Person from Porlockis right. You should minimise any time above 25000 - above that there is a real and serious riskof decompression sickness. Any subaqua divers around? They'll tell us all about bends, chokes, creeps, staggers etc caused as the N2 in the blood comes out of solution on depressurisation.
The only time I did a chamber run above 25000, we prebreathed 100% O2 for 60 minutes to purge N2.
You really don't want to be mucking around up there.