Does Abnormality In ECG Show Up While On Beta Blockers?

My partner is having SVT symptoms but her ecg scans showed no problems with her heart and she has been refered to a specialist, i questioned the fact that was taking beta blockers at the time to counter the raised BPM - she was admitted to hospital with a rate 270bpm release with a course of beta blockers and to have a ultrasound and a ECG the doctors are saying there is nothing showing up on the ecg but surely if beta blockers level and lower the heart BPM then surely they wouldnt see the issues while on beta blockers can you help this has been going on for months now since january - UK westmidlands she is 24 Female extremely health eater , was suffering with acute stress from work but has been off work now for 3 months