I Have Been Waking From Deep Sleep With Rapid Heart

I have been waking from deep sleep with rapid heart beats 138-140. I have been on atenolol beta blocker for 30years for migraines. This only ever happens in early hours of mornings an hav now had 4episodes 10 days apart. My blood pressure is normal but rises considerably with these attacks. I went to the ER an was monitored but heart had slowed by the time anything was recorded. 2 attacks have been 30-40 mins and 2 10mins long. I have no other health issues an not over weight