OMG I regularly went up to 63,000' on test flights, without too much blood bubbling I'm pleased to report. The physiological effects of altitude was well known to most of us, and dictated just about everything about the design philosphy for the environmental systems on Concorde.
These test flight excursions at 63,000 were when we used to check the intake surge margins by doing a near zero G pushover at Mach 2 from a zoom climb. (Which I mentioned in another Concorde thread). At the top of the 'bunt' the throttles would be pulled back, to prevent overspeeding; at the point of zero alpha the local Mach number at the intake face would be far higher than any other time, as well as wing flow distortion would also be at maximum, so hence the surge margin check. With all this going on you really did ask yourself if this was really an airliner.
Dude
