmia, the diagnosis of WPW may not be known or apparent due to the presence of a non-sinus rhythm 12-lead ECG (i.e., the standard electrocardiographic criteria of WPW are not observed). Electrocardiographic issues to consider include the regularity of the rhythm, the rate of the ventricular response, and the width and morphology of the QRS complex.

If one considers this case, electrocardiographic features suggestive of WPW-related atrial fibrillation include a widened QRS complex (> 0.12 sec); wide QRS complex tachycardia with rapid ventricular response (> 180 to 200/min); beat-to-beat variation in QRS complex morphologies; bizarre QRS complex morphologies; and rhythms with initial slurring of QRS complex (the delta wave).

The presenting ECG (Figure 5) and rhythm strips (Figures 1, 2, and 4) provided several electrocardiographic clues to the diagnosis, including the rapid, irregular rate, the beat-to-beat variation in the QRS complex morphology, and the delta wave (Figure 6).

Figure 5

Figure 5

Figure 6

Figure 6

Urgent treatment of atrial fibrillation in the hemodynamically unstable WPW patient is direct current cardioversion. In the hemodynamically stable patient, agents able to reduce anterograde conduction down the WPW and also restore sinus rhythm should be utilized. These agents include procainamide and amiodarone. Of note, amiodarone should not be the clinician's first choice because its initial action when given intravenously is vagomimetic, and it can speed bypass conduction.5

In fact, such an agent is not recommended by many authorities in the initial treatment of WPW-related atrial fibrillation.5,6–8 AV node blocking agents (calcium antagonists, beta-adrenergic blocking agents, and digoxin) are clearly contraindicated in this setting because they facilitate conduction down the accessory pathway, increasing the ventricular response with the risk of ventricular fibrillation.3,9–10

Adenosine provides no benefit because the pathophysiology lies outside the AV node, and adenosine may potentiate conduction down the accessory pathway due to AV node blockade. Admission should be considered for these patients to allow for continuous electrocardiographic monitoring and possible electrophysiologic testing and pathway ablation.

Long-term medical therapy is problematic for a number of reasons. Such antiarrhythmic agents are reasonably ineffective in arrhythmia suppression; the proarrhythmic effects of these medications also must be considered. Medical management is even less efficacious when the patient exerts himself or experiences increased level of endogenous catecholamines for other reasons.1,5

Long-term medical management should be limited to older, sedentary individuals with infrequent and non-life threatening arrhythmias.11 The preferred alternative therapy for younger patients with frequent symptomatic occurrences or those individuals with life-threatening arrhythmia (such as atrial fibrillation, wide complex tachycardia, or compromising narrow complex tachycardia) is surgical or transcatheter accessory pathway ablation.

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