Dear Tibber's Mom,
Without evaluating your son, I cannot say for sure exactly what is going on with him. However, I can say that supraventricular tachycardia (SVT) is an infrequent but well known (to us cardiologists, at leasts) finding in newborns as well as in older children. In the face of a structurally normal heart, it does not cause significant problems UNLESS it is allowed to continue for many hours at a time without a break. The medicaitons that we use for it, including beta blockers such as propranolo, are quite well tolerated and are not known to cause any developmental problems, even with long term use.
As a pediatrician, I am a somewhat concerned about your response to his SVT. Again, without evaluating him specifically, I cannot speak to your son's individual response. But, typically SVT does not cause sudden cardiac death. Therefore, quitting your job and making sure that you are no more than 20 feet from him seems to be a response of hyper-concern. If your cardiologist is not familiar with neonatal SVT, it may be worthwihle obtaining a second opinion, so that you can get the appropriate guidance and reassurance in the care of your child and so that you and he can have a normal growth relationship without him feeling like he is overprotected.