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The drugs within this group are short acting and have been classically used agents for anaesthesia. They produce a state of hypnosis and higher doses are required to produce an anaesthetised state but are poor '''analgesics'''.
==Mechanism of Action==
Barbituates act by depressing the central nervous system (CNS) by acting at the ''Gamma Aminobutyric Acid A'' receptors (GABAa). They mimic and enhance GABA, which is the principle inhibitory neurotransmitter in the CNS. Once bound to the GABAa receptor they reduce the rate of GABA dissocation and thereby ''increase chloride conductance'' is maintained resulting in hyperpolarisation of the membrane and reduced neuronal excitability. However, as the concentration of barbituate increases, it starts to have a direct effect on the chloride conductance and it is this that is thought to bring about the anaesthetic effects, while the GABA related increases causes a sedative effect. They act to depress the motor centres allowing there use as an ''anticonvulsant'' agent, as well as depressing the sensory centres and inducing an anesthetised state.
==Pharmacological Considerations==
Barbituates are usually powders of the salt that require reconsititution using sterile water or saline. Onset of action and doses depends upon the amount of unbound and unionised form of the barbituate in the circulation. This is due to the ability of the drug to cross the blood-brain barrier. Barbituates are ''cumulative'' making them unsuitable to maintain anaesthesia.
==Side Effects and Contraindications==
*Patients with an acidaemia and hypoproteinaemia often require lower doses to produce an anaesthetised state due to the increase in unbound and unionised forms.
*Causes respiratory depression, particularly in the cat.
*Dose and rate depended cardiovascular depression.
*Hypotension due to peripheral vasodilation and reduction in cardiac output from myocardial depression and tachyarrhythmias.
==Drugs in this Group==
===Thiopental===
Thiopental (also ''Thiopentone'') causes a rapid loss of conciousness with time of onset being influenced by premedication agents. It is considered a '''ultra short acting''' barbituate, with effects seen within 15-30s following injection, and has a rapid recovery period, commonly 10-15 minutes. The duration and depth of anaesthetic, however, depends upon the ''amount'' of drug injected, ''speed'' of injection, and ''rate of distribution'' in non-fatty and fatty tissues.
Thiopental comes as a yellowish powder, and once reconsituted, can be used for up to 24 hours if refridgerated.
===Pentobarbital===