View source for Puzzle 2005 4 156 - Answer

{{NHJ|
|mainauthor= '''A.A.M. Wilde, R.B.A. van den Brink'''
|edition= 2005:4,156
}}
[[Image:Puzzle_2005_4_156_fig1.jpg|Figure 1|thumb]]
[[Image:Puzzle_2005_4_156_fig2.jpg|Figure 2|thumb]]

An otherwise healthy 32-year-old male was referred
with palpitations. Attacks had been occurring
monthly for several years, predominantly at the start
of exercise (he is an enthusiastic sportsman), lasting
minutes to days. Sotalol had been prescribed but
almost immediately discontinued several months
previously due to bradycardia. The patient was on no
other drugs, and rejected further pharmacological
treatment. Physical examination, stress-test and cardiac ultrasound were all normal. Atrial fibrillation (AF) had
been documented repeatedly, and the patient was
scheduled to undergo pulmonary vein (PV) isolation.
Figure 1 shows his preablation ECG; figure 2 is typical
of all postablation ECGs during the three months of
follow-up. 

'''What is the obvious difference, and how
could you explain it?'''