C340 pilot reported a pressurization failure resulting in a hypoxic condition.

Attributes
ACN	1569896
Time
Date	201808
Local Time Of Day	1201-1800
Place
Locale Reference	ZKC.ARTCC
State Reference	KS
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Cessna 340/340A
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	Pressurization Control System
Person 1
Function	Captain Single Pilot
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Commercial
Experience	Flight Crew Last 90 Days 16.25 Flight Crew Total 727 Flight Crew Type 125.1
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Published Material / Policy

Narrative:

Was enroute at 23;000 ft MSL over kansas when I started to feel strange. I looked at the pressure differential and it was I think at cabin pressure of 10;000 ft. I was at 23;000 ft for a little over an hour when the controller thought I was getting hypoxic and had me descend to 10;000 feet MSL. In the descent I put on supplemental oxygen from an oxygen tank on board with a mask. At 10;000 feet I felt normal again and continued the flight at 10;000 ft. I have never been to altitude chamber that was recommended to me but now I agree that was a terrific suggestion. I checked the pressurization system and it is not working correctly and am getting it repaired. I think the controller was well trained to realize hypoxia conditions and was correct in having me descend to a level that doesn't require supplemental oxygen. I now know what hypoxic conditions fell like and will be more aware on other flights.

Original NASA ASRS Text

Title: C340 pilot reported a pressurization failure resulting in a hypoxic condition.

Narrative: Was enroute at 23;000 ft MSL over Kansas when I started to feel strange. I looked at the pressure differential and it was I think at cabin pressure of 10;000 ft. I was at 23;000 ft for a little over an hour when the controller thought I was getting hypoxic and had me descend to 10;000 feet MSL. In the descent I put on supplemental oxygen from an oxygen tank on board with a mask. At 10;000 feet I felt normal again and continued the flight at 10;000 ft. I have never been to altitude chamber that was recommended to me but now I agree that was a terrific suggestion. I checked the pressurization system and it is not working correctly and am getting it repaired. I think the controller was well trained to realize hypoxia conditions and was correct in having me descend to a level that doesn't require supplemental oxygen. I now know what hypoxic conditions fell like and will be more aware on other flights.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.