My son has Supraventricular Tachycardia SVT, he also has benign hypotonia?

Hi all, my son was born 4 weeks early via emergency c-section (he is now 4 months). After 45 minutes of not finding a heartbeat using the dopla machine and ultrasound, being told that i had lost him and enduring much dispair, I felt him kick. I asked to be checked again and his heartbeat was detected and strong. At birth his apgar score was 7 and 8 (at 5 minutes) and his blood gasses were fine.
On his second day of life his heart started to beat up to 280 beats per minutes which has been diagnosed as supraventricular tachycardia (SVT). His SVT is now successfully controlled by Atenolol (hopefully he will grow out of it in one year). He also has benign low tone/hypotonia which tends to get worse when his dose of Atenolol is increased.He is reaching most of his milestones and appears to have intact intelligence. When I hold him he seems really wriggly.He is seeing a physio.
Has any one had any similar experiences? Or a baby on atenolol and did it have any drowsy effects?
Thanks J

Atenolol does have side effects such as drowsiness but does its job well. As for the SVT, he might never have an attack again, or, if he does, it can easily be treated with a catheter ablation - a 2-5 hour procedure in which a catheter (small narrow line) is directed up to the heart via a vein in the groin area. Once there, they restart the arrhythmia and ablate (destroy) a small area that is responsible for the problem.

Has the cardiologist ruled out Wolfe-Parkinson-White? There are in many cases surgical ways of correcting frequent spells of SVT. They thread a wire into the heart and cauterize the area of the hearts electrical conduction system that is causing the reentry SVT to happen. Good Luck!