What Causes Frequent Variation In Heart Rates?

hi my name is denise, my son chad has had pericarditis 3 times in the past 2 years........they have only put him on toradol and some other medicine can not think of the name.........the first time with pericarditis he had a small ulcer, the second time his gallbladder was removed, and the third just stayed in with medication for a day, and sent home.......went to cleveland clinic sent home with a 48 hour heart monitor, came back with admorailites, his heart rate at the hosp would go into the low 40's and up into the 70's and a few times in the hundreds. he has horrible pain while he was in the hosp. always knowing when his heart rate was low,and can still tell when it goes down not sure what to expect, we are seeing a electropsyolgist next wk......my concern is i am not sure what to expect, worried about a pace maker he is only 23