Anesthesia/anesthetics adjuncts

Fentanyl (0.001 mg/kg i.v.) can be used with xylazine (0.44 mg/kg i.v.) for anesthetic premedication. Fentanyl is sometimes used as an anesthetic adjunct during inhalation anesthesia to improve analgesia. The pharmacokinetics of fentanyl make it ideal for administration by constant rate infusion and it can be administered intra-operatively at a rate of 0.001-0.004 mg/kg/h after a 0.001 mg/kg loading dose. To prevent excitement or locomotory stimulation in recovery, the infusion should be discontinued 30 min prior to recovery or the horse should be sedated with xylazine (0.1 mg/kg i.v.) prior to recovery. 

One of the earliest demonstrations of polymorphic metabolism was made by Kalow and colleagues at the University of Toronto. They were interested in examining why some patients appeared to be exquisitely sensitive to succinylcholine, a skeletal muscle relaxant that was widely used in anesthesia as an anesthetic adjunct. They found that BChE activity (then referred to as... 

Midazolam is a rapidly metabolized benzodiazepine (p. 228) that is used for induction of anesthesia. The longer-acting lorazepam is preferred as adjunct anesthetic in prolonged cardiac surgery with cardiopulmonary bypass its am-nesiogenic effect is pronounced. 

For use as a narcotic analgesic supplement in general or regional anesthesia. For administration with a neuroleptic such as droperidol as an anesthetic premedication, for induction of anesthesia and as an adjunct in maintenance of general and regional anesthesia. 

Benzodiazepines—including diazepam, lorazepam, and midazolam—are used intravenously in anesthesia (see Chapter 25), often in combination with other agents. Not surprisingly, benzodiazepines given in large doses as adjuncts to general anesthetics may contribute to a persistent postanesthetic respiratory depression. This is probably related to their relatively long half-lives and the formation of active metabolites. However, such depressant actions of the benzodiazepines are usually reversible with flumazenil. 

In the last two decades there has been increasing use of intravenous anesthetics in anesthesia, both as adjuncts to inhaled anesthetics and as part of techniques that do not include any inhaled anesthetics (eg, total intravenous anesthesia). The properties of some of the commonly used intravenous anesthetics are summarized in Table 25-1. Unlike inhaled anesthetics, intravenous agents do not require specialized vaporizer equipment for their delivery or facilities for... 

Recovery is sufficiently rapid with most intravenous drugs to permit their use for short ambulatory (outpatient) surgical procedures. In the case of propofol, recovery times are similar to those seen with sevoflurane and desflurane. Although most intravenous anesthetics lack antinociceptive (analgesic) properties, their potency is adequate for short superficial surgical procedures when combined with nitrous oxide or local anesthetics, or both. Adjunctive use of potent opioids (eg, fentanyl, sufentanil or remifentanil see Chapter 31) contributes to improved cardiovascular stability, enhanced sedation, and perioperative analgesia. However, opioid compounds also enhance the ventilatory depressant effects of the intravenous agents and increase postoperative emesis. Benzodiazepines (eg, midazolam, diazepam) have a slower onset and slower recovery than the barbiturates or propofol and are rarely used for induction of anesthesia. However, preanesthetic administration of benzodiazepines (eg, midazolam) can be used to provide anxiolysis, sedation, and amnesia when used as part of an inhalational, intravenous, or balanced anesthetic technique. 

Respiratory depression becomes an important side-effect when opioids are used for postoperative pain treatment, since the anesthetic agent and most adjuncts of anesthesia induce a long-lasting depressant effect on respiration, which can increase the opioid effects up to respiratory arrest. Therefore careful supervision of respiration during the postoperative period is mandatory (Mulroy, 1996). Opioid-induced respiratory depression can be interrupted by the opioid antagonist naloxone. 

Analgesic efficacy and clinical use Fentanyl (Clotz and Nahata, 1991) is a potent analgesic and anesthetic compound. It is used for the treatment of severe acute and chronic pain, as a pre-medication or adjunct to anesthesia and as a primary anesthetic for the induction or maintenance of anesthesia. In combinations with neuroleptics e.g. droperidole, it induces a pain free and calm state known as neuroleptanalgesia (Foldes, 1973). In this condition, surgery can be performed in an awake patient, who is able to cooperate with the surgeon. 

Analgesic efficacy and clinical use Sufentanil (Rosow, 1984 Monk et al., 1988) is a very potent fentanyl analog with analgesic and anesthetic properties and a more rapid onset and a shorter duration of action. It is used for perioperative analgesia, short duration anesthesia and as an adjunct to various anesthetic procedures including neuroleptanalgesia (Isaacson, 1992). 

Adjunctive use of potent opioids (eg, fentanyl and related compounds) contributes cardiovascular stability, enhanced sedation, and profound analgesia. Other intravenous agents such as the benzodiazepines (eg, midazolam, diazepam) have slower onset and recovery features and are rarely used for induction of anesthesia. However, preanesthetic administration of benzodiazepines can be used to provide a basal level of sedation and amnesia when used in conjunction with other anesthetic agents. 

General anesthetics are rarely given alone. In addition to the analgesic agents just mentioned, benzodiazepines (midazolam, Versed diazepam, Valium ) are commonly used as adjuncts for the relief of anxiety, amnesia, and sedation prior to induction of anesthesia. Neuromuscular blockers (e.g., succinylcholine or pancuronium) can also be administered during the induction of anesthesia to relax skeletal muscles. 

Phencyclidine (PCP) is a potent veterinary analgesic and anesthetic it is sometimes used iUicitly by humans in cases of drug abuse, leading to serious psychological disturbances. Ketamine is a rapid-acting general anesthetic and anesthesia adjunct, administered intramuscularly and intravenously. 

The ability to dry up bronchial secretions and reduce laryngospasms (induced by some general anesthetics) has been the reason for using atropine and scopolamine as presurgical medication. It should be mentioned that scopolamine, which differs from atropine by the (3-6,7-epoxy bridge (Fig. 8-13), while it generally parallels atropine s pharmacological spectrum, does not share its cerebral and medullary stimulation rather it exhibits CNS depression and amnesia, properties applied to anesthesia in an adjunct capacity. 

Alfentanil, an opiate analgesic (8 to 50 mcg/kg IV), is indicated as an adjunct to general anesthetic in the maintenance of general anesthesia with barbiturate, nitrous oxide, and oxygen. In addition, it is used as a primary anesthetic for induction of anesthesia when endotracheal intubation and mechanical ventilation are required. 

Droperidol is used as an adjunct for induction and maintenance of general anesthesia and as an anesthetic in diagnostic procedures. Droperidol, which has antiemetic properties, causes marked sedation and potentiates the CNS depressant effects of alcohol, hypnotic-sedatives, and numerous psychoactive agents. Droperidol is absorbed well through an IM injection—sedation begins in 3 minutes, peaks at 30 minutes, and lasts for 2 to 4 hours. Droperidol is metabolized by the liver to p-fluoro-phenylacetic acid and p-hydroxypiperidine, and its metabolites are excreted in urine and feces. 

Methohexital can be used alone as an anesthetic for short procedures. It may also he used as an inducing agent or adjunct to regional anesthesia. 

A general anesthetic is usually given with adjuncts that augment specific components of anesthesia, permitting lower doses of general anesthetics with fewer side effects. 

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