Centrally Acting Anti-Hypertensives

Property Clonidine Methyldopa
Class Central α2-agonist (200:1 α21) Phenylalanine derivative
Uses Analgesia, sedation, anti-hypertensive Antihypertensive (especially in pregnancy)
Presentation Clear colourless solution at 150μg.ml-1 Tablets - not appropriate for urgent blood pressure reduction
Route of Administration PO/IV at 10-200mcg up to QID. Can be added to neuraxial blockade at 1-2mcg.kg-1 to decrease opioid requirement. PO/IV.
Dosing 50-200μg QID. 250-500mg PO BD/TDS.
Absorption 100% PO bioavailability with rapid absorption Highly variable PO bioavailability
Distribution 20% bound, VD 2L.kg-1 50% protein bound, VD 0.3L.kg-1
Metabolism 50% hepatic to inactive metabolites, t1/2β 9-18 hours Intestinal and hepatic
Elimination 50% renal elimination unchanged 40% renal elimination unchanged
Mechanism of Action Agonist of central α2 receptor, ↓ SNS tone via decreased NA release from peripheral nerve terminals. Metabolised to α-methyl-noradrenaline in the CNS, which agonises central α2 receptors.
CVS Initial ↑ in BP due to α1 stimulation, evident with bolus dosing. Followed by prolonged ↓ in BP, ↑ PR, ↓ AV conduction, ↑ baroreceptor sensitisation (lower HR for a given increase in BP). Cessation may cause rebound HTN. SVR with unchanged HR or CO
CNS Sedation, analgesia due to ↓ NA release which ↓ opioid requirement. Adjunct in chronic pain and in opioid withdrawal. Anxiolysis at low doses. Central antiemetic effect. May ↓ MAC
Metabolic Stress response to surgical stimulus is inhibited
Renal Diuresis secondary to inhibition of ADH


  1. Peck TE, Hill SA. Pharmacology for Anaesthesia and Intensive Care. 4th Ed. Cambridge University Press. 2014.
  2. Smith S, Scarth E, Sasada M. Drugs in Anaesthesia and Intensive Care. 4th Ed. Oxford University Press. 2011.
Last updated 2017-09-12

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