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Timing of women's labor may determine effectiveness of pain medication

September 16, 2005

WINSTON-SALEM, N.C. - Natural daily body rhythms may influence the effectiveness of spinal-epidural pain medication for women in labor, according to new research from Wake Forest University Baptist Medical Center. The study found that women who had day labor got longer pain relief than women with night labor with the same amount of labor pain medicine.

"Our research shows that the time of day that the medication is delivered is important in determining its effectiveness," said Peter H. Pan, M.D., an obstetrical anesthesiologist. "In the future, doctors may consider time of day, as well as patient's weight and other factors, when determining the best dose or method of drug delivery."

The study's goal was to determine if the body's natural internal rhythms can affect the effectiveness of pain medication for laboring women. The study, reported in the current issue of Anesthesiology, involved 77 women who were in labor with their first babies and requested spinal-epidural pain medication.

The research is part of the field of chronobiology that studies biologic rhythms. These rhythms include the circadian rhythm, the 24-hour body clock, and the ultradian rhythm, shorter-than-24-hour-cycles, which determine sleeping and eating patterns and can also affect blood pressure, heart rate and body temperature. Some rhythms are controlled by sunlight and others are controlled internally.

The researchers found that women who labored between noon and 6 p.m. got an average of 27 percent longer pain relief from a single dose of the spinal-epidural medication fentanyl than women whose labor was between 8 p.m. and 2 a.m. Fentanyl is one of the commonly used drugs for labor pain relief in the United States. However, this is one of the first studies to examine whether there are chronobiological variations in its effectiveness for labor pain relief.

"The right treatment given at the wrong time can be ineffective or create a crisis of escalating toxicity," wrote Dominique Chassard, M.D., and colleagues with Hotel-Dieu Hospital in France in an editorial accompanying the study. "Conversely, even a weak treatment, if given at the right moment, could prove surprisingly effective."

Pan said that knowing more about how time of administration influences the drug's effects could have significant practical implications for how it is prescribed and delivered. For example, the dose could be tailored to the time of day or a smart, programmable, time-released version of the drug could be developed to optimize desired drug effects and minimize undesired side effects both for women in labor and patients with other pain syndromes.

Pan's co-investigators were Sherman Lee, M.D., and Lynn Harris, B.S.N, both with Wake Forest Baptist.

Wake Forest University Baptist Medical Center