Question 7

Compare and contrast the pharmacokinetics and pharmacodynamics of midazolam and dexmedetomidine.

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College Answer

Most candidates used the effective tabular format presenting pharmacokinetics and pharmacodynamics of each drug side by side. Many answers demonstrated a lack of correct detail with respect to the pharmacokinetics and pharmacodynamics of these two level 1 drugs. Many included pharmaceutics which attracted no marks as it was not asked.


Name Dexmedetomidine Midazolam
Class Sedative Sedative
Chemistry Imidazoline derivative Benzodiazepine
Routes of administration IV is the usual route of administration, but it can also be given IM, buccally, intranasally, and intrathecally IV, IM, subcutaneously, intranasally buccally and orally (though the oral dose required is about doubled)
Absorption 16% oral bioavailability; undergoes extensive first-pass metabolism 44% bioavailability; well absorbed, but also undergoes significant first-pass metabolism
Solubility pKa 7.1; freely soluble in water, but also has excellent fat solubility. pKa 6.7; good water solubility at pH <4 (as a hydrochloride salt) -0 when injected, it becomes lipid-soluble at physiologic pH
Distribution VOD = 1.3-2.5L; highly protein-bound (96%) VOD = 0.8 to 1.5 L/kg; 96% protein-bound
Target receptor Presynaptic α2 noradrenaline receptors, as well as imidazoline receptors GABA-A channel (a separate binding site from GABA)
Metabolism N-glucuronidation and hydroxylation by CYP450. Hepatic metabolism to α-hydroxymidazolam (which is active), and then an inactive renally excreted glucouronide. α-hydroxymidazolam can accumulate in renal failure
Elimination All the metabolites are inactive and excreted renally, which can give the urine a healthy green tinge. Both the active metabolite and the inactive glucouronide are renally excreted
Time course of action Redistribution half-life is 6 minutes; elimination half life coming off a high dose infusion after steady state is reached is usually 2.2-3.7 hrs in critically ill patients Redistribution half-life is 15 minutes; elimination half-life is 1.5-3.5 hours
Mechanism of action α2 receptor effects: by hyperpolarising the presynaptic membrane, α2 receptor activation creates negative feedback which suppresses further noradrenaline release from the presynaptic nerve terminal (responsible for some of the sedating and analgesic effects)
Imidazoline receptor effects are poorly understood, but are at least equally important to the analgesic haemodynamic and sedating effects
Allosteric modulator of the GABA-A receptor: acts on GABA-A chloride channels, where it binds to a site distinct from the GABA binding site, and potentiates the effects of GABA, this increasing the chloride current and hyperpolarising the cell membrane of the neuron
Clinical effects Sedation (which resembles natural sleep) and a minor analgesic effect (likely related to opioid potentiation and anxiolysis). No effect on airway reflexes, apart from what is expected with normal sleep. No depression of the respiratory drive, even at high doses. Bradycardia and hypotension with decreased cardiac output, due to its sympatholytic effects. Sedation, amnesia, anticonvulsant effect, mild decrease in cerebral oxygen demand, no effect on ICP.
Respiratory response to raised CO2 is flattened, but respiratory drive is not as suppressed as it would be with opioids. Airway reflexes are depressed.
Haemodynamic effects (decreased blood pressure and heart rate) are related to its suppression of the sympathetic nervous system. These are less pronounced than those of propofol.
Single best reference for further information Weerink et al (2017) Okkola & Ahonen (2008)


Weerink, Maud AS, et al. "Clinical pharmacokinetics and pharmacodynamics of dexmedetomidine." Clinical pharmacokinetics 56.8 (2017): 893-913.

Afonso, Joana, and Flávio Reis. "Dexmedetomidine: current role in anesthesia and intensive care." Revista brasileira de anestesiologia 62 (2012): 125-133.

Bousquet, Pascal, et al. "Imidazoline receptor system: the past, the present, and the future." Pharmacological reviews 72.1 (2020): 50-79.

Dundee, J. W., et al. "Midazolam." Drugs 28.6 (1984): 519-543.

Olkkola, Klaus Tapio, and Jouni Ahonen. "Midazolam and other benzodiazepines." Modern anesthetics (2008): 335-360.

Gerecke, M. "Chemical structure and properties of midazolam compared with other benzodiazepines." British journal of clinical pharmacology 16.S1 (1983): 11S-16S.