As we know people who stay in coma or unconsciousness "experience" timeless,space-less, absolute emptiness and total detachment from physical world. Even though I used the word "experience" will not suitable to describe the situation in coma or unconsciousness state. It is a condition with no happiness or sorrow, no pain,no darkness or light,no noise or sound etc....and finally we can say "unconditional state of mind." and closer to description of "Nirvana"
In theravāda, this question's base-knowledge is ucchedadiṭṭhi, diṭṭhadhammanibbāna [nihilism&hold the doctrine of happiness in this life], rūpa-attato-sakkāyadiṭṭhi [form (the body) to be the self].
- what happen when person die during the coma or unconsciousness state?
In abhidhamma, person still has mind's arising until person's citta-ja-rūpa, such as breathe, still going on. But because of karmma (kamma), temperature (utu), and nutrient (āhāra) are not good enough to support mind to done mind's job, such as making-nerve-impulse (citta-ja-rūpa), etc. So, the tool of the scientist can't catch mind's arising of this person.
That's the reason why some vegetative state can awake again, after long sleeping.
- Is this similar to Nirvana? (as total detachment of world.)
Definitely no. Because buddhā said in Sutta. Saṃ. Sa. Dvādasamaṃ āḷavakasuttaṃ:
(The Blessed One:)
- “With faith you cross the flood
and with diligence the ocean
With effort end unpleasantness
and with wisdom purify yourself.”