The actions of
benzodiazepines are due to the potentiation of the neural inhibition that is mediated by
gamma-aminobutyric acid (
GABA). Practically all effects of the
benzodiazepines result from their actions on the ionotropic
GABA(A) receptors in the central nervous system.
Benzodiazepines do not activate
GABA(A) receptors directly but they require
GABA. The main effects of
benzodiazepines are sedation,
hypnosis, decreased anxiety,
anterograde amnesia, centrally mediated muscle relaxation and anti-
convulsant activity. In addition to their action on the central nervous system,
benzodiazepines have a dose-dependent ventilatory depressant effect and they also cause a modest reduction in arterial blood pressure and an increase in heart rate as a result of a decrease of systemic vascular resistance. The four
benzodiazepines, widely used in clinical anaesthesia, are the agonists
midazolam,
diazepam and
lorazepam and the antagonist
flumazenil.
Midazolam,
diazepam and
flumazenil are metabolized by
cytochrome P450 (CYP)
enzymes and by
glucuronide conjugation whereas
lorazepam directly undergoes
glucuronide conjugation.
CYP3A4 is important in the biotransformation of both
midazolam and
diazepam.
CYP2C19 is important in the biotransformation of
diazepam. Liver and renal dysfunction have only a minor effect on the pharmacokinetics of
lorazepam but they slow down the elimination of the other
benzodiazepines used in clinical anaesthesia. The duration of action of all
benzodiazepines is strongly dependent on the duration of their administration. Based on clinical studies and computer simulations,
midazolam has the shortest recovery profile followed by
lorazepam and
diazepam. Being metabolized by CYP
enzymes,
midazolam and
diazepam have many clinically significant interactions with inhibitors and inducers of
CYP3A4 and 2C19. In addition to pharmacokinetic interactions,
benzodiazepines have synergistic interactions with other
hypnotics and
opioids.
Midazolam,
diazepam and
lorazepam are widely used for sedation and to some extent also for induction and maintenance of anaesthesia.
Flumazenil is very useful in reversing
benzodiazepine-induced sedation as well as to diagnose or treat
benzodiazepine overdose.