Ultra-short-acting methylated oxybarbiturates (methohexital and hexobarbital) and thiobarbiturates (thiopental, thialbarbital, and thiamylal), short-acting oxybarbiturates (pentobarbital), and long-acting oxybarbiturates (phenobarbital).
The barbiturates are widely used and versatile. They are classified as sedative-hypnotics on the basis of their use in humans to produce sedation. In veterinary anesthesia, they are used primarily to induce and maintain general anesthesia. An important distinction between the actions of the sedative-hypnotic barbiturates and the inhalational anesthetics and the actions of other drugs described in this chapter is that progressive increases in the dose of barbiturates and inhalational anesthetics produces progressive depression of the CNS and all physiologic functions, which leads to the loss of consciousness and even to death.
The barbiturates are classified according to their duration of action. The ultra-short-acting barbiturates are commonly used to induce anesthesia before maintenance with the inhalational anesthetics. The amount necessary is less than that required for surgical intervention; all that is necessary is anesthesia of adequate depth to produce sufficient muscle relaxation for tracheal intubation. A likely dose for this purpose is approximately 8–12 mg/kg. The final dose depends on the degree of sedation produced by drugs used for preanesthetic medication. Deep sedation produced by opioids requires lower induction doses of ultra-short-acting barbiturates than are required when no preanesthetic is used.
The ultra-short-acting thiopental, thialbarbital, and thiamylal are used principally to induce anesthesia; they are followed by maintenance with an inhalational agent or used alone for short periods.
The long-acting phenobarbital is used primarily in anticonvulsant therapy or for prolonged sedation.
The short duration of the ultra-short-acting thiobarbiturates is the result of a rapid decrease in plasma and therefore brain concentration. The rapid decrease is due to rapid distribution into nonnervous tissue—initially into highly perfused visceral tissues (heart, brain, liver, spleen, etc.) and then into muscle and fat. The final elimination of the drug is through metabolism and renal excretion of metabolites. The ultra-short action of the methylated oxybarbiturate methohexital is attributed to rapid metabolism, rather than redistribution into nonnervous tissues. The short-acting barbiturate pentobarbital is biotransformed in the same way as