What Causes Low Heart Rate And Dizziness?

I am currently being monitored with a Zio (portable heart rate monitor) for two weeks for syncopal episode last Friday. At that time I was given fluids and sent home. Heart rate dropped into 50s and blood pressure increased to 140 sys. This kept happnening periodically so I went to the ER on Christmas day. My pulse got down to 41 at the lowest. They sent me home with a monitor. I keeping having waves of dizziness/lightheaded and I can see my pulse dropping into the 50s. This eventually passes after about a half hour. I have an appt tomorrow am with a primary care doctor to talk about this. It is believed to be caused from withdrawing from Effexor. I m wondering if it is dangerous for this to be happening so frequently. Doesn t it mean I m not getting enough oxygen if I m light headed? I don t have much fair in the ER I went to. Thanks