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General anaesthetic

A general anesthetic is an anaesthetic (or anesthetic) that brings about a reversible loss of consciousness.

Two types of general anesthetic drugs are used: inhalation anesthetics and injection anesthetics.

Inhalation anesthetics are liquid or gaseous and are usually delivered using an anesthesia machine[?]. An anesthesia machine allows composing a mixture of oxygen, anesthetics and ambient air, delivering it to the patient and monitoring patient and machine parameters. Liquid anesthetics are vaporized in the machine.

Various compounds have been used for inhalation anesthesia, but only a few are still in use. nitrous oxide, isoflurane[?], desflurane[?] and sevoflurane[?] are probably used most widely today.

Injection anesthetics are used for induction and maintenance of a state of unconsciousness. Among the most widely used substances are propofol[?], etomidate[?], barbiturates such as methohexital and thiopental, benzodiazepines such as midazolam and diazepam, and ketamine.

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Pharmacology of general anesthetic drugs

The sites of action of general anaesthetics have proved difficult to find, because they have many different structures, ranging from complex steroids to the inert monatomic gas xenon. It is clear, however, that general anaesthetics must act upon the central nervous system by modifying the electrical activity of neurons. A change in electrical activity must be brought about at the molecular level by modifying the function of ion channels. This could occur by anaesthetic molecules binding directly to ion channels or by their disrupting the function of molecules that maintain ion channels.

Many forms of ion channel have been cloned in the past decade, adding greatly to our knowledge of proteins involved in neuronal excitability. These range from voltage-gated ion channels[?], such as sodium, potassium, and calcium channels, to the ligand-gated ionotropic ion channel[?] superfamily and the G protein-coupled ion channel[?] superfamily. Although researchers generally agree that ion channels are the ultimate site of action of general anaesthetics, there is considerable disagreement about the molecular mechanisms. Some hold that anaesthetics disrupt the actions of ion channels indirectly, by modifying physical properties of the lipid bilayer. Others believe that anaesthetics act by directly binding to specific sites on ion channel proteins.

Lipid solubility

Von Bibra and Harless, in 1847, were the first to suggest that anaesthetics may act by dissolving in the fatty fraction of brain cells. They proposed that anaesthetics dissolve and remove fatty constituents from brain cells, changing their activity and inducing anaesthesia. The first report of anaesthetic potency being related to lipid solubility was published by H. H. Meyer in 1899, entitled "Zur Theorie der Alkoholnarkose". Two years later a similar theory was published independently by Overton.

Meyer and Overton had discovered the most striking correlation observed between the physical properties of general anaesthetic molecules and their potency. Meyer compared the potency of many agents, defined as the reciprocal of the molar concentration required to induce anaesthesia in tadpoles, with their olive oil/water partition coefficient[?]. He found a nearly linear relationship between potency and the partition coefficient for many types of anaesthetic molecules such as alcohols, aldehydes, ketones, ethers, and esters. Meyer and Overton also found that the anaesthetic concentration required to induce anaesthesia in 50% of a population of animals (the EC50) was independent of the means by which the anaesthetic was delivered, i.e., the gas or aqueous phase.

Protein binding sites

Two classes of proteins are inactivated by clinical doses of anaesthetic in the total absence of lipid. These are luciferases[?], which are used by bioluminescent[?] animals and bacteria to produce light, and cytochrome P450[?], which is a group of heme proteins that hydroxylate a diverse group of compounds, including fatty acids, steroids, and xenobiotics[?] such as phenobarbital[?]. These proteins bind general anaesthetics and are inhibited with a potency that is approximately equal to their potency for general anaesthesia and also proportional to the anaesthetic molecule's lipid solubility.

From the correlation between lipid solubility and anaesthetic potency, both Meyer and Overton had surmised that anaesthesia occurs when the anaesthetic reaches a critical concentration in some lipid phase within the body. However, these results on lipid-free proteins show that the correlation between lipid solubility and potency of general anaesthetics is a necessary but not sufficient condition for inferring a lipid target site; general anaesthetics could equally well be binding to hydrophobic target sites on proteins in the brain.

The cutoff effect

There is a limitation to the Meyer-Overton correlation. As one ascends a homologous series of anaesthetics, such as the n-alcohols, one would expect from the Meyer-Overton correlation that the alcohols would become increasingly potent as the carbon chain length increases because the alcohols grow more hydrophobic. Instead of becoming increasingly potent without limit however, at certain chain lengths the addition of just one methylene group causes the molecule to lose its ability to anaesthetise. For the n-alcohols the cutoff occurs at a carbon chain length of about 13, and for the n-alkanes at a chain length of between 6 and 10, depending on the species.

If general anaesthetics disrupt ion channels by partitioning into and perturbing the lipid bilayer, then one would expect that their solubility in lipid bilayers would also display the cutoff effect. However, partitioning of alcohols into lipid bilayers does not display a cutoff for long-chain alcohols from n-decanol[?] to n-pentadecanol[?]. A plot of chain length vs. the logarithm of the lipid bilayer/buffer partition coefficient K is linear, with the addition of each methylene group causing a change in the Gibbs free energy of -3.63 kJ/mol.

The cutoff effect is easily interpreted if target sites for general anaesthetics are hydrophobic pockets of fixed dimensions in proteins. As the acyl chain[?] grows, the anaesthetic fills more of the hydrophobic pocket and binds with greater affinity. When the molecule is too large to be entirely accommodated by the hydrophobic pocket, the binding affinity no longer increases with increasing chain length. When the aqueous solubility of the of the molecule exceeds that of the hydrophobic pocket, cutoff occurs.

 

Uses in surgery

Uses in emergency medicine

Uses in intensive care medicine

Uses in diagnostics

 

Related topics Local anesthesia



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