I used to work in cardiology/pacemakers/defibs - so yep, I'm familiar
. Ideally, you'd want more info than what the vet gave you, but it's probably okay or the vet would've said something. There is a little node in the heart called the Sinus Node - this is where electrical activity begins (in a healthy heart) to cause a heart beat. Sinus Arrhythmia, in a nutshell and w/out getting too techy, is when there is a variance in cycle-lengths (i.e. duration of ONE heartbeat) upon inhaling and exhaling. In other words, the beat is slightly faster when inhaling, than when exhaling.
I'm sure the vet will always keep an eye on it, and it really is not much to worry about unless there is a change.
Wylie had some dysrhythmias as a puppy and actually outgrew them, which is not uncommon.