Halothane characteristics

Enflurane has practically all the same characteristics as halothane and is used in the same sitnations. It is poorly absorbed. It is also prescribed under the name ethrane. 

Halothane was introduced into clinical practice in 1956. It was not the first fluorinated anaesthetic— fluoroxene (Fluoromar) holds that distinction—but it was the first to achieve widespread acceptability. Halothane is a fluorinated alkane 1-bromo, 1-chloro -2,2,2-trifluoroethane (Figure 3.2). It has a characteristic odour, similar to chloroform, and requires a stabiliser, thymol (0.01%), to prevent degradation by light. Halothane has a blood/gas partition coefficient of 2.4 able 3.2) but its lack of irritant qualities makes possible the use of relatively high inspired concentrations (2-4%). For that reason, inhalation induction is characteristically smooth and rapid. Compared to sevoflurane, and possibly isoflurane, recovery from halothane anaesthesia is delayed. 

The newer volatile anesthetics, desflurane and sevoflurane, have physicochemical characteristics (ie, low blood gas partition coefficients) that are favorable to a more rapid onset and shorter duration of anesthetic actions compared with isoflurane and halothane. However, both of these newer agents also have certain limitations. The low volatility of desflurane necessitates the use of a specialized heated vaporizer, and the pungency of the drug leads to a high incidence of coughing and sympathomimetic side effects that make it less than ideally suited for induction of anesthesia. 

The induction of unconsciousness may be the result of exposure to excessive concentrations of toxic solvents such as carbon tetrachloride or vinyl chloride, as occasionally occurs in industrial situations (solvent narcosis). Also, volatile and nonvolatile anesthetic drugs such as halothane and thiopental, respectively, cause the same physiological effect. The mechanism(s) underlying anesthesia is not fully understood, although various theories have been proposed. Many of these have centered on the correlation between certain physicochemical properties and anesthetic potency. Thus, the oil/water partition coefficient, the ability to reduce surface tension, and the ability to induce the formation of clathrate compounds with water are all correlated with anesthetic potency. It seems that each of these characteristics are all connected to hydrophobicity, and so the site of action may be a hydrophobic region in a membrane or protein. Thus, again, physicochemical properties determine biological activity. 

Crawford MW, Lerman J, Sloan MH, Sikich N, Halpern L, Bissonnette B. Recovery characteristics of propofol anaesthesia, with and without nitrous oxide a comparison with halothane/nitrous oxide anaesthesia in children. Paediatr Anaesth 1998 8(l) 49-54. 

Hilgenberg JC, Stoelting RK. Characteristics of succinyl-choline-produced phase 11 neuromuscular block during enflurane, halothane, and fentanyl anesthesia. Anesth Analg 1981 60(4) 192-6. 

Answer D. Saturation of the blood with inhaled anesthetics is more rapid if they have a low blood-gas partition coefficient. This results in the more rapid achievement of a partial pressure of the dissolved anesthetic molecules commensurate with their movement out of the blood into the alveolar spaces of the lung, where they are eliminated. Note that the same physicochemical characteristic is responsible for the rapid onset of the anesthetic action of sevoflurane. Although redistribution of anesthetics between tissues occurs, it is not responsible for rapid recovery. MAC values are a measure, of anesthetic potency. With the exception of halothane (and methoxyflurane), inhaled anesthetics are not metabolized to a significant extent. Naloxone is an opioid receptor antagonist. 

It is pertinent to observe here that a number of volatile anaesthetics viz., halothane, isoflurane, enflurane and the like essentially contain in each of them an asymmetric carbon atom i.e., a chiral centre) therefore, may invariably occur both as (+)-or (-)-enantiomers. It has been a common practice to make use of these volatile anaesthetics as their racemates commercially however, another school of thought devised a mean to establish and determine the anaesthetic characteristics of individual enantiomers. 

Further studies haye been conducted on compound (XXXV), now called Ethrane The compound was administered to 250 patients as the major anesthetic, Its physical properties and clinical characteristics are similar to those of halothane. 

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