Patients who have Wolf-Parkinson-White syndrome (WPW) , have a condition in which the electrical impulse may return to the upper chamber of the heart over an abnormal extra connection that typically has been present since birth. The returning electrical impulse may once again go down the "normal" conduction system, creating an additional heartbeat. Each time this cycle repeats itself, the heart beats. Rapid heart beating is simply the repeating of this kind of cycle many times. There are many ways to treat it, 1)beta blockers or you can get an ablation which burns away the extra pathway and is curative whereas the medication is just preventative but doesn't always stop an attack of a racing heart. My uncle has WPW and I was recently diagnosed with avnrt which I have had successfully removed with an ablation. He takes a beta blocker a day and this seems to control his symptoms although he has been left with side effects.
With WPW it is possible for the heart to reach really high levels of beating, (not the case in the majority of cases)
The best way to determine a patient's level of risk is to initiate this chaotic rhythm controlled setting of an in the electrophysiology laboratory and determine its effect.
If she is young I would recommend going for an ablation to get rid of this problem and help her to resume a normal life without having to take drugs. If I can be of any more help please shout...
Hope this helps.