We had one do that a year or so back and he recovered brilliantly.
The Vet, the one and only Alistair McLean, simply crushed up a whole heap of Quinine Tablets, added some water and periodically tubed him with the resultant mixture until his heart went back to it's normal beat.
Amongst its many handy aspects including anti-malarial as well as anti-inflamatory and anelgesic properties, Quinine is also an anti-arrhythmic which means that it supresses/corrects abnormal heart rhythms, the dreaded atrial fibrilation being one such example. Here ends today's excerpt from Chemistry 101.