EXTRADURAL BUPIVACAINE-LOADED MICROSPHERES AND SPINAL-CORD BLOOD-FLOWIN THE CHRONICALLY INSTRUMENTED RABBIT

The local anaesthetic agent bupivacaine decreases spinal cord blood fl ow. This study documents the effect of bupivacaine-loaded polylactic a cid microspheres, a formulation allowing sustained release, in the awa ke rabbit. Animals received at random extradural injection (1 ml) of e ither 5 mg of plain bupivacaine (n=6) or 5 mg of bupivacaine-loaded mi crospheres (n=6). Arterial blood pressure, cardiac output and spinal c ord blood flow, using radioactive microspheres, and arterial blood gas tensions were measured before (T0) and 30 (T30) and 120 (T120) min af ter the extradural injection. Motor blockade was assessed on a four-po int scale every 5 min until complete recovery. All rabbits experienced paralysis of hindlimbs. Those receiving bupivacaine recovered complet ely before T120, and the block duration was 86+/-23 min (mean+/-SD), w hereas the block duration was 210+/-60 min (P<0.05) in those receiving the microsphere formulation. Left renal blood flow baseline was 470+/ -179 (mean+/-SD) and right renal blood flow was 464+/-177 ml min(-1) 1 00g(-1) and did not vary significantly throughout the experiment. Lumb ar spinal cord blood flow decreased in both groups, at T30 with bupiva caine (-16+/-7%) and at T120 with microspheres (-12+/-8%). Arterial bl ood pressure decreased more rapidly with bupivacaine (-20+/-14% within 15 min) than with microspheres (-19+/-11% within 30 min) and returned to baseline at T120 in both groups. Cardiac output and arterial blood gas tensions did not change significantly. We conclude that polylacti c acid microspheres can prolong motor blockade from bupivacaine. Whate ver its formulation, and independently of systemic haemodynamic and bl ood gas variables, bupivacaine leads to a decrease in spinal cord bloo d flow, which persists for the duration of motor blockade. This sugges ts that motor block must be avoided in clinical practice when extradur al bupivacaine is used for pain relief.