Ep.43 –Clonidine For The FRCA Primary

© GasGasGas – The FRCA Primary Anaesthetic Sciences Podcast 2025

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Reference Tables

Table 1: Basic Drug Properties

Property	Detail
Generic Name	Clonidine
Drug Class	Alpha-2 Adrenoceptor Agonist (Imidazoline derivative)
Chemical Stem	Aniline derivative
Year Synthesised	1962
Molecular Weight	230.093 g/mol
Log Octanol/Water Coefficient	2.174 (moderately lipophilic)
pKa	8.2
Protein Binding	20%
Receptor Selectivity	Alpha-2:Alpha-1 ratio = 220:1

Table 2: Formulations and Routes

Route	Presentation	Licensed Status	Notes
Intravenous	Clear, colourless solution, 150 mcg/ml	Licensed	Onset 10 minutes, offset 3-7 hours
Oral	Tablet form	Licensed	Bioavailability ~100%, onset 60-90 minutes
Transdermal	Patch (100/200/300 mcg/24hr)	Licensed	Onset 48 hours
Epidural	Using IV preparation (avoid preservatives!)	Unlicensed (not advocated)	Up to 150 mcg
Intrathecal	Using IV preparation (avoid preservatives!)	Unlicensed (not advocated)	Approximately 30 mcg

Table 3: Dosing Regimens by Clinical Indication

Indication	Route	Dose	Frequency/Duration	Notes
Analgesia	IV	1-3 mcg/kg (75-225 mcg for 75kg patient)	Slow Aliquots dose	Onset 10 mins, duration 3-7 hours
ITU Sedation	IV infusion	0-2 mcg/kg/hr	Continuous	See preparation details below
Neuraxial Anaesthesia	Epidural	Up to 150 mcg	Single dose	‘thought to’ Prolong block duration
Neuraxial Anaesthesia	Intrathecal	~30 mcg	Single dose	‘thought to’ Prolong block duration
Paediatric Pain	Oral	1 mcg/kg at 8am and 2pm; 2 mcg/kg at night	Three times daily

Table 4: ITU Sedation Infusion Preparation

Concentration	Clonidine Amount	Diluent Volume	Final Concentration	Example Rate (75kg patient at 1 mcg/kg/hr)
Single Strength	750 mcg	50 ml	15 mcg/ml	5 ml/hr
Double Strength	1500 mcg	50 ml	30 mcg/ml	2.5 ml/hr

Calculation for 75kg patient requiring 1 mcg/kg/hr:

• Hourly requirement: 1 mcg × 75 = 75 mcg/hour
• Using single strength (15 mcg/ml): 75 ÷ 15 = 5 ml/hour
• Using double strength (30 mcg/ml): 75 ÷ 30 = 2.5 ml/hour

⠀For 2 mcg/kg/hr (maximum rate):

• Single strength: 10 ml/hour
• Double strength: 5 ml/hour

Table 5: Cardiovascular Effects Profile

Parameter	Effect	Mechanism	Clinical Significance
SVR – Early Phase	↑ (transient hypertension)	Alpha-1 receptor stimulation during peak plasma levels	Brief, usually insignificant clinically
SVR – Later Phase	↓ (potential hypotension)	Central sympatholysis predominates	Dose-dependent, monitor carefully
Chronotropy	↓↓ (bradycardia)	Reduced sympathetic outflow + baroreceptor sensitisation	Common, may be significant
Inotropy	↔ (unaffected)	No direct cardiac effect	Cardiac output typically maintained
Dromotropy	↓ (slowed conduction)	Reduced sympathetic drive to conduction system	PR prolongation, slowed AV conduction
Cardiac Output	↔ (maintained)	Balance of effects	Generally well-tolerated

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Classification & Historical Context

Chemical Class: Clonidine belongs to the imidazoline class of alpha-2 adrenoceptor agonists, chemically derived from aniline.

Historical Development: First synthesised in 1962, clonidine’s journey through clinical medicine demonstrates serendipitous drug discovery. Initially developed as a nasal decongestant, clinicians observed its antihypertensive properties, leading to its use in blood pressure management. Subsequently, its utility expanded to include migraine prophylaxis before the anaesthetic community recognised its value as a premed and volatile-sparing sedative agent. This evolution mirrors other drugs with multiple applications, such as gabapentin’s transition from anticonvulsant to analgesic adjunct.

