THE EFFECT OF PRIOR DURAL PUNCTURE ON CISTERNAL CEREBROSPINAL-FLUID MORPHINE CONCENTRATIONS IN IN SHEEP AFTER ADMINISTRATION OF LUMBAR EPIDURAL MORPHINE

Combined spinal epidural anesthesia has become increasingly popular as a method of providing rapid onset of analgesia or surgical block with access for further administration of analgesics or anesthetics. No in vivo studies have evaluated the relationship between dural puncture a nd drug transfer from the epidural space to the cerebrospinal fluid (C SF). To determine whether morphine administered in the epidural space adjacent to a dural puncture results in increased CSF concentrations a t the cisterna magna (CM), 12 adult ewes were studied. Each animal was assigned to one of three groups. Animals in Group 1 served as a contr ol and received no dural puncture. Animals in Group 2 received a dural puncture with a 25-gauge (G) Whitacre needle, while Group 3 animals r eceived a dural puncture with an 18-G Tuohy needle. One hour after dur al puncture, each animal was given epidural morphine, 0.2 mg/kg. Six h ours after the administration of epidural morphine, CSF from the CM wa s sampled and analyzed by gas chromatography-mass spectrometry for mor phine concentration. The mean morphine concentration at the CM for Gro up 1 (control) was 22 +/- 12 ng/mL, whereas animals with 25-G and 18-G dural punctures had concentrations of 154 +/- 32 ng/mL and 405 +/- 53 ng/mL, respectively (P = 0.0005). These data demonstrate that a signi ficant increase in CSF morphine concentration at the brainstem will oc cur when lumbar epidural morphine is administered adjacent to a dural puncture. Furthermore, the increase in CSF morphine concentration is p ositively correlated with the size of the needle producing the dural p uncture. These findings highlight the potential for delayed respirator y depression when epidural opiate administration follows a dural punct ure.