Barbiturate Anesthesia*

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Barbiturate Anesthesia

• Pentobarbital Thiopental
• Methohexital
• Adapted from Muir, Hubbell, Skarda, Bednarski
  Veterinary Anesthesia, Third Edition

2
Effects on the CNS

• reacts with the inhibitor GABA receptor
• decreases cerebral blood flow, metabolism, and
  intracranial pressure
• poor analgesia at sub-hypnotic levels

3
Effects on respiration

• respiratory depressants
• couphing, sneezing, hiccoughing and laryngospasm
  occur frequently may be related to excessive
  salivary secretion
• laryngospasm common in cats
• short apnea after IV bolus (relative overdose)

4
Effects on respiration

• respiratory arrest can be treated by intubation
  and controlled ventilation
• respiratory stimulant - doxapram

5
Effects on cardiovascular system

• significant CV depression if rapid administration
  or large dose
• arrhythmias such as VPCs and ventricular
  bigeminy
• increase in both parasympathetic (bradycardias -
  heartblocks) and sympathetic tone (ventricular
  arrhythmias)

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Effects on cardiovascular system

• may be initial increase in blood pressure from
  tachcardia and increased peripheral vascular
  resistance
• drop in BP and cardiac depression if patient
  already anesthetized
• lower dose in sick, debilitated or depressed
  animals

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Effects on cardiovascular system

• concentrations greater than 2.5 may cause
  thrombophlebitis
• tissue slough probably does not occur at 2.5
  concentration

8
GI effects

• initial depression of GI motility then increase
  in both tone and motility
• sometimes vomiting during induction if no
  premedication with tranquilizer

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Kidney and liver effects

• no effect on kidney unless large dose, then
  reduction in renal blood flow
• single bolus dose has no effect on liver function
  but large dose may injure patients with liver
  disease

10
Effects on uterus and fetus

• pentobarbital contraindicated
• thiopental crosses the placenta within 45 seconds

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Absorption, elimination and excretion

• IV pentobarbital requires 5 -10 minutes for full
  effect
• IV thiopental maximal effect in 30 seconds
• are absorbed from the GI tract

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Elimination
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Elimination

• redistribution to lean body tissues
• repeated doses have cumulative effect
• thin, heavily muscled animals (greyhounds) have
  prolonged recovery
• acute tolerance rare but when it happens may be
  related to excitement or cardiac output
  distribution

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Elimination

• the amount of nonionized, nonprotein-bound drug
  is increased by acidosis
• liver disease may slow recovery after large doses
• hypothermia slows recovery

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Pentobarbital (Nembutal)

• dose 3-13 mg/lb depending on condition of the
  animal
• inject 1/2 rapidly, the rest in small increments
• administration of 50 glucose will lengthen
  effect
• minimum lethal dose 23 mg/lb IV

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Thiopental (Pentothal)

• dose 3-8 mg/lb IV
• discard solution after 3 days at room temp
• subcutaneous injection may cause necrosis of skin
  (higher concentrations only) treat with saline
  and lidocaine

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Thiopental (Pentothal)

• 10 concentration used in horses
• dog anesthesia in 20-60 seconds
• ventricular bigeminy may occur
• apnea more common after rapid injection
• treat overdose with oxygen, controlled
  ventilation, fluids, alkalinizing solutions and
  diuretics

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Methohexital (Brevane)

• similar to thiopental except not cumulative
  because of rapid metabolism
• dose 3 -7 mg/lb
• recommended for sighthounds
• respiratory depression and apnea are common
• involuntary excitement and convulsions (treat
  with diazepam)

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The End