General Anaesthetics and its types

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Chapter: Medicinal Chemistry : General Anaesthetics

General anaesthetics are group of drugs that produces loss of consciousness, and therefore, loss of all sensations. The absolute loss of sensation is termed as anaesthesia.


General Anaesthetics

INTRODUCTION

General anaesthetics are group of drugs that produces loss of consciousness, and therefore, loss of all sensations. The absolute loss of sensation is termed as anaesthesia. General anaesthetics bring about descending depression of the central nervous system (CNS), starting with the cerebral cortex, the basal ganglia, the cerebellum, and finally the spinal cord. These drugs are used in surgical operations to induce unconsciousness and, therefore, abolish the sensation of pain.

Horace Wills, a dentist, in 1844 successfully used N2O as an anaesthetic for tooth extraction. Mortan, a dentist, demonstrated ether as an anaesthetic agent and it became popular. In 1847, chloroform was used by Simpson in Britain for obstetrical purposes. The first intravenous anaesthetic, thiopentone, was introduced in 1935. In 1901, Mayer and Overton pointed out a direct parallelism between lipid/water partition coefficient of general anaesthetics and their anaesthetic property known as minimal alveolar concentration (MAC).

MAC is the lowest concentration of an anaesthetic in pulmonary alveoli that is needed to produce immobility in response to a painful stimulus in 50% of the individuals. MAC of a number of general anaesthetics shows excellent correlation with their oil/gas partition coefficients. However, this only reflects the capacity of anaesthetics to enter into the CNS and attain sufficient concentration in the neuronal membrane.

The basic molecular targets show that the ligand-gated ion channels are the major target of anaesthetic action. Many inhalation anaesthetics, such as barbiturates, benzodiazepines, and propofal potentiate the action of inhibitory transmitter GABA to open chloride channels. The action of glycine transmitter, which also activates chloride channels in the spinal cord and medulla, is augmented by barbiturates, propofol, and many other inhalation anaesthetics. N2O and ketamine do not act on GABA or glycine, but they selectively inhibit the excitatory N-methyl D-aspartate (NMDA) type of glutamate receptor.


Types of General Anaesthetics

General anaesthetics are usually given through inhalation or by intravenous injection.

Inhalation anaesthetics: Nitrous oxide, a gas at ambient temperature and pressure, continues to be an important compound of many anaesthesia regimens. Halothane, enflurane, isoflurone, desflurane, sevaflurane, and methoxyflurane are volatile liquids.

Intravenous anaesthetics: Several drugs are used intravenously, alone, or in combination with other drugs to achieve an anaesthetic state for minute surgery of the patients in the intensive care unit. These drugs include the following:

Barbiturates (thiopental, methohexitol) 

Benzodiazepines (midazolam, diazepam)

Opiod analgesics (morphine, fentanyl, sulfentanyl, afentanil, remifentanil) 

Propofol

Ketamine 

Miscellaneous:

droperidol, etomidate, dexmedetomide.

Mode of action: General anaesthetics target the ligand gated ion channels and produce the anaesthetic action. The GABA receptor gated chloride channels are the most important sites and opens to perform the inhibitory action. N2O and ketamine do not affect the GABA or glycine gated Cl–channel, but they selectively inhibit the excitatory NMDA-type of glutamate receptor, which belongs to calcium-gated channels in the neurons and leads to neuronal hyper-polarization.


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