VPT 321: Veterinary Neuropharmacology (2+1)

INDIVIDUAL BARBITURATES AND PROPOFOL

• Pentobarbital sodium — was widely used for anaesthesia. But now used for control of convulsions.
• Thiopental sodium
  • Most popular agent.
  • Used for induction of anaesthesia followed by maintenance with inhalational agents.
  • Given as a 2.5% solution in dogs and 1.25% in small dogs and cats at a dose of 20 – 25mg / kg dose (in non premedicated animals) but higher concentration in large animals (upto 10% in horses)
  • After a single intravenous dose, effect lasts for 10-15 min.
  • Apnea on rapid injection common.
  • Dose given slowly over 30-40 seconds till desired effect is obtained.
  • Another option is to give only 2/3^rd the calculated dose and observe for the reactions till the entire dose is given
  • Maintenance not recommended since it accumulates in tissues
  • Care to be taken in Cardiovascular patients.
  • If Xylazine is used as premedication, dose is decreased by 75 – 90 %
  • Poor muscle relaxation
  • Weak analgesia
  • Half the dose sufficient with 2.5% solution
  • Severe sloughing, necrosis on perivascular injection if not given properly.
• Thiamylal Na – Ultrashort acting – not used.
• Methohexital Na —not a thiobarb.. but 2-3 times more potent and shorter acting. More rapid recovery
  • Rapid induction, good surgical anaesthesia and fast recovery. But induces tremors and even convulsions during recovery.
  • Tremors reduced by opioid or acepromazine premedication
  • Used as 1% soln. but in large animals up to 2.5% soln. can be used.
  • Though advantageous, superseded by newer agents
  • Other barbiturates includes phenobarbitone, Secobarbitone, hexobarbital. etc..) which are used as sedative-hypnotics
• Propofol (2,6- diisopropylphenol)
  • An oily solution – highly lipid soluble, Available as an aqueous emulsion containing propofol (10 mg / ml), soybean oil (100 mg / ml), glycerol (2.5 mg / ml), egg lecithin (12 mg / ml) and Na OH (to adjust pH)
  • Induces effects by enhancing inhibitory effects of GABA (from a different pathway from that of Benzodiazepines)
  • On intravenous injection rapidly taken in by CNS -- but rapidly redistributed to other parts of the body – high Volume of distribution
  • Metabolized in liver and pulmonary tissues to respective  glucuronides and sulfates
  • On CVS – mild hypotension, may sensitize heart to adrenaline arrhythmia
  • On lungs – short periods of apnea
  • Uses – for induction of anaesthesia—with intravenous bolus or infusion with dextrose
  • In dogs – 5-7 mg/kg, dose  can be reduced  if premedicated with tranquilizers
  • Cats – 5 – 8 mg /kg