ACR HAS ALTDEV OF 500 FT DURING EMER DSCNT.

Narrative

CONTACTED CTR AND DECLARED MEDICAL EMER ON BOARD AND REQUESTED IMMEDIATE DSCNT FOR LNDG AT CHS. CTR CLRED US TO FL240 (FROM FL330) THEN TO FL230. I IMMEDIATELY INITIATED AT FLT IDLE DSCNT. THE FLT ATTENDANT THEN CAME TO THE FLT DECK FOR THE EMER MEDICAL KIT WHICH IS LOCATED BEHIND MY SEAT (FO SEAT). SHE COULDN'T RELEASE THE KIT AND NEEDED ASSISTANCE. CTL OF THE ACFT WAS THEN POSITIVELY SWITCHED TO THE CAPT WHILE I HELPED THE FLT ATTENDANT. THE KIT WAS RELEASED; THE FLT ATTENDANT EXITED THE FLT DECK AND I WAS THEN GIVEN CTL OF THE ACFT; STILL IN THE DSCNT TO FL230. THE AUTOPLT WAS ENGAGED. HOWEVER; I WAS UNAWARE THAT THE CAPT EXTENDED THE SPD BRAKES. THE AUTOPLT INITIATED A SLOW RATE OF LEVEL OFF WHICH DID NOT SUCCESSFULLY 'CATCH' THE PROGRAMMED FLT LEVEL OF FL230. I DISENGAGED THE AUTOPLT AT APPROX 200 FT LOW (22800 FT) AND EXCESS NOSE DOWN TRIM CONTINUED THE ACFT TO 22500 FT AT WHICH TIME A CLB BEGAN BACK TO FL230. 500 FT WAS THE EXTEND OF THE OVERSHOOT AND THE ENTIRE DEV LASTED APPROX 10-15 SECONDS (FROM FL230 BACK TO FL230). AFTER LEVELOFF THE CAPT POINTED OUT THE SPD BRAKES AND THEY WERE RETRACTED. NOTHING OF THE DEV WAS MENTIONED BY CTR. THE SUBSEQUENT DSCNT AND LNDG WAS UNEVENTFUL. SUPPLEMENTAL INFO FROM ACN 272913: HOWEVER; THAT ITEM SLIPPED BY CALLING A DEV OF 500 FT. NEXT TIME I WILL TELL FO IF SPD BRAKE IS OUT WHEN TRANSFERRING ACFT CTL. OVERALL WE WERE VERY PLEASED WITH THE RAPID DSCNT/DIVERSION.