BE35 PLT ATTRIBUTES HDG AND ALTDEVS AT 13000 FT TO STRESS AND HYPOXIA.

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I WAS RATHER ANXIOUS ABOUT A PREVIOUS ENG SPUTTERING INCIDENT BUT DECIDED IT WAS BETTER TO FLY NOW WITH GOOD WX THAN TO WAIT IN HOPES THAT MY ANXIETY WOULD SUBSIDE. I ACCORDINGLY PLANNED A 1-DAY SKI TRIP TO A HIGH ALT ARPT. ON PREFLT; I NOTICED THAT MY OXYGEN BOTTLE WAS EMPTY BUT I KNEW THAT MY DEST ARPT COULD BE REACHED COMFORTABLY WITH ONLY A 10 MIN VISIT TO 13000 FT. I TOOK OFF AND 45 MINS INTO MY IFR FLT PLAN WAS DIRECTED TO CLB AND MAINTAIN 15000 FT. I TOLD ATC THAT I NEEDED A LOWER ALT BECAUSE I DID NOT HAVE OXYGEN AND WAS VECTORED TO THE 13000 FT AIRWAY AND TOLD TO TURN TO 080 DEGS. BELIEVING THAT THE VOR WAS TO MY L; I RECALL THINKING; '080 DEGS IS 10 DEGS L OF N' AND PROCEEDED TO STEER 350 DEGS. WHEN CTR ADVISED I WAS WELL OFF COURSE; I COULD NOT UNDERSTAND HOW AND CONTINUED TO MORE CAREFULLY MAINTAIN MY HDG OF 350 DEGS. CTR ADVISED THAT MY DIRECTIONAL GYRO MUST NOT BE WORKING PROPERLY AND PROCEEDED TO GIVE ME RELATIVE HDG CHANGES TO GET ME IN SIGHT OF MY DEST ARPT AND HANDED ME OFF TO THE TWR. WHEN I LANDED; THERE WAS A MESSAGE WAITING FOR ME TO CALL THE CTR SUPVR WHO ADVISED ME HOW BADLY I HAD ADHERED TO ASSIGNED HDGS AND ALTS. UNDER NORMAL CIRCUMSTANCES; I KNOW THAT I CAN FUNCTION WELL AT 12000 FT OR EVEN 13000 FT WITHOUT OXYGEN. I DISCOVERED THAT; WHEN ANXIOUS OR STRESSED; THE LEVEL AT WHICH I CAN FUNCTION WITHOUT OXYGEN IS APPARENTLY A LOT LOWER.