Chronobiology of epidural ropivacaine: variations in the duration of action related to the hour of administration.

BACKGROUND A temporal pattern of the kinetics of local anesthetics is demonstrated in dental and skin anesthesia, with an important variation in the duration of action related to the hour of administration. The aim of this study is to determine whether the hour of injection influences the duration of epidurally administered ropivacaine during labor. METHODS One hundred ninety-four women in the first stage of labor were assigned to one of four groups throughout the day period: group 1 (night: from 1:01 to 7:00 am), group 2 (morning: from 7:01 am to 1:00 pm), group 3 (afternoon: from 1:01 to 7:00 pm), and group 4 (evening: from 7:01 pm to 1:00 am). Each patient received 14 ml ropivacaine, 0.17%, epidurally, and analgesia duration was measured. RESULTS Pain assessed by a visual analog score was not differ-ent among groups before the first injection of local anesthetic. Analgesia duration was greater in the diurnal period (group 2: 110 +/- 25 min and group 3: 117 +/- 23 min) compared with the nocturnal period (group 1: 94 +/- 23 min and group 4: 91 +/- 23 min) (P < 0.01). The largest intraday variation of analgesia duration among groups reached 28%. CONCLUSIONS Epidural analgesia duration exhibits a temporal pattern with important differences among diurnal and nocturnal phases. The authors emphasize that the lack of consideration of the chronobiologic conditions in epidural analgesia studies may create significant statistical bias. Future studies dealing with epidural local anesthetics should consider the time of drug administration.