Chronobiology of subarachnoid fentanyl for labor analgesia.

BACKGROUND Chronobiology studies the recurrent biologic rhythms that directly affect how an organism interacts with its environment and how its environment affects the organism. The purpose of this study is to determine whether the time of administration influences the analgesic duration of the commonly used subarachnoid fentanyl for labor analgesia. METHODS After institutional review board approval and informed consent were obtained, 77 healthy nulliparous women in active labor requesting neuraxial analgesia were assigned to one of two groups, based on the time of combined spinal-epidural analgesia placement: the day group, for the time period from 12:00 to 18:00, and the night group, for the period from 20:00 to 02:00. Combined spinal-epidural analgesia was performed with 20 microg subarachnoid fentanyl. An epidural catheter was inserted but not dosed until patients requested further analgesia. Dynamic data were recorded at 5-min intervals for 20 min initially and then every 15 min. The analgesic duration was defined as the time from subarachnoid fentanyl injection to the time the patient requested further analgesia. RESULTS Seventy evaluable patients completed the study, with 35 per group. Patient demographics, visual analog pain scale scores, and labor characteristics were similar between groups, but the duration (mean +/- SD) for subarachnoid fentanyl labor analgesia was 92 +/- 34 min for the day group and 67 +/- 21 min for the night group (P < 0.001). CONCLUSIONS The results indicate that chronobiology of subarachnoid fentanyl plays a significant role of up to 27% difference in labor analgesic duration between the two administration time periods studied. Chronobiology should be incorporated in future comparative studies or analysis of previous studies on subarachnoid fentanyl.