Has anyone ever experienced a shut down of part of their autonomic nervous system while on DMT ? If so were you made consciously aware of it ? Do you think this is possible ?

what do you mean with this? autonomic nervous system is responsible for loads of things like breathing, heart rate and so on.. what do you think dmt could 'shut down' ?

From personal experience, I thought this was possible only with ketamine or cocaine.

Sniff through one nostril; make half of the brain asleep while the other half remains conscious.

But now that I think of it; autonomous system shutdown ? I think you mean consciousness shutdown, if the autonomous were to shutdown, you'd stop breathing.

But, whatever the substance, I think the autonomous goes in hyper-drive rather than down. When the adrenaline kicks in (as it does in most out of body experiences), everything sorts of accelerates. The autonomous system is what regulates our body automatically. You can loose track of your breathing, forget your body and all, but I think it's the consciousness that slips away from your body. Also, in medical terms; the autonomous is more synonym with hormone regulation than breathing or heart beating. So I'm wondering if you don't mean something else than the autonomous system ?

But yeah ! Definitely, if your body shuts down, I'm convinced any decent entity noticing that happening will bring you back home. But maybe that depends on where you roam... ?

there are times it seems your breathing is stopping
you must remember when you feel that not to get scared
just consciously breath deep breaths in and out and you will get through it
i know it can be unnerving but dont let it scare you

Exactly what I mean , this happened to me and I was told by some "entities" to breath , I even argued with them that it was not possible for the very reasons you gave , they insisted I was wrong . I was immediately conscious of my body and sure enough I wasn't breathing . I don't know for how long I was "away" for , but I started to gasp for air and was fine . I am not sure if I was holding my breath or what ?
Fear never entered into this equation nor did I feel I was in danger at any time . Considering how many times I have oobied over the years I found this event interesting .

Years ago when I used to practice Raja Yoga , part of the exercise was to practice stopping the breath and heart . The best I could do after years of practice was to stop breathing for an extended period of time and get my heart rate way , way down . I know we can consciously intervene with our autonomic system through practice , but to have this happen on it's own was strange indeed.

A further update on this as to what really happened . Let me state right upfront it was NOT a autonomous system shutdown , so no worries there , your all safe at least for now.

I went back in today to find out what went down yesterday with the breathing thing , it turns out the reason I wasn't breathing was because my airway was restricted due to the odd position I was laying in . I was on a bed with the back of my head propped up too much with a pillow . When I entered hyperspace relaxation my head went down enough to block my airway , like what happens when you snore .

Kinda strange the more I think about it , how these entities knew I wasn't breathing and I didn't yet . I don't know if I would have come out of it by myself or what , lesson learned though.

Be careful in there !

you have to 'turn off' your brainstem (with all medulas oblongatas), this would mean death.
other question, is it possible to get a heart failure during the experience?
think it's more possible, cause it all depends on your psychological traits.