What Causes Possible Vagus Nerve Fainting?

I have Marfan syndrome and possible vagus nerve fainting and almost fainting where by I get pounding in back of head, weakness in legs and almost black out, I have had my Mitral valve replaced, I have a defribullator due to having v-tach also had 3 cardiac ablations for afib and aflutter with rapid ventricular response. Could vagus nerve be causing fainting? Maryeliz