Physical Appearance: Clonidine for injection appears as a clear, colourless solution at 150 mcg/ml concentration. Oral tablets and transdermal patches (providing 100, 200, or 300 mcg over 24 hours) offer alternative delivery methods.

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Pharmacodynamics Of Clonidine

Mechanism of Action

Clonidine functions primarily as a partial agonist at presynaptic alpha-2 adrenoceptors. This distinction from full agonists like dexmedetomidine is important: even with complete receptor occupancy, partial agonists produce submaximal responses. The clinical implication is a ceiling effect on therapeutic benefits while dose-dependent side effects persist.

Molecular Mechanism: Alpha-2 receptors are G-protein coupled receptors (GPCRs). Agonism leads to reduced intracellular cyclic AMP (cAMP), subsequently decreasing calcium entry and neurotransmitter (noradrenaline in this case) release.

Primary Sites of Action:

1. Lateral Reticular Nucleus (Brainstem): Stimulation of postsynaptic alpha-2 receptors reduces sympathetic nervous system outflow, creating the sympatholytic state
2. Spinal Cord (Dorsal Horn): Alpha-2 receptor activation modulates pain transmission, though the exact mechanism remains debated—either through inhibiting substance P release or stimulating endogenous opioid pathways
3. Peripheral Nervous System: Presynaptic alpha-2 stimulation inhibits noradrenaline release across synaptic clefts
4. Vasculature: Initial alpha-1 receptor stimulation (during peak plasma concentrations) causes transient vasoconstriction, followed by predominant central sympatholytic vasodilation

Clinical Applications

Anaesthetic Applications:

• Perioperative Sedation & Anxiolysis: Low-dose clonidine provides anxiolysis without excessive sedation, though paradoxically causes anxiety at higher doses due to alpha-1 receptor effects
• Analgesia Enhancement: Reduces opioid requirements through spinal and supraspinal mechanisms
• Sympatholysis: Attenuates the surgical stress response, particularly useful during tourniquet inflation
• MAC Reduction: Decreases volatile anaesthetic requirements by up to 50% (dose-dependent ceiling effect)
• Post-Anaesthetic Shivering:
• Anti-Emetic Effects: Modest reduction in postoperative nausea and vomiting
• Emergence Delirium: Particularly effective in paediatric patients following sevoflurane anaesthesia

Intensive Care Applications:

Clonidine offers distinct advantages for ITU sedation, creating “green rolling hills of ventilator liberation” rather than turbulent emergence. It facilitates smoother weaning by maintaining sedation without respiratory depression.

Non-Anaesthetic Applications:

• Opioid and alcohol withdrawal symptom mitigation
• Chronic pain management (adjunct)
• Cancer pain (adjunct)
• Migraine prophylaxis
• Menopausal symptom management (hot flushes)

Side Effects By System Of Clonidine

Cardiovascular System (Most Clinically Significant):

Systemic Vascular Resistance

Biphasic Blood Pressure Response:

• Early (minutes): Transient hypertension from peripheral alpha-1 stimulation
• Late (hours): Hypotension from central sympatholysis

Rebound Hypertension: More common than with dexmedetomidine, particularly after abrupt cessation of doses exceeding 1.2 mg/day for several days. Mechanism likely involves sudden catecholamine surge from withdrawn central sympathetic suppression

Negative Chronotropy (Bradycardia)

• Reduced sympathetic outflow
• Increased vagal tone
• Baroreceptor sensitisation (enhanced bradycardic response to given blood pressure)

Negative Dromotropy

• Conduction Effects: PR interval prolongation and slowed AV nodal conduction (negative dromotropic effect)

Inotropy

• Preserved

Coronary Circulation:

• Complex—local vasoconstriction balanced by reduced central sympathetic drive

Central Nervous System:

• Cerebral blood flow reduction
• Dose-dependent sedation
• Biphasic anxiety response (anxiolysis at low doses, anxiety at high doses)
• Dry mouth (antisialagogue effect)
• Reduced intraocular pressure

