Thiopentone

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Thiopentone = Sodium Thiopental
Thiopental, a barbiturate, is used for the induction of anesthesia

First used in 1934 by Lundy' and Waters

Yellow amorphous powder.

pH 10.5 - 11 , therefore not to be given with Normal Saline & RL

Dose - 3-5mg/kg ;  Route: IV

Adequate induction dose will cause loss of eyelash reflex.

Used in concentration of 2.5%

Given Commonly at outer aspect of forearm (Never in anticubital fossa)

If Concentration >2.5%

• Pain on injection
• Venous thrombosis

Mechanism of action :

• depress RAS
• binds at a distinct binding site associated with a Cl- ionopore at the GABA_A  receptor, increasing the duration of time for which the Cl- ionopore is open. The post-synaptic inhibitory effect of GABA in the thalamus is, therefore, prolonged.

Duration of action 

• Determined by redistribution (not by elemination)
• Patient looses consciousness in 30 sec and regains in 20 min.
• With repeated doses - duration depends on elimination (Metabolized in Liver)

Systemic effects:

• Decrease BP (Compensatory increase in Heart Rate)
• Decreased response to hypoxia & hypercarbia.
• Apnea (Treat with Positive pressure resp. -  IPPV)
• Decrease Tidal Volume and Respiratory Rate 
• Can cause bronchospasm & laryngospasm
• Decreases Cerebral blood flow
• Decreases Cerebral O2 consumption ( Therefore Cerebroprotective ; DOC - in Head Injury )
• Lowers threshold for pain ( it is AntiAnalgesic)
• Does not cause Muscle relaxation
• Powerful anticonvulsant (refractory seizures can respond to it)
• Decreases Renal blood flow and GFR
• Crosses Placenta and has some antithyroid activity.

It is Microsomal enzyme inducer.

Induction of hepatic enzymes increase the rate of metabolism.

Contraindications:

Acute Intermittent Porphyria and Variegate Porphyria

Can be used in Porphyria Cutaneous Tarda

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In case of inadvertent intra-arterial injection (eg: if in anticubital fossa)

• pain , 
• blanching hand , 
• absent radial pulse , 
• Secondary thrombosis

Treatment - 

• Stop injection & Leave the needle in situ (most important)
• Flush with saline
• Give Vasodialator
• Brachial or stellate ganglion block
• Anticoagulant

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