If one is not able to sit, then he may pray lying down.  In that case, he should make gestures using his hands and head to indicate various Salah movements.  If it is too difficult to make any movement, then he can pray using his tongue abd eye movements, and if he is unable to do so but is still concious, then he should make his Slah in mind.

One shoud remind himself of a recitation that Allah has tought us in the Quran: 

Say: "Truly, my prayer and my service of sacrifice, my life and my death, are (all) for Allah, the Cherisher of the Worlds"  The (Quran - Al-An'am: 162)