Clonidine

• Antihypertensive:

  • Clonidine (Catapres) acts in the brain, inhibiting adrenergic outflow from the brainstem. Inhibition of sympathetic outflow results in a decrease in blood pressure.

  • Mechanism of action: centrally acting selective a₂ adrenergic agonist.

  • Especially effective in

    • management of severe hypertension or

    • in renin-dependent hypertension

  • Transdermal clonidine (Catapres) patch: useful for surgical patients unable to take oral formulation

  • Clonidine (Catapres) reduces cardiac output (by reducing both stroke volume and heart rate) and peripheral resistance.

  • Reduction in stoke volume occurs due to increased venous pooling (decreased preload).

  • Clonidine (Catapres) does not interfere with cardiovascular responses to exercise.

  • Renal blood flow and function is maintained during clonidine treatment.

  • Clonidine (Catapres) has minimal or no effect on plasma lipids.

• Other Clinical Uses

• Analgesia--

  • Preservative-free clonidine administered into epidural/subarachnoid space (150-450 micrograms)-- dose-dependent analgesia

  • No respiratory depression, nausea, vomiting, delayed gastric emptying or pruritus -- effects associated with opioids

    • Probable Mechanism: activation postsynaptic a₂ receptors in the spinal cord substantia gelatinosa

      • Clonidine (Catapres) & morphine: no cross-tolerance when used concurrently in neuraxial analgesia

  • Side effects of neuraxial clonidine (Catapres)

    • hypotension, sedation, dry mouth

• Preanesthetic Medication:

  • Oral clonidine (Catapres) (preanesthetic medication):

    • Enhances intrathecal morphine + tetracaine (pontocaine) for postoperative analgesia (no increase in morphine-related side effects)

    • Preanesthetic clonidine (Catapres) also:

      • Reduces reflex tachycardia that may be caused by direct laryngoscopy for tracheal intubation

      • Reduces intraoperative blood-pressure heart rate lability

      • Reduces plasma catecholamines levels

      • Significantly decreases anesthetic requirements for inhaled (MAC) and injected agents.

• Adverse Effects

  • Dry Mouth (xerostomia)

  • bradycardia (in patients with SA nodal abnormality)

  • Withdrawal syndrome upon abrupt discontinuation (increased blood pressure, headache, tachycardia, apprehension, tremors

Category: Pharmacology Notes