Beta blockers were a type of drug that slowed down a person's heartrate and pulse. A certain class of them could be used to slow down a pulse to the point that it would appear that someone was dead.
Metroprolol and atenolol were commonly available beta blockers that would slow down heartrate. Tetrodotoxin was a neurotoxin derived from a puffer fish that could cause the consumer to appear in a death-like state for days. (CTU Operations Manual)