Anesthesia/anesthetics analgesics

Ketamine is an excellent anesthetic, analgesic, and sedative in specialized settings. It can be used in patients who cannot tolerate barbiturates, in settings where cardiovascular depression must be avoided, and in patients with refractory bronchospasm. It is useful in one-lung ventilation, asthmatic patients, or when there is need for an intramuscular route of administration. It is very helpful in specific situations such as the need for anesthesia in uncontrollable, mentally retarded patients. It can be simply mixed with syrup if oral premedication is desired. 

Etomidate (Amidate), a nonbarbiturate, is used for induction of anesthesia Etomidate also may be used to supplement other anesthetics, such as nitrous oxide, for short surgical procedures. It is a hypnotic without analgesic activity. 

TIVA has become feasible thanks to the introduction of agents with a suitably short duration of action, including the injectable anesthetics propofol and etomidate, the analgesics alfentanil und remifentanil, and the muscle relaxant mivacurium. These drugs are eliminated within minutes after being adminster-ed, irrespective of the duration of anesthesia. 

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. 

Antipsychotic agents are also used for a diversity of other indications like hiccups, Tourette s syndrome, aggressive behavior in children and the elderly and alcohol withdrawal syndrome. Some of them are also used in anesthesia as they can potentiate the sedative, analgesics or anesthetic effects of other agents. Antipsychotics which are mainly active by blocking dopamine activity have also an effect on chemoreceptor trigger zone and may therefore be used as anti-emetics. 

Propofol can be used for induction as well as maintenance of anesthesia. It is very lipophilic and induction of anesthesia takes place within 30 seconds. After a single dose the patient awakes in approximately 5 minutes and after anesthesia by continuous intravenous administration of longer duration recovery may take 10-15 minutes. It can be used in combination with the usual range of premedications, analgesics, muscle relaxants and inhalation anesthetic agents. 

Halothane, isoflurane, and enflurane have similar depressant effects on the EEG up to doses of 1-1.5 MAC. At higher doses, the cerebral irritant effects of enflurane may lead to development of a spike-and-wave pattern and mild generalized muscle twitching (ie, myoclonic activity). However, this seizure-like activity has not been found to have any adverse clinical consequences. Seizure-like EEG activity has also been described after sevoflurane, but not desflurane. Although nitrous oxide has a much lower anesthetic potency than the volatile agents, it does possess both analgesic and amnesic properties when used alone or in combination with other agents as part of a balanced anesthesia technique. 

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. 

Etomidate is a carboxylated imidazole that can be used for induction of anesthesia in patients with limited cardiovascular reserve. Its major advantage over other intravenous anesthetics is that it causes minimal cardiovascular and respiratory depression. Etomidate produces a rapid loss of consciousness, with minimal hypotension even in elderly patients with poor cardiovascular reserve. The heart rate is usually unchanged, and the incidence of apnea is low. The drug has no analgesic effects, and coadministration of opioid analgesics is required to decrease cardiac responses during tracheal intubation and to lessen spontaneous muscle movements. Following an induction dose, initial recovery from etomidate is less rapid (< 10 minutes) compared with recovery from propofol. 

Analgesic efficacy and clinical use Alfentanil is a shortacting potent opioid with analgesic and anesthetic properties (Larijani and Goldberg, 1987). It is less potent than fentanyl but administration can be better controlled. It is mostly used as a supplement to general anesthesia or as a primary anesthetic e.g. in cardiac surgery. Intra-... 

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). 

Additions of 1/10 to 2 1/2% of urea to dilute procaine solutions increase materially their potency, to ten times, on direct application to nerve trunks, presumably by favoring penetration. The effect is not due to alkalinization. The action of local anesthetics is reported to be increased by intravenous injection of methylene blue, by the local application of caffeine or theophylline, by morphine, and by the antipyretic analgesics. Cocaine anesthesia is said to be ineffective in tissue impregnated with oxalic acid. Inflamed tissues are less susceptible to local anesthesia, probably because of their difficult penetration. 

Several drugs are used intravenously, alone or in combination with other drugs, to achieve an anesthetic state (as components of balanced anesthesia) or to sedate patients in intensive care units who must be mechanically ventilated. These drugs include the following (1) barbiturates (thiopental, methohexital) (2) benzodiazepines (midazolam, diazepam) (3) opioid analgesics (morphine, fentanyl, sufentanil, alfentanil, remifentanil) (4) propofol (5) ketamine and (6) miscellaneous drugs (droperidol, etomidate, dexmedetomidine). Figure 25-2 shows the structures of... 

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. 

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