Extradural bupivacaine-loaded microspheres and spinal cord blood flow in the chronically instrumented rabbit

The local anaesthetic agent bupivacaine decreases spinal cord blood flow. This study documents the effect of bupivacaine-loaded polylactic acid microspheres, a formulation allowing sustained release, in the awake rabbit. Animals received at random extradural injection (1 ml) of either 5 mg of plain bupivacaine (n = 6) or 5 mg of bupivacaine-loaded microspheres (n = 6). Arterial blood pressure, cardiac output and spinal cord blood flow, using radioactive microspheres, and arterial blood gas tensions were measured before (T0) and 30 (T30) and 120 (T120) min after the extradural injection. Motor blockade was assessed on a four-point scale every 5 min until complete recovery. All rabbits experienced paralysis of hindlimbs. Those receiving bupivacaine recovered completely before T120, and the block duration was 86 +/- 23 min (mean +/- SD), whereas 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) 100g(-1) and did not vary significantly throughout the experiment. Lumbar spinal cord blood flow decreased in both groups, at T30 with bupivacaine (-16 +/- 7%) and at T120 with microspheres (-12 +/- 8%). Arterial blood 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 polylactic acid microspheres can prolong motor blockade from bupivacaine. Whatever its formulation, and independently of systemic haemodynamic and blood gas variables, bupivacaine leads to a decrease in spinal cord blood flow, which persists for the duration of motor blockade. This suggests that motor block must be avoided in clinical practice when extradural bupivacaine is used for pain relief.