Anaesthesia inhalation anaesthetics

It was William Morton, in 1846, who first noted that ether could be used as an anaesthetic for dentistry and surgery. Before that all surgery was conducted on conscious patients. Chloroform, CHCI3, was developed later and Queen Victoria gave the process of anaesthesia credence when she gave birth to a child in 1853 and suffered little or no pain. More recent inhaled anaesthetics include isofluorane , CF3 CH(C1) O CHF2. Draw its structure (hint it is also an ether). 

DIRECT ANAESTHETICS-GENERAL 1. Risk of arrhythmias when inhalational anaesthetics are coadministered with epinephrine or norepinephrine 2. Case report of marked t BP when phenylephrine eye drops given during general anaesthesia... 

Desflurane has the lowest blood/gas partition coefficient of any inhaled anaesthetic agent and thus gives particularly rapid onset and offset of effect. As it undergoes negligible metabolism (0.03%), any release of free inorganic fluoride is minimised this characteristic favours its use for prolonged anaesthesia. Desflurane is extremely volatile and caimot be administered with conventional vaporisers. It has a very pimgent odour and causes airway irritation to an extent that limits its rate of induction of anaesthesia. 

Gepts E 1998 Pharmacokinetic concepts for TCI anaesthesia. Anaesthesia 53 (SI) 4-12 Harper N 2001 Inhalational anaesthetics. 

The correlation between anaesthetic potency and lipid solubility shown in Fig. 2.10 is valid for most inhaled anaesthetics and the product MAC X oil/gas partition coefficient (which should of course be a constant) varies by only a factor of 2 or 3 for potencies ranging over 100 000-fold. This constancy implies that inhaled anaesthetics act in the same manner at a specific hydrophobic site (the so-called unitary theory of anaesthesia). This has been challenged by more recent work that has identified compounds, including alkanes and poly-halogenated and perfluorinated compounds, which do not obey the Meyer- Overton hypothesis. It has been suggested that a contributory cause of deviation from this hypothesis may be the choice of lipid to represent the anaesthetic site of action of these compounds, implying that there may be multiple sites of action for inhaled anaesthetics. 

Halsey. M.J. (1989) Physicochemical properties of inhalation anaesthetics, in General Anaesthesia, (eds J.F. Nunn eta/.). Butterworth. London. 

Maintenance of anaesthesia is then usually by administration of an inhalation anaesthetic in a mixture of air or oxygen. 

Nitrous oxide has a low potency and must be used in combination with other inhalation anaesthetics for general anaesthesia. Nitrous oxide provides rapid induction and recovery. It also has an analgesic action and is used as a 50% mixture with oxygen to provide analgesia without loss of consciousness during labour and manipulations of injured body parts. 

Intravenous anaesthetics can be used for short surgical procedures of 10-20 minutes. Their elimination from the body is too slow to allow rapid control of the depth of anaesthesia needed for long procedures. They are most often used for induction because, even the fastest acting inhalation anaesthetics take a few minutes to act and cause excitement before anaesthesia is produced, which would be unpleasant for the patient. It also means that the amount of inhalation anaesthetic can be reduced. 

Intravenous anaesthetics are used to induce general anaesthesia, followed by an inhalation anaesthetic for maintenance, and for short surgical procedures. Propofol is commonly used for both induction and maintenance anaesthesia. 

Basal anaesthetics are agents which induce a state of unconsciousness but the depth of unconsciousness is not enough for surgical procedures. They are often used to induce basal anaesthesia before the administration of inhalation anaesthetics. They are also used for repeated short procedures in children like the changing of painful dressings. Basal anaesthetics offer three cardinal merit points, namely devoid of mental distress, pleasant induction and lesser respiratory irritation. They are often administered through the rectum. Few deserve mention. 

In a double-blind study in 11 healthy subjects there were several instances when alcohol 0.25 to 5 g/kg (equivalent to 1 to 3 drinks) enhanced the effects of nitrous oxide 30% in oxygen, inhaled for 35 minutes. Some effects were seen with the drug combination, which were not seen with either drug alone these included subjective effects and delayed free recall. For mention of the effect of alcohol following anaesthesia, see Anaesthetics, general -I- Alcohol , p.92. 

