Studies on the Distribution of Lidocaine in Tissue After Spinal and Intravenous Application in Rabbits: Clinical and Forensic Toxicological Aspects

Distribution of lidocaine in tissue was investigated after spinal anesthesia and intravenous application in rabbits to examine aspects of forensic and clinical toxicology. Respiratory arrest occurred at 5-15 min after injection of 5 mg/kg hyper-baric lidocaine solution into the subarachnoid space followed by standing in the head down position. The minimum concentration of lidocaine in the middle part of the cervical spinal cord of the dead rabbits was 197 micrograms/g. On the other hand, maximum concentration in the same site at the critical state of almost complete respiratory arrest was 175 micrograms/g. The time at which the concentration of lidocaine reached a maximum in plasma and various tissues after subarachnoid administration was 10-15 min. The concentrations in the lumbar spinal cord, a site of administration, were 1200-1500 micrograms/g 5 min after the injection, which decreased gradually to about 180-250 micrograms/g at 60 min when paralysis of the hind legs began to recover. According to these data, the concentration of lidocaine in the spinal cord of rabbits required for neural blockade was presumed to be about 200 micrograms/g. Since the maximum concentrations of lidocaine in each tissue was already observed at 2.5 min after intravenous injection, transport of lidocaine from plasma to tissues was considered to be very rapid. Lidocaine accumulates most rapidly in the spleen and kidney, followed by the brain, lung, heart, spinal cord and plasma. The concentration in the liver was the least because of its hepatic metabolism. Distribution to adipose tissue was slower than that to other tissues, but the highest accumulation was observed at the late stage. Elimination rates of plasma and tissues other than adipose were almost the same.