In EMS we use a stair chair:

http://www.med-worldwide.com/media/JU-JSA800B.JPG

It's somewhat more maneuverable than a standard wheelchair, and significantly less weight.

Typical chairs such as the one above are rated at 400-500 lbs.

As the name implies, it can be used (by either 2 or 4 rescuers, preferably with a spotter in addition though this is not necessary) to carry a patient up or down a flight of stairs, wheel the patient along a flat surface, or carry the patient across an uneven surface.

But as another poster mentioned, if the patient is dependent on oxygen that is likely to be a much greater limitation. A standard D tank will only last 15 minutes or so if operating into a non-rebreather. Using a nasal cannula, maybe 45 minutes to an hour (I've never fully emptied a D tank through a cannula so I don't know for sure).