Review of Pharmacology - 9E (2015)

Review of Pharmacology
Note:
• Trielene has maximum analgesic activity.
• Ether has maximum muscle relaxant activity.
• All inhalational agents increase cerebral blood flow (maximum with halothane) as well as intracranial
tension (maximum with halothane and ether).
• Halothane, chloroform and methoxyflurane are hepatotoxic whereas methoxyflurane and sevoflurane
are nephrotoxic.
Intravenous Agents
These may be fast acting (used for induction) or may be slow acting.
Anaesthesia
Thiopentone is used as a 2.5%
solution for i.v. induction of
anaesthesia
Action of thiopentone terminates
very quickly due to redistribution.
1. Inducing Agents
A. Thiopentone
• It is an ultra short acting barbiturate and is the most commonly used intravenous inducing
agent. It is used as a 2.5% solution.
• Sulphur is added to pentobarbitone to increase the lipid solubility.
• Due to high lipid solubility, it is very fast acting drug.
• Action of this drug terminates very quickly due to redistribution (although half life
is longer).
• It also possesses anticonvulsant action (another barbiturate methohexitone increases
the risk of convulsions, therefore used for electroconvulsive therapy).
• It is the agent of choice for cerebral protection (decreases cerebral oxygen consumption,
decreases intra-cranial tension and decreases cerebral metabolic rate).
• It causes peripheral vasodilatation and also depresses cardiovascular system,
therefore can cause hypotension.
• Instead of producing analgesic effect, it can produce hyperalgesia at subanaesthetic
doses.
• Respiratory depression and transient apnea are other problems seen with this agent.
• On accidental injection of thiopentone in the arteries, it can lead to thrombosis and
vasoconstriction that may progress to ischemia and gangrene. It is accompanied
by very severe pain. This condition is treated by leaving the needle in situ (needle
should not be withdrawn), dilution of injected thiopentone with saline, immediate
heparinization and papaverine injection to relieve spasm. Vasodilators, steroids,
lignocaine and urokinase can also be employed. Brachial plexus and stellate ganglion
block should be performed.
• Barbiturates are absolutely contra-indicated in acute intermittent porphyria.
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B. Methohexitone
• It is also an ultra short acting barbiturate.
• It is 3 times more potent than thiopentone.
• It induces seizures; therefore, it is the agent of choice for electroconvulsive therapy.
C. Ketamine
• It is a phencyclidine (hallucinogenic) derivative that is administered in a dose of 2
mg/kg.
• Its onset of action is 30-60s whereas action terminates in 15-20 min. due to
redistribution.
• It acts by blocking NMDA receptors of glutamate.
• It is a very strong analgesic agent but lacks muscle relaxant property.
• It is used for producing dissociative anaesthesia (state of profound analgesia,
amnesia with light sleep, immobility, feeling of dissociation from one’s own body
and the surroundings).
Contd...
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