How does this have anything to do with their liability? If they used excessive force under the circumstances (and if the guy didn't resist then they did) then their actions were the proximate cause of his heart attack. It doesn't matter if he would have had one next week or next year.
One would have to argue that police acted with an almost-unprecedented degree of excessive force to transform a healthy heart into a damaged heart in the course of an armed traffic stop-and-hold.
That would very dramatic courtroom fare and would definitely enhance the plaintiff's chances of a favorable verdict as well as a much larger award.