Update is slow

 Hi,

The following query takes more than 50 mins to update 8000 rows,it has index on all the columns  in the query,but however it does full table scan in ECG_STUDY...any idea in rewriting this query,

Tuning advisor has no recommendatins for this query ,..,the version is 11.2.0.,3

UPDATE ecg_study es

   SET es.r_mc_ecg_confirm_status =

       (SELECT DECODE(ps.ps_ecg_confirm_type,

                      NULL,

                      173,

                      DECODE(ps.ps_ecg_confirm_type, 168, 172, 173))

          FROM /*ecg_study es,*/ protocol_study ps

         WHERE es.ps_protocol_id = ps.ps_protocol_id(+)

           AND es.r_mc_ecg_confirm_status NOT IN (172, 173, 174))

-Thanks