Cardizem lowers the HR, and could help potentiate vagally mediated AFIB, in that it could accentuate HR drops after exertion or upon eating/digestion.
This drug also slows the excitabilty of the electrical signal being fired through the SA node, and decreases electrical conduction through AV node. So coming out of AFIB having too much Cardizem in your system can cause too low a HR upon restoration of NSR, even to the point of necessitating an emergent pacemaker.