Blood Pressure, but Not Cerebrospinal Fluid Fentanyl Concentration, Predicts Duration of Labor Analgesia from Spinal Fentanyl

Background: There is a wide variability in dilution of drugs in cerebrospinal fluid after spinal injection, as measured near the site of injection. With local anesthetics, there is a wide variability in speed of onset, which correlates with block duration. The authors tested whether local cerebrospinal fluid drug concentrations and onset time would predict duration of analgesia from spinal fentanyl in laboring women. Methods: After written informed consent, fentanyl (50 mu g) was injected using the combined spinal epidural method in 56 women requesting analgesia for labor, The stylet was reinserted in the spinal needle, and 60 s later, the cerebrospinal fluid was aspirated for fentanyl assay. Time to analgesia and duration of analgesia were recorded, and data were analyzed by linear regression. Results: Fifty-two women were included for data analysis. The cerebrospinal fluid fentanyl concentrations were 3.1 +/- 5.9 mu g/ml, with a 7-fold range (0,9-5.9 mu g/ml). Fentanyl concentration did not correlate with onset, initial sensory level at 5 and 10 min, or duration of analgesia. Decreased diastolic and increased systolic blood pressure and lower parity, but not fentanyl concentrations, correlated with longer labor analgesia. The resultant model was predictive when applied to data from four previous studies of spinal opioid analgesia duration. Conclusions: Contrary to our hypothesis, the local concentration of fentanyl in the cerebrospinal fluid 1 min after injection was not correlated with onset or duration of labor analgesia. The unexpected but consistent relationship between blood pressure and combined spinal epidural analgesia duration suggests that resting hemodynamic state affects the distribution and/or clearance of intrathecally administered opioids.