INTRAVENOUS
ANESTHETIC AGENTS
Important pharmacological
characteristics for anes-thetic management using IV anesthetics are shown in
Table 25.1. The principal use of these drugs is to induce anesthesia. In
typical anesthetic management, the last event that the patient remembers is the
insertion of the needle. Patients are often unaware that other anesthet-ics,
most frequently in the form of inhalational drugs, are necessary to maintain
the anesthesia.
Among the barbiturates ,
three com-pounds, thiopental sodium (Pentothal
Sodium), thiamy-lal sodium (Surital),
and methohexital sodium (BrevitalSodium),
are useful as induction agents, as supplemen-tal drugs only during short
periods when surgery re-quires increased depth of anesthesia, or as maintenance
hypnotics for short surgical procedures. These drugs are termed ultra–short-acting agents, since their
rapid entry into the CNS is followed by relatively rapid redistribu-tion of the
drug to indifferent tissues, such as skeletal muscle. Because of their slow
rate of metabolism, these agents, when used in large repeated doses or by
contin-uous infusion, cause persistent hypnosis or subtle men-tal cloudiness.
All three IV barbiturates
rapidly produce unconscious-ness. 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 maneu-vers with facial grimaces or arm and leg
movements and with potentially dangerous changes in blood pressure and heart
rhythm. Consequently, induction of anesthe-sia may be nearly the only
indication for thiopental. However, if thiopental is to be used to maintain
anes-thesia 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 practi-tioner. 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 bar-biturates are similar, methohexital in particular may
provide some advantages in selected situations. Its du-ration 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 ex-ample, it can be used for basal sedation in the few mo-ments
that a very painful stimulus is applied, and then, as consciousness is quickly
regained, the surgeon can as-sess the results by talking to the patient.
Cardiovascular depression may
occur after the admin-istration of barbiturates by IV bolus. The hemodynamic
changes are transient in the healthy patient with good cardiovascular reserve,
but they may be prolonged and/or not well tolerated in elderly patients or
those with poorly compensated myocardial function. For ex-ample, thiopental
decreases myocardial contractility and dilates capacitance vessels, thereby
reducing ve-nous return to the heart. The healthy normovolemic patient may
compensate for these changes by an in-crease in heart rate to maintain stroke
volume and blood pressure. The patient with myocardial disease or hypovolemia
may not be capable of appropriate com-pensation. Serious ischemic impairment of
the my-ocardium may occur in patients with coronary artery disease.
Respiratory depression also
may occur after the ad-ministration of barbiturates by IV bolus. Respiration
may be further compromised by barbiturate-induced laryngospasm, as it is with
most anesthetics.
There is some tendency of the
ultra–short-acting bar-biturates to precipitate at biological pH once they are
in-jected, especially if the injection solution is not given slowly enough to
allow the drug to be diluted by the ve-nous blood. If inadvertent intraarterial
injection occurs and drug precipitates are formed, arterial thrombosis,
vasospasm, local ischemia, and possibly tissue sloughing may occur.
Methohexital precipitation is less common, since it is a more potent
barbiturate and can be provided in a more dilute solution. Barbiturate
solutions must not be coadministered with acidic solutions, such as those
containing meperidine, morphine, or ephedrine.
Most of the adverse reactions
associated with the use of the intravenous barbiturates are predictable and
therefore can be controlled or avoided. Some reactions, such as
hypersensitivity, are entirely unpredictable. Particularly patients with
asthma, urticaria, or an-gioedema may acquire allergic hypersensitivity to the
barbiturates. Acute intermittent porphyria is an ab-solute contraindication to
the use of barbiturates.
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