2008B03 List the anaesthetic-related uses of clonidine.
What are the effects of clonidine on the cardiovascular and central nervous
systems and how are these effects mediated?

 

List:

·      Receptor effects

·      CVS

·      CVS

·      Other

 

Receptor effects:

Adrenoceptors

·   Partial agonist (ceiling effect)

·   α2:α1 = 200:1

·   α2A > α2B > α2C

·   Pre-synaptic α2: ↓noradrenaline release

·   Post-synaptic α2: activation, like α1

Imidazoline receptor

·   Agonist (may account for many effects!)

 

Central nervous system:

Effects

·   Mainly post-synaptic

·   Locus coeruleus:

o ↓Activity of ARAS, ↓SNS outflow

o ↑descending inhibition of nociceptors

·   Dorsal horn:

o ↓glutamate/substance P release by nociceptors

o ↓activation of WDR projection neurons

·   Brainstem (imidazoline)

o ? May mediate many effects

Uses

·   Pre-medication, anxiolysis

·   Sedation, MAC and propofol-sparing

·   Analgesia, opioid-sparing

·   ↓CMRO2 -> ↓CBF

·   Augment and prolong neuraxial blockade

 

·   *Note slower onset than CVS effects

 

Cardiovascular:

Biphasic effects

·   Initial ↑SVR, ↑mAP +/- reflex ↓HR

o Peripheral post-synaptic α2B

o Occurs only with IV bolus

·   Then ↓SVR, ↓mAP +/- reflex ↑HR

o Central post-synaptic > peripheral pre-synaptic α2A

o Hypotension may be prolonged

·   Minimal effect on cardiac output

Uses

·   Antihypertensive

·   Help obtund pressor response to laryngoscopy

·   ?↓Risk peri-op myocardial ischaemia:

o ↓SNS is protective

o Reflex ↑HR is harmful

 

Other:

Uses

·   ↓SNS activation in opioid withdrawal

·   ↓Shivering

·   ↓Salivation

 

 

 

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