Absorption of bupivacaine, etidocaine, lignocaine and ropivacaine into n-heptane, rat sciatic nerve, and human extradural and subcutaneous fat.

Absorption of four amide local anaesthetics, including a new experimental agent, ropivacaine, in n-heptane, rat sciatic nerve and human extradural and subcutaneous fat was studied in vitro. The relative n-heptane/buffer (37 degrees C) partitioning of bupivacaine:etidocaine:lignocaine:ropivacaine was 10:39:1:2.9. The absorption of bupivacaine and etidocaine into nerve tissue was identified at steady state, but in extradural and subcutaneous fat etidocaine was taken up significantly more than bupivacaine. The lowest uptake was with lignocaine, and at steady state its mean concentrations were similar in all three tissues. There was a significantly higher concentration of ropivacaine than lignocaine in all tissues. Mean tissue concentrations of bupivacaine in sciatic nerve and subcutaneous fat, on the other hand, were significantly higher than those of ropivacaine. The ratios of relative mean uptake (mg g-1), at steady state, of bupivacaine, etidocaine, lignocaine and ropivacaine were: sciatic nerve 3.3:4:1:1.8, extradural fat 4.1:8.3:1:2.3 and subcutaneous fat 3.8:10.6:1:1.9, all of which were clearly lower than the theoretical n-heptane/buffer partitioning ratios.