i have this condition, was born with an extra nerve in my heart which sometimes kicks in, top speed is about 200 bpm. i grew up in formby so was seen at alder hey as a child for diagnosis. i used to have quite a few attacks as a child, some just seconds, or minutes, a couple lasting hours. they used to stop by me just calming down, my mum used to help sooth me through an attack.
once, though, but i was a bit older and not living at home, i couldn't get it to stop, and ended up going to hospital and having an injection to stop it. i did take verapamil once (i don't think verapamil is a beta blocker, beta blockers are usually taken daily, i only took verapamil to stop an attack), i didn't like it at all. normally, when it stops, it's straight away. the verapamil slowed it down gradually, i remember feeling really strange when it was happening, quite scared and ill for the next 24 hours.
the attacks are much less frequent nowadays, if it's ever a problem again, they now do keyhole surgery to burn away the extra nerve. i didn't have any issues with it during childbirth either (not that your ds will have to worry about that!) hth