Barbiturates Flashcards
(36 cards)
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
