[Are combinations of local anesthetics dangerous?]

Although local anaesthetic mixtures are often used, there is often no scientific basis for this. Interactions between the local anaesthetics in the mixture can even increase their toxicity. Such interactions may be due to the physical chemical properties of each constituent (this is not a real problem for amide local anaesthetics), to a mutual displacement from the binding sites on the plasma proteins or to an additional effect on membrane receptors. This displacement would seem to benefit the least toxic drug: the toxicity of local anaesthetic mixtures may therefore be reduced; for example, lidocaine and mepivacaine are displaced by bupivacaine. Moreover, the fraction of drug which leaves the blood vessel is greater than the serum free fraction. During chronic administration, neurological toxicity and free fraction are narrowly linked. When using local anaesthetic mixtures, it must be remembered that potency and toxicity of a local anaesthetic are closely correlated and that the toxic effects are, at least, additive.

Pharmacokinetics of unchanged morphine in normal and cirrhotic subjects

Published reports of morphine pharmacokinetics differ, perhaps because of analytical discrepancies and different study protocols. A highly specific radioimmunoassay (N-carboxymethylnormorphine RIA) was used to measure unchanged morphine in plasma in six normal subjects and in eight cirrhotic patients with hypoalbuminemia, hyperbilirubinemia, and prolonged prothrombin time. All subjects received a single intravenous bolus of morphine hydrochloride (0.1 mg/kg). Peripheral blood samples were collected at frequent intervals from 2 min to 36 hr after injection. In cirrhotic patients as compared with normal subjects, terminal half-life of unchanged morphine was longer (201 +/- 39 vs 111 +/- 32 min (mean +/- SD), P less than 0.01) and total body clearance was slower (21 +/- 7.5 vs 33.5 +/- 8 ml X min-1 X kg-1, P less than 0.05). The apparent volume of distribution and the steady-state volume of distribution did not differ in the two groups. In conclusion, cirrhotic patients have a prolonged terminal half-life of unchanged morphine that is attributable to a decrease in total body clearance.