What Causes Rapid Heart Rate Along With Fatigue And Reduced Oxygen Levels?

Hello, For the past year, my 17 year old daughter has had episodes of fainting, heart pain, rapid heart rate with oxygen levels that drop. Her school caught one episode where her heart rate went up to 179 and her oxygen level went down to 80%; her doctor caught another episode where her heart went up to 237 without her fainting. She it taking 25 mgs of atenolol, but still does not feel well and has fainting spells. She has gone to a doctor that is conducting POTS research, who said she does not have the 3 antibodies that POTS patients have. Is this her heart? She wore a heart monitor for a month (last April), but the heart monitor was never on during an episode (also the fainting & rapid heart rate were not occurring as frequently last year). Do you have any suggestions? We don t know what to do next. I appreciate your help. Thank you, Teresa