Can this technology eventually detect their heartbeat, or is it just looking at slower changes over time? If the latter it sounds much simpler to defeat, if the former that would have many repercussions.

Live heart rate by video analysis would make things like televised court proceedings, congressional hearings, and news interviews much more invasive. Elevated heart rate is a sign of stress, and it wouldn't be long before people were jumping to conclusions over whether someone was lying or hiding their true feelings/intentions.