I Have Been Diagnosed By A Heart Specialist As Having

I have been diagnosed by a Heart specialist as having Tachycardia, where my heart beats extra 3 beats in between beats I've had a cardio exercise stress test which produced ECg changes ST drpession & Chest pain I cant remember why he has given me a script to take Atenolol AN Tablets 50mg .05 daily
I am having a CTCA investigation tomorrow Is there any need to start the Atenolol tonight or should I wait
The pain in my chest eases with rest