Barbiturate drugs are defined as any drug derived from barbitunc acid. Barbituric acid, which lacks central nervous system (CNS) activity, is a cyclic compound obtained by the combination of urea and malonic acid.
Barbiturate drugs with sedative hypnotic properties result from substitutions at the number 2 and 5 carbon atoms of barbituric acid. Barbiturates that retain an oxygen atom on the number two carbon atom of the barbituric acid ring are designated as oxybarbiturates.
Replacement of this oxygen atom with a sulfur atom results in thiobarbiturates, which are more lipid soluble than oxybarbiturates and is associated with greater hypnotic potency and a more rapid onset but shorter duration of action.
Barbiturate Drugs with a phenyl group in the number 5 carbon position, such as phenobarbital, have enhanced anticonvulsant activity.
Here the most commonly used Barbiturate drugs thiopentone will be discussed with reference to other barbiturates as relevant. Thiopental sodium is a thiobarbiturate, the sulfur analogue of sodium pentobarbital.
It is an ultrashort-acting depressant of the central nervous system which induces hypnosis and anesthesia, but not analgesia.
Barbiturate drugs cause dose-dependent depression of the respiration center. Increase in the dose of barbiturates initially results in abolition of the ‘neurogenic drive’ followed by a decrease in the chemical drive.
With toxic doses, the respiration is maintained mainly by the ‘hypoxic drive’. A further increase in the barbiturate drugs concentration abolishes the hypoxic drive and also causes a direct paralysis of the medullary centre.