Why does Rust keep checking for his pulse?

It's a grounding technique I've seen suggested for people who have survived traumas. Basically a bio-feedback mechanism to remind you that you are alive/settle you after an emotionally volatile experience.

Rust is obviously messed up because of things he saw as a cop. His daughter was also killed in a car accident. Rust in general shows signs of maladaptive coping, such as his copious use of drugs and alcohol, never sleeping, and so on. He's a guy who has seen a lot of shit but who is too stubborn to seek help or express how he feels...hence why the season finale was so effective, because it's one of the few times we see him open up.

I always saw this as a part of Rust's detached, analytical behavior. He's checking his pulse to get an objective idea of himself and the state he's in.

For instance, for the first scene mentioned (altercation with Marty), I can imagine a few different interpretations:

• Rust is willing himself to calm down. Though he didn't show it (at all), the confrontation actually got his heart racing. But Rust is committed to being almost pathologically detached. Being flustered won't do. So he checks his pulse to see when he's back to his normal heart rate.

• Rust is curious: Did this tense situation even register? Is his pulse any higher than if he was staring out a window? Rust being Rust, he doesn't actually know if it affected him. So he seeks an objective metric: Is my heart racing?
  I actually imagine him finding that he's completely unaffected, and concluding (cynically but correctly) that he really is pretty messed up. It's not that he's committed to being detached; he's incapable of being otherwise.

• A mix of the two above: Rust knows he's been affected by the situation, but how much? How much impact did this have on a scale of 60-100 bpm? He's trying to supplement his subjective experience with objective measurement. He's not trying to tamp down the emotional/physiological response as in the first interpretation, nor is he unaware of it, but he is trying to reason about it.

For the bikers's raid, the same three interpretations could work.

For the last scene, I'd say the first interpretation seems more likely. He messed up: he didn't act rationally. In a sense, his body betrayed his mind. So he's trying to find his way back to his level-headed, intellectual self.

Regardless of his reasons for checking his pulse, it's a fantastic "mannerism" to give the character. It seems like a very clinical thing to do in emotionally-charged situations, and so underscores how Rust just isn't like other people.