Not that I've actually ever had to do what you describe but were I in your shoes I would tend to gloss over the moment of the traumatic event happening quite quickly. Making the happening quite matter of fact will get you (and your protagonist) through chronologically. Then, the detail, the horror, the physical impact, can be examined at length as the protagonist's brain allows the full detail to leak back into the brain in dreams, moments of reflection, unwanted recollection of detail etc.
The traumatic experience is not the trauma. It's an experience, in circumstance, that leads to trauma. Often in the middle of such events adrenaline and instinct kick in to get the physical body out of immediate danger. Peppering a matter-of-fact detail of "what is happening" with soupy visceral descriptions of heart-beating, mouth-drying adrenaline high: semi-automatic scrambling away from the next source of danger; the constant head-whipping scan for the next dangerous noise; trying to take in everything whilst painfully conscious of the poor vision provided by everything not in the direct field of vision, should get you through the chronicle of the times.
The bite is going to settle in when all is calm, after the fact. That's when the horror starts.
The key word here is "extreme" when applied to the trauma. I think a lot of genre writers, particularly in the horror arena try to deal with common or garden trauma and Indoril's reply gives you the dramatic convention for dealing with trauma. I would warn you that there's been a lot of horror written in the last thirty years and this method of conveying trauma has become something of a trope. This could be what's troubling you so I'll move on.
You specifically refer to extreme trauma, and the scene you describe reminds me of the rather operatic murder of Frank Castle's entire extended family in the movie "The Punisher" from a few years ago. To a certain extent that movie is about the operatic notion of The Punisher as a character. The idea that his mission is born out of circumstances so ridiculously heightened that it is almost impossible for the human brain to contemplate a reaction to them.
I think that there's a scale of trauma which really depends upon the cause of that trauma. The state of mind where one finds oneself obsessing over little details is part of the process of discovering the body of one's loved one hacked up in your kitchen upon returning from work (for example). Your system floods with adrenaline but it has no where to go, you're on high alert but all is quiet. This is the experience of PTSD where people's minds have conditioned them to go into high alert at the slightest stimulus.
If danger is present many descriptions of perception centre on people becoming robotic, although they still get the PTSD effect later at the time survivors tend to describe a mental space of "just acting".
That's in the case of a single dangerous traumatic situation being dealt with by a human being with a strong survival instinct.
In "extreme" circumstances, I would imagine that the personal danger and threat would escalate to a level where rational thought would cease to be possible if one is to survive. In such event I would imagine that perception would shut down into a sort of systemic tunnel vision where, if someone is to survive, their hindbrain just kicks in and wipes out both ego and superego. The really intense part about such a situation would be that a person would be capable of doing anything that furthered their survival while in that state. To a certain extent their own identity would be compromised.
A person in this state of mind would be able to just kill any person that threatened them, ignore the dire straits of loved ones, and lose all awareness of pain.
Whether such a mental state exists is debateable, even if anyone has a capacity for it they wouldn't be the norm. Not only that but the likely effect on the psyche when the sense of identity returns would probably leave someone who experiences it mentally ill for the rest of their life, possibly unable to effectively communicate what happened to them.
However, we are writers and we lie to entertain others. The business of dramatic license gives us the power to examine such cases as if they could be sensibly articulated; we could even invent a person who eventually bounced back from, or even leveraged, such a mental state, such as we are told Viking warriors experienced the baresark during battle. The question of what happens when humanity tries to cultivate such an animalistic state has been explored in some werewolf fiction, these are places to consider the extremity of your protagonist's mental state upon a scale from "heightened awareness" to "running on instinct".