A320 SHORTLY AFTER LAST MIN APCH ARR CHANGE ON DSCNT INTO LAX.

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ARR TO LAX; ORIGINALLY FILED THE CIVET 4 ARR TO LAX. SHORTLY BEFORE CIVIT INTXN WE WERE ADVISED TO EXPECT THE MITTS ONE ARR. I BELIEVE WE WERE AT OR HAD BEEN CLRED TO FL180 PRIOR TO BEING ADVISED OF THE MITTS ARR. SHORTLY AFTER THE MITTS ADVISORY WE WERE CLRED BY SOCAL APCH TO DSND ON THE MITTS ONE ARR. I WAS PNF THE CAPT WAS PF. THE CAPT SELECTED 8000 FT IN THE ALT WINDOW. THIS WAS THE DSCNT LIMIT ON THE ARR AT DENAY INTXN. I CONFIRMED 8000 FT. I CONFIRMED AT OR ABOVE 12000 FT AT SNRKL INTXN THE FIRST ALT CONSTRAINT AFTER CIVIT. THE COCKPIT WAS A BUSY PLACE AT THIS POINT. WE HAD JUST SCRAMBLED TO REPROGRAM THE MITTS ARR AND RWY 24R INSTEAD OF CIVIT AND RWY 25L. BOTH OF US WERE STILL SHUFFLING APCH CHARTS. AFTER CIVET INTXN AT APPROX 14000 FT I WAS MOSTLY HEADS DOWN PERFORMING DSCNT AND APCH CHK DUTIES AND CONFIRMING THE CHANGES PROGRAMMED INTO THE MCDU. WHEN I LOOKED UP WE WERE A COUPLE OF MI BEFORE SNRKL INTXN AT APPROX 11000 FT INSTEAD OF 12000 FT. THE CAPT CAUGHT THE ALTDEV AND MENTIONED IT. I CONFIRMED 12000 FT AT SNRKL. WE CROSSED SNRKL AT APPROX 11300 FT. APCH CTL THEN MENTIONED THE ALTDEV. THE CAPT THEN ANSWERED THE RADIO CALL ACKNOWLEDGING THE DEV TO APCH CTL. I'M NOT SURE BUT THE CAPT MAY HAVE GONE TO AN OPEN DSCNT INSTEAD OF A MANAGED DSCNT. ON THE A320 AN OPEN DSCNT WOULD BE AN IDLE DSCNT TO THE 8000 FT IN THE ALT WINDOW. A MANAGED DSCNT WOULD HAVE HONORED CONSTRAINTS. HIGH WORKLOAD TIMES IN THE COCKPIT REQUIRE EXTRA VIGILANCE IN PERFORMING YOUR DUTIES.