Barbiturates Flashcards
Is thiopental a weak acid or weak base, and what is it’s pKa?
Weak acid with a pKa of 7.6.
What is the volume of distribution of thiopental?
2.2 L/kg
What is the clearance rate of thiopental?
3.5 ml/kg/min
What is the half-life of thiopental?
6-15 hours. After an induction dose of thiopental, the short duration and rapid emergence is caused by redistribution of the drug to less vessel-rich areas of the body, and not by metabolism. However, after repeated administration of the drug, the metabolism and clearance play a more important role in duration of action than redistribution. At this point, the drug displays zero‑order kinetics, which results in a longer lasting effect and delayed recovery. This is why thiopental is never used for maintenance of anaesthesia.
What percentage of thiopental is protein bound?
80%
What is the induction dose of thiopental?
3-7 mg/kg
What is the pKa of methohexital?
7.9 (it is like all barbiturates a weak acid)
What is the volume of distribution of methohexital?
2.0 L/kg
What is the clearance rate of methohexital?
11 ml/kg/min.
What is the half-life of methohexital?
3-5 hours. Methohexital has an elimination half-life of 3-5 hours with a faster clearance, at 11 ml/kg/min, than thiopental. It causes less cumulation than thiopental and can be used for maintenance of short procedures. Hydroxymethohexitone is its main metabolite, which only has minimal hypnotic activity.
What percentage of methohexital is protein bound?
60%. The onset of action of a drug depends on its proportion of unbound drug, the ionized fraction and its lipid solubility. Methohexital has a faster onset of action than thiopental because it has a higher proportion of drug unionized in plasma, and a lower percentage that is protein‑bound.
What is the induction dose of methohexital?
1-2 mg/kg. It is 2.7 times more potent than thiopental. Methohexital is used for induction anaesthesia. It is ultra‑short‑acting and is used for short procedures, day case surgery and, most commonly, in ECT. It also has an emerging role in pre-hospital care.
What is the volume of distribution of phenobarbital?
0.7 L/kg
What is the clearance rate of phenobarbital?
11 ml/kg/min. Ionization and renal elimination are increased after alkalinization of the urine.
What is the half-life of phenobarbital?
24-96 hours. Approximately 25% is excreted unchanged in the urine and 75% metabolized in the liver.
What percentage of phenobarbital is protein bound?
50-65%.
Regarding the pharmacology of the barbiturates (true or false):
Thiopental is the sulphur analogue of pentobarbital
True
Regarding the pharmacology of the barbiturates (true or false):
All barbiturates are derived from barbituric acid
True
Regarding the pharmacology of the barbiturates (true or false):
Thiopental is an oxybarbiturate
False. Thiopental is an thiobarbiturate. Methohexital is an oxybarbiturate.
Regarding the pharmacology of the barbiturates (true or false):
Barbituric acid is a condensation product of urea and lactic acid
False. Barbituric acid is a condensation product of urea and malonic acid.
Regarding the pharmacology of the barbiturates (true or false):
The water solubility of barbiturates increases on transformation from the keto to enol form, called tautomerism
True
Regarding the pharmacokinetics and mechanism of action of the barbiturates (true or false):
Thiopental has a pKa of 7.6 and a pH 10.5 in solution
True
Regarding the pharmacokinetics and mechanism of action of the barbiturates (true or false):
Thiopental can undergo tautomerism from the enol to the keto form, which is more water-soluble
False. The enol form is more water-soluble in alkaline solutions.
Regarding the pharmacokinetics and mechanism of action of the barbiturates (true or false):
Methohexital has a faster clearance than thiopental
True
Regarding the pharmacokinetics and mechanism of action of the barbiturates (true or false):
Methohexital is 60% protein-bound, mainly in albumin
True
Regarding the pharmacokinetics and mechanism of action of the barbiturates (true or false):
Barbiturates suppress the action of GABA
False. Barbiturates enhance the action of GABA by direct receptor binding and augmentation.
Regarding the clinical uses of thiopental, methohexital and phenobarbital (true or false):
Thiopental is used for the induction of anaesthesia at a dose of 1-2 mg/kg
False. The dose of thiopental for induction is 3-7 mg/kg. Methohexital has an induction dose of 1-2 mg/kg.
Regarding the clinical uses of thiopental, methohexital and phenobarbital (true or false):
Thiopental, methohexital and phenobarbital are used to treat status epilepticus
False. Methohexital is not used to treat status.
Regarding the clinical uses of thiopental, methohexital and phenobarbital (true or false):
Phenobarbital can precipitate seizures and is used for anaesthesia in ECT
False. Methohexital is used in ECT.
Regarding the clinical uses of thiopental, methohexital and phenobarbital (true or false):
Thiopental elicits a smooth induction with a clear endpoint and is often used for RSI
True
Regarding the clinical uses of thiopental, methohexital and phenobarbital (true or false):
Methohexital is ultra-short acting and has an emerging role in pre-hospital care
True
Regarding the clinical effects of thiopental, methohexital and phenobarbital (true or false):
Methohexital does not cause pain on injection
False. Methohexital can often cause pain on injection. However, this can be alleviated by mixing with 1% lidocaine.
Regarding the clinical effects of thiopental, methohexital and phenobarbital (true or false):
Thiopental may elicit laryngospasm and bronchospasm on induction
True. Thiopental may elicit laryngospasm and bronchospasm on induction, thereby creating a difficult environment for placement of a supraglottic airway.
Regarding the clinical effects of thiopental, methohexital and phenobarbital (true or false):
Thiopental can cause osteomalacia and hypersensitivity reactions
False. Phenobarbital is known to cause osteomalacia. Phenobarbital and thiopental have a risk of hypersensitivity reactions.
Regarding the clinical effects of thiopental, methohexital and phenobarbital (true or false):
Thiopental and methohexital cause a decrease in mean arterial blood pressure on induction
True. Thiopental and methohexital both cause a decrease in mean arterial blood pressure on induction, thiopental to a greater extent.
Regarding the clinical effects of thiopental, methohexital and phenobarbital (true or false):
Phenobarbital reduces intraocular pressure
False. Thiopental reduces intraocular pressure.