Cerebrospinal fluid and spinal cord distribution of baclofen and bupivacaine during slow intratbecal infusion in pigs

Background: Increasing numbers of patients are receiving chronic intrathecal infusions of local anesthetics, baclofen, opioids, and other analgesics via implanted pumps. These infusions typically deliver drugs at rates measured in microliters per hour. However, to date, there have been no studies aimed at characterizing drug distribution within cerebrospinal fluid (CSF) and spinal cord during these slow infusion rates. Therefore, this study was designed to address this knowledge gap. Methods: Anesthetized pigs were instrumented with eight intrathecal microdialysis probes placed at multiple points along the neuroaxis in both the anterior and posterior intrathecal space to permit continuous CSF sampling for measurement of bupivacaine and baclofen concentrations. Animals were divided into three groups and received bupivacaine and baclofen infusions at 20 or 1,000 mu l/h or as a 1,000-mu l bolus over 5 min every hour. Drug administration continued for 8 h, at which time the animals were killed, and the spinal cord was removed and divided into 1-cm-long sections that were further divided into anterior and posterior portions for measurement of bupivacaine and baclofen concentrations. Results: In all groups, drug concentration in CSF and spinal cord decreased rapidly as a function of distance from the site of administration, with most drug found within a few centimeters. In addition, there were significant anterior-posterior differences in both CSF and spinal cord drug concentrations. Conclusions: During slow intrathecal infusion, drug distribution in CSF and spinal cord is severely limited in all groups, although significantly more so in the 20-mu l/h infusion group.