Safe and Toxic Dosage of Local Anesthetics

Acute poisoning by local anesthetics is fairly common, especially with cocaine, and is apt to be fatal. Some accidents are due to excessive doses through mistakes or faults of technique, but the susceptibility varies greatly owing to differences of absorption and of destruction, and to varying response to the actions. Fright plays a considerable part. 

Probably all commonly used local anesthetics have produced fatal accidents. These depend not only upon the absolute dose, but also on the rate of absorption, on idiosyncrasy, and on other conditions. The Local Anesthetic Committee of the American Medical Association has made recommendations that should minimize the accidents. The following are the most important. Procaine appears the safest of the more widely used local anesthetics and may be employed for subcutaneous and submucosal injections, but the concentration should not exceed 1%. Cocaine and butyn should not be injected under the skin or mucous membranes but restricted to surface application. The total quantity of cocaine should not exceed 0.06 to 0.1 g (1 to 1 1/2 grains). The patient should be recumbent if the operation permits. With nervous patients, it is advisable to inject morphine 15 min before the local anesthetic and to delay the start of the operation until 20 min after the injection of the local anesthetic. Urethral injections are especially dangerous, and should be avoided if there is trauma or stricture. 

Strict attention should be given to prevent the confusion of procaine and cocaine. The solutions should be kept in different kinds of bottles. Cocaine solution may be distinguished by tinting with sodium fluorescein, 1 mg/g of cocaine. This would also show the concentration of the solution. 

The treatment of poisoning by local anesthetics should begin with prevention and the selection, dosage, and technique of the administration gross errors and carelessness have caused many deaths. The previous administration of a sedative, especially of the barbituric series, diminishes the risk by suppressing the convulsions and their interference with respiration, so that animals survive one and a half to four times the ordinary fatal dose of cocaine or procaine if administered hypodermically. 

Clinically, 0.2 g of phenobarbital or 0.6 g of sodium barbital may be administered an hour before operation. Barbiturates do not prevent the direct circulatory collapse and depression of respiration that occur on intravenous injection of procaine, and are useless or harmful for either prophylaxis or treatment if the anesthetic agent is rapidly absorbed. However, if the symptoms develop slowly, the suppression of the convulsions is at least helpful. 

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