Respiratory System:

• Minimal impact on minute ventilation at clinical doses
• Respiratory depression only at supraclinical doses

Gastrointestinal System:

• Reduced motility
• Antisialagogue effects

Endocrine System:

• Reduced insulin secretion with potential mild hyperglycaemia (usually clinically insignificant except during prolonged infusions)

Renal System:

• Mild diuretic effect
• Reduced renovascular resistance
• Glomerular filtration rate typically maintained

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Pharmacokinetics Of Clonidine

Absorption

• Oral Bioavailability: Approaching 100%, making oral-to-IV conversion straightforward
• Onset Time:
  • Oral: 60-90 minutes
  • IV: 10 minutes
  • Transdermal: 48 hours

Distribution

• Volume of Distribution (Vd): 2.1 L/kg (range 1.7-2.5 L/kg)
  • For a 70kg adult: Vd = 147 litres (range 119-175 litres)
• Lipid Solubility: Moderately lipophilic (log octanol/water 2.174), facilitating CNS penetration though less than dexmedetomidine (2.8)
• Protein Binding: Only 20% (predominantly unbound in plasma)
• pKa: 8.2 (at physiological pH 7.4, relatively more ionised than unionised)

Clinical Correlation: The relatively large volume of distribution and moderate lipophilicity explain both its CNS effects and prolonged duration of action. The low protein binding means fewer drug:protein displacement issues (quite a lot is free anyway).

Metabolism

• Hepatic Metabolism: Approximately 50% undergoes hepatic transformation
• Primary Enzyme: CYP2D6 (66% of metabolism)
• Secondary Enzyme: CYP3A4 (minority contribution)
• Active Metabolite: 4-hydroxyclonidine (retains alpha-2 activity but reduced lipophilicity limits CNS penetration)
• Inactive Metabolites: Multiple

Enzyme Interactions to Consider:

• CYP2D6 Inhibitors: Fluoxetine, paroxetine (may increase clonidine levels)
• CYP3A4 Inhibitors:
  • Strong: Clarithromycin, ketoconazole, ritonavir
  • Moderate: Diltiazem
  • Weak: Cimetidine

Clinical Implication: Patients on CYP2D6 or 3A4 inhibitors may experience enhanced clonidine effects and require dose adjustment.

Elimination

• Renal Excretion: 40-60% eliminated unchanged in urine
• Renal Clearance: 133 ml/min
• Faecal Excretion: ~20%
• Elimination Half-Life: 6-23 hours (commonly cited as 9-18 hours)
• Context: Not dialysable

Renal Impairment Considerations: Significant accumulation occurs with severe renal dysfunction (GFR <10 ml/min). In renal failure, rely on hepatic clearance (50%) for drug elimination. Dose reduction and extended dosing intervals recommended.

Converting from IV to Oral (ITU Step-Down):

1. Calculate total IV dose over 24 hours
2. Divide by 3 to account for dosing frequency
3. Administer every 8 hours orally
4. Maximum oral dose: 600 mcg three times daily

Example for 75kg patient on 1 mcg/kg/hr:

• 24-hour IV dose: 75 mcg/hr × 24 = 1800 mcg
• Oral dose: 1800 ÷ 3 = 600 mcg every 8 hours (at maximum recommended dose)

Important: Gradual weaning over 36+ hours essential to prevent rebound hypertension and sympathetic surge.

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4. Special Clinical Applications

ITU Sedation Protocol

Clonidine offers a unique sedation profile in intensive care, providing:

• Sedation without significant respiratory depression
• Maintained responsiveness (patients more rousable than with propofol/benzodiazepines/alfentanil)
• Facilitated ventilator weaning
• Reduced delirium incidence….?

Initiation Strategy:

• Start with 10 mcg boluses if rapid effect desired
• Alternatively, commence infusion at 0.5-1 mcg/kg/hr and titrate upward
• Down-titrate other sedating agents (propofol, benzodiazepines) as clonidine effect establishes

Maintenance: 0-2 mcg/kg/hr (most patients maintained at 1-1.5 mcg/kg/hr)

Monitoring Requirements:

• Continuous cardiac monitoring (bradycardia