In a study in 20 healthy patients the concurrent use of either halothane or isoflurane increased the serum concentrations of propofol by about 20% during the maintenance of general anaesthesia. The US manufacturer of propofol notes that inhalational anaesthetics (such as halothane or isoflurane) would be expected to increase the effects of propofol. The man-... 

Nitrous oxide usually reduces the MAC of inhalational anaesthetics in a simple additive manner an inspired concentration of 60 to 70% nitrous oxide is commonly used with volatile anaesthetics. Similarly, the concurrent use of nitrons oxide reduces the dose of intravenous barbiturate anaesthetics and sevoflurane required for anaesthesia. 

A randomised, double-blind, placebo-controlled study involving 44 patients found that lidocaine epidural anaesthesia (15 mL of 2% plain lidocaine) reduced the MAC of sevoflurane required for general anaesthesia by approximately 50% (from 1.18 to 0.52%). This implies that a lower dose of inhalational anaesthetic provides adequate anaesthesia during combined epidural-general anaesthesia than for general anaesthesia alone. ... 

Anaesthesia in the presence of beta blockers normally appears to be safer than withdrawal of the beta blocker before anaesthesia, provided certain inhalational anaesthetics are avoided (methosy-flurane, cyclopropane, ether, trichloroethylene) and atropine is used to prevent bradycardia. Bradycardia and marked hypotension occurred in a man using timolol eye drops when he was anaesthetised. 

Most major and widely-used inhalant anaesthetics are regarded as safe or safer than general opiods and relaxant anaesthetic [176], but ever since Pringle, Maunsell and Pringle [177] and Thompson [178] in 1905 reported that diethyl ether and chloroform anaesthesia reduced urine output closer attention has been paid to the effect of all anaesthetics. Hence, an important application of the fluoride ion-selective electrode combines fluoride levels in blood and other fluids in connection with the administration of fluorine-containing inhalant anaesthetics. The wide range of such studies merits the detailed discussion in the section following the non-blood fluids. 

The early members of the homologous series of ethers are low boiling, flammable liquids. Diethylether was an early anaesthetic but it is little used now, having been replaced with non-flammable inhalation anaesthetics such as enflurane, which is a halogenated ether (Fig. 5.21). Such anaesthetics act via a non-spedlic mechanism where they dissolve in lipid membranes and, in the case of anaesthesia, they dissolve in neurological membranes, thus preventing them from functioning properly. 

There have been many attempts to produce a unified theory detailing the mechanism of action of inhalation anaesthetics but no single theory has been accepted. Early theories on the mechanism of action of inhalation anaesthetics can by summarised by means of the Meyer-Overton theory, which indicated that the potency of anaesthetic action was related to the lipophilicity of an anaesthetic compound. The Meyer-Overton theory suggested that lipids within the brain could be dissolved by anaesthetic agents, thereby interfering with brain cell activity, leading to anaesthesia. 

Two methods of anaesthesia are currently in use, the application of inhaled gaseous or volatile anaesthetics such as halothane, sevoflurane and isoflurane to maintain a level of anaesthesia. Older compounds in this category include nitrous oxide and chloroform. 

Intravenous anaesthetics are mainly used for rapid induction of anaesthesia, which is then maintained by an inhalational agent. They also serve to reduce the amount of maintenance anaesthetics. 

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. 

INHALATIONAL AND INTRAVENOUS ANAESTHETICS DIURETICS t hypotensive effect Additive effect as the anaesthetics cause vaiying degrees of myocardial depression and/or vasodilatation, while diuretics tend to 1 circulatory volume Monitor BP closely, especially during induction of anaesthesia... 

An anaesthetic that has high solubility in blood, i.e., a high blood/gas partition coefficient, will provide a slow induction and adjustment of the depth of anaesthesia. This is because the blood acts as a reservoir (store) for the drug so that it does not enter the brain easily imtil the blood reservoir has been filled. A rapid induction can be obtained by increasing the concentration of drug inhaled initially and by hyperventilating the patient. 

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