Pharmacological Actions

All three IV barbiturates rapidly produce unconsciousness. Since unconsciousness is attended by amnesia without either analgesia or skeletal muscle relaxation, anesthetized patients may react to painful stimuli but are unaware and do not remember the procedure. For example, patients undergoing short surgical procedures with thiopental alone may respond to surgical maneuvers with facial grimaces or arm and leg movements and with potentially dangerous changes in blood pressure and heart rhythm. Consequently, induction of anesthesia may be nearly the only indication for thiopental. However, if thiopental is to be used to maintain anesthesia for short operative procedures, analgesia should be provided with other drugs.

Thiopental remains the most popular IV induction agent. Its rapid and pleasant induction of anesthesia and its relatively low cost are among the reasons for its high acceptance rate by both the patient and the practitioner. Also, it does not induce obstructive secretions in the airway, produces little or no emesis, and does not sensitize the myocardium to endogenous cate-cholamines that may be released in response to the stress of surgery. It can, however, cause cardiovascular depression.

Although the pharmacological actions of the IV barbiturates are similar, methohexital in particular may provide some advantages in selected situations. Its duration of action is only half as long, and it exerts fewer cumulative effects than does thiopental. The occasional requirement of intraoperative communication between the patient and surgeon is easily satisfied with metho-hexital because of its short duration of action. For example, it can be used for basal sedation in the few moments that a very painful stimulus is applied, and then, as consciousness is quickly regained, the surgeon can assess the results by talking to the patient.

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