Yeah, no, he wasn't on any drugs... that's the first thing I looked for, because between coke and PCP, the officer may well have been justified in tasing him repeatedly in three minutes. Drugs're bad, mmkay? 'Specially for cops.
But drugs like that also do bad things to the heart, and what the flying hell was he doing tasing someone directly on the chest? You don't introduce voltage to the heart if you can avoid it, and according to the story (which may or may not be correct, since I don't trust third-party reporting fully, especially in cases like this) he was on the ground, prone, and cuffed. If you assume he was dragged out onto his back, and you assume they didn't drag him out on his stomach, roll him over, and then tase him, a leg would still have been a safer bet. It would have kept you away from a possible headbutt or teeth. I would be interested to hear his excuse for hitting the chest, as basic electrode safety states, again, that you do not introduce voltage to the heart, and the only excuse I can think of for doing so would be a rush attack... not because the perp didn't get up quick enough.
Note that it's the cop's boss that is claiming they thought he was on drugs. Given the allegations of corruption, that smells like CYA to me - especially since the bossman is possibly responsible for putting this guy on the road with a physically restricted partner. That's another thing I can't get over - why would you do that? Why would you send a cop out with another cop who has physical restrictions due to recent bypass surgery? Why? That's just asking for trouble!
*grumbles, settles down, and tries to disengage rant mode*