[rafe33]

There is a theory that the rapid ascent to 45,000 feet was done to deprive the passengers of oxygen and incapacitate them.

[jimktrains2]

From what I've read that theory is bullocks because 45k is still in the safety margins the plane was designed for.

[knowaveragejoe]

Wikipedia puts the service ceiling at 43,100 ft (13,140 m).

I imagine it could surpass that though without depressurization.