If he were waiting for Saïx's heart rate to increase he would be waiting days at the very least. Another of those odd quirks of not having a proper heart, and one that he, at least, didn't mind.

He hadn't quite been expecting the feint however, and while he didn't have enough time to properly bring his sword around it was easy enough to put his arm in between Remus and his intended target. He wasn't about to let his throat be torn out that easily - or indeed, at all, even if he would probably manage to live through it.