Combinations of fentanyl and bupivacaine produced greater effects than those predicted by additivity at 15 min ( P < 0.001 ) and 30 min ( P = 0.015 ) ( mean differences , 9.1 [ 95 % CI , 4.1-14 .1 ] and 6.4 [ 95 % CI , 1.2-11 .5 ] units of the normalized response , respectively ) , indicating a synergistic interaction .