Nonvolatile or intravenous anaesthetics

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Chapter: Medicinal Chemistry : General Anaesthetics

a. Ultra short-acting barbiturates b. Arylcyclohexylamines c. Benzodiazepines d. Narcotic analgesics e. Miscellaneous

Nonvolatile or intravenous anaesthetics

a. Ultra short-acting barbiturates

Metabolism of Barbiturates: This is discussed in Section III, Chapter ‘Sedatives and Hypnotics’.

1. Thiopentone sodium (Thiopental)


Properties and uses: A yellowish-white powder, hygroscopic, freely soluble in water, and partly soluble in ethanol. These are usually administered intravenously for the production of complete anaesthesia of a short duration. It belongs to the category of ultra short-acting barbiturates. Onset is rapid (about 30 sec) and duration is brief (10–30 min). By rectal route it is administered as a solution, suspension, or suppositories as basal anaesthetic. It is also used as a sedative, hypnotic, and anticonvulsant.

Assay for sodium: Dissolve the sample in water, add 0.1 ml of methyl red solution, and titrate with 0.1 M hydrochloric acid until a red colour is obtained. Boil the mixture gently for 2 min, cool it, and, if necessary, continue the titration with 0.1 M hydrochloric acid until the red colour is again obtained.

Assay for thiopental: Dissolve the sample in water, add 2 ml of dilute sulphuric acid, and shake with chloroform. Filter and evaporate the filtrate to dryness on a water-bath. Dissolve the residue in 30 ml of previously neutralized dimethylformamide and add thymol blue in methanol. Titrate immediately with 0.1 M lithium methoxide until a blue colour is obtained.

Storage: It should be stored in well-closed airtight containers and protected from light.

Dosage forms: Thiopental injection B.P.

2. Methohexital sodium


Properties and uses: White to off-white hygroscopic powder, essentially odourless, and the solution is alkaline to litmus, soluble in water. Methohexital produces more rapid recovery from unconsciousness than thiopental. It is more potent and has shorter duration of action. It is used for the induction of anaesthesia through the intravenous administration. It has two advantages over thiopental sodium. First, being it has less affinity towards fatty tissues and second, it has a greater potency. Its onset of action is quite speedy comparable to thiopental sodium while its recovery is more rapid. For these reasons, this intravenous anaesthetic is specifically useful for short surgical operations, such as oral surgery, gynaecological investigation, genitourinary procedures, and electroconvulsive therapy.

3. Thiomylal Sodium


Properties and uses: Thiomylal is a highly hydrophobic thiobarbiturate having its structural features very much related to thiopental. Its biological activities are almost identical to thiopental. Used as intravenous anaesthetic.

b. Arylcyclohexylamines

1. Ketamine HCl


Properties and uses: It is a white or almost white crystalline powder, freely soluble in water, methanol, and ethanol. Its another name is ‘dissociative anaesthetic’ because it produces unpleasant hallucinations and strong feelings of dissociation from the environment. It is a rapidly acting nonbarbiturate general anaesthetic that produces anaesthesia and is characterized by profound analgesia.

Assay: Dissolve the substance in methanol and add 1.0 ml of 0.1 M hydrochloric acid, and perform potentiometric titration, using 0.1 M sodium hydroxide.

Storage: It should be stored in well-closed airtight containers, protected from light.

Dosage forms: Ketamine HCl injection I.P., B.P.

c. Benzodiazepines

Metabolism: This is discussed in Sec III, Chapter ‘Sedatives and Hypnotics’.

1. Midazolam


Properties and uses: It is a white or yellowish, crystalline powder, soluble in acetone, ethanol, and methanol, but insoluble in water. Midazolam has been used adjunctively with gaseous anaesthetics. The onset of CNS effect is slower than that of thiopental and it has a longer duration of action.

Assay: Dissolve the sample in anhydrous acetic acid and add aceticanhydride, titrate with 0.1 M perchloric acid and determine the end point potentiometrically.

Dosage forms: Midazolam injection B.P.

d. Narcotic analgesics

1. Fentanyl

Synthesis and drug profile is discussed in Sec IV, Chapter ‘Narcotic Analgesics’.

2. Alfentanyl

Properties and uses: It is closely related to fentanyl. It is a potent analgesic used as a primary anaesthetic or as an adjunct in the maintenance of anaesthesia. It has the same properties and side effects as fentanyl. It relieves moderate to severe break through pain.


e. Miscellaneous

1. Etomidate


Properties and uses: It contains a 4-carboxylic acid ester-substituted imidazole moiety, which is also present in a number of compounds that are structural variants of the triazolo and imidazolo benzodiazepines. It is a positive allosteric modulator of GABA receptors.

2. Propofol


Properties and uses: It is colourless or is a very light yellow in colour, clear liquid, very slightly soluble in water, miscible with hexane and with methanol. Propofol is useful for induction and maintenance of anaesthesia.

Assay: It is assayed by adopting liquid chromatography technique

Storage: It should be stored in well-closed airtight containers and protected from light under an inert gas.

Dosage forms: Propofol injection B.P.

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