This 25-year-old woman in the past 4 years has had 5-6 episodes of fast regular palpitation, starting and ending abruptly, lasting 45-60 minutes:

• Sinus rhythm
• Wolff-Parkinson-White
• EP studies demonstrated an anteroseptal bypass tract.  

Comment: The bypass tract was very close to the AV node. Cryoablation was attempted but was unsuccessful. The procedure was stopped, because of the high risk of inducing AV block.  

Six months later the patient is seen in the Emergency Department complaining of palpitation and faintness that had started one hour earlier:

• Supraventricular tachycardia, 196/min
• Atrioventricular re-entrant tachycardia (more likely) or AV nodal re-entrant tachycardia
• Right bundle branch block aberration

There was no response to vagal stimulation. The arrhythmia subsided after IV injection of Adenosine:

• Sinus rhythm has been restored. Heart rate 83/min
• Right bundle branch block in no longer present
• There are two wide QRS complexes. Comparison with ECG #1 suggests intermittent conduction through a bypass tract (although premature ventricular complexes originating from the right ventricle cannot be ruled out}

Comment: 12 leads of rhythm ECG display would allow us to see the wide QRS complexes in all leads and be helpful in the differential diagnosis (see next tracing)

Twelve lead rhythm strip:

• The pattern of the two wide QRS complexes is the same as that of the QRS in ECG  #1. This favours the diagnosis of intermittent conduction though the anteroseptal bypass tract.