The post cardioversion ECG of the second case is unavailable but might look similar to the one above, depending on the location of the bypass tract.Literature: This paper established adenosine as safe in ventricular tachycardia and led to new ACC/AHA guidelines which state that adenosine may be given in regular wide complex tachycardia:
Marill KA et al. Adenosine is safe and effective for differentiating wide-complex supraventricular tachycardia from ventricular tachycardia. Crit Care Med 2009 Sep 37:2512.
Summary of points
1) Any fast rhythm which worries you may be treated with electrical cardioversion. If confused, use electricity. If the patient is unstable, use electricity.
2) AV nodal blockers are only contraindicated when there is atrial fib with WPW
3) In regular tachycardias due to WPW (even wide ones!), AV nodal blockers are safe and effective. They block the limb of the re-entrant rhythm which goes through the AV node, thus interrupting the circuit.
4) Atrial fib with WPW is very recognizable: there are bizarre QRS with multiple morphologies, and very fast rhythms with short R-R intervals. If you can find any R-R interval shorter than 240 ms, then AV nodal blockers are definitely dangerous.