A 52-year-old man presents to the Emergency Department with severe palpitations, dyspnea, and presyncope for the past hour. He reports a thirty year history of episodic palpitations occurring once or twice annually and subsiding spontaneously. Otherwise, he is healthy and physically active without symptoms:

• Atrial fibrillation with rapid ventricular response (186/min)
• Left bundle branch block pattern

Consider conduction through a bypass tract because of: 1) his longstanding history of episodic palpitations, 2) atrial fibrillation with very rapid ventricular rate and unusual wide-QRS  pattern in an apparently healthy individual, 3) variation in the QRS pattern and width, suggesting possible fusion (conduction through a bypass tract and the normal conduction pathways), 4) some of the RR intervals (measured from the onset of the QRS, not from the peak of the R waves) are 200 msec, corresponding to an average ventricular rate of 300/min. The AV node does not usually conduct at such a rate.

The patient is given IV procainamide, and the arrhythmia subsides:

• Sinus rhythm, 90/min
• Wolff-Parkinson-White