This 45-year-old man was referred to the EP clinic because of episodic palpitations since he was a teenager, occuring once or twice a year and lasting from a few minutes to one hour. Recently, his palpitations have been more frequent, and are now occuring once monthly. While in the waiting area, the patient experiences a typical episode of his palpitations:

• Narrow QRS tachycardia, 180/min
• Incomplete right bundle branch block (iRBBB)

The P wave appears closely after the QRS, negative in the inferior and precordial leads. 
Consider Atrioventricular Re-entrant Tachycardia (AVRT) or AV Nodal Re-entrant Tachycardia (AVNRT).

With carotid sinus stimulation, the patient converted to sinus rhythm and immediately felt more comfortable:

• Sinus rhythm with one premature atrial complex
• Wolff-Parkinson-White

The arrhythmia in the first ECG was Atrioventricular Re-entrant Tachycardia (AVRT). The presence of a positive delta wave in V1, negative delta wave and QRS in III and aVF, and a positive delta wave in II identifies a right lateral bypass.