Answer - Case of the month (Dec 2011)
Q: After the first two complexes the QRS complex changes. What could explain this?
A. Axis rotation due to left anterior hemiblock
B. Intermittent pre-exitation
C. Sodium channel blocker infusion
D. Intermittent right bundle branch block
Answer: B. Intermittent pre-exitation. See http://en.ecgpedia.org/wiki/Wpw for more information about pre-exitation.