A 39-year-old limousine driver had complained of episodes of rapid palpitation for the past 15 years, starting and stopping suddenly, lasting 1-2 hours. On the way to a client, he developed a much more severe episode, accompanied by dyspnea and weakness. An ambulance is called and he is taken to the Emergency Department of a nearby hospital where an ECG is recorded:

• Atrial fibrillation with conduction most likely through a bypass tract 
  
• Wolff-Parkinson-White (WPW), Ventricular rate 220/min
  

The rhythm is atrial fibrillation with very rapid ventricular response and wide QRS. with right axis deviation and right bundle branch block pattern. A very important finding is that several successive R-R intervals measure 200 msec., which corresponds to AV conduction rate of 300/min. The AV node does not  conduct at such a rapid rate, as its effective refractory period is longer than 200 msec. The findings strongly suggest the presence of a bypass tract.  

In the Emergency Department the patient was promptly intubated and cardioverted. ECG post-cardioversion is as follows: 

• Typical WPW pattern (left antero-lateral bypass tract)

Two days later EP studies confirmed the presence of a left lateral bypass tract, that was successfully ablated.
The following is the post-ablation ECG. The WPW pattern is no longer present.