FLC OF CPR LTT HAS PRESSURIZATION FAILURE. EMER DSCNT ACCOMPLISHED.

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CRUISING AT FL370 A CHANGE IN PRESSURIZATION WAS NOTED BY EAR. CHKING CABIN ALT GAUGE; CABIN RATE PEGGED AT 2000 FPM CLB. ATC WAS ADVISED AND LOWER ALT WAS REQUESTED PENDING COCKPIT INVESTIGATION FOR CAUSE. FL350 WAS ASSIGNED AND DSCNT TO SUCH WAS ACCOMPLISHED. AS COCKPIT INVESTIGATION REVEALED NO SOLUTION TO THE PROB; AN EMER WAS DECLARED WITH ATC. AT FL350 WE MAY HAVE DEVIATED 200- 250 FT BELOW ASSIGNED ALT TRYING TO CTL PRESSURIZATION LEAK. ATC ASSIGNED 13000 FT AND AN EMER DSCNT AS PER CHKLIST WAS CONDUCTED. PLTS WERE ON OXYGEN AND PAX MASKS WERE DEPLOYED AUTOMATICALLY AS CABIN CLBED PAST 10000 FT IN DSCNT. NO INJURIES WERE INCURRED. IN MY OPINION; BOTH CREW AND PAX HANDLED EMER ABOVE AND BEYOND NORMAL EXPECTATIONS. ALL REMAINED CALM AND COLLECTED THROUGHOUT A DIFFICULT SIT. CAUSE OF PRESSURIZATION FAILURE WAS ECU DUCTING DETACHMENT THEREBY ALLOWING LOSS OF CABIN PRESSURE VENTING OUT IN TAIL CONE. HOPEFULLY THIS INCIDENT WILL REINSTILL THE IMPORTANCE OF CHK AND DOUBLECHK TO MAINT RELATING TO HOW SERIOUS OVERSIGHTS CAN BE. EVEN THOUGH I BELIEVE THAT WE DID NOT DEV FROM OUR ASSIGNED ALT IN ATC EYES; HINDSIGHT WOULD SUGGEST USING THE AUTOPLT WHEN TROUBLESHOOTING A PROB. WITH REGARD TO POSSIBLE TFC CONFLICT ON DSCNT; I KNOW THERE WERE NO RA'S ON THE TCASII; BUT THAT IS ONLY A 6 MI CIRCLE AND ATC MAY HAVE DEVIATED OTHER ACFT. IF SO; I AM UNAWARE. I DO FEEL OUR ANNUAL SIMULATOR TRAINING PROVED VERY USEFUL IN DEALING WITH THIS EMER.