A backup life support system in his chair kept him alive but in suspended animation.
Quote:
“ DOCTOR: No, no, no, we had that conversation last time we met. I'm more interested in your survival. I saw you destroyed. The Daleks blasted you at point-blank range.
DAVROS: Ha! There was damage to my primary life support system. The secondary and back-up circuits switch in immediately. Synthetic tissue regeneration took place whilst bodily organs were held in long-term suspension.”
