Never found altitude much of a problem from the body point of view although some helicopters didn't like it. In New Guinea we use to regularly land at altitudes up to 12000 and for kicks I did one at 14000 in Bell 47s and Hiller 1100s. In my stupid youth I was once persuaded by a senior pilot at night to see how high a twin engined Wessex would go. At 18500 my sphincter muscles decided it was time to come down! The rotor rpm was a problem as on lowering the lever, the rpm wanted to shoot up. But to answer your query, I never found operating up to 12000 a problem although I was young and quite fit then plus we lived at 5000. However the lack of puff was noticeable but I never experienced vision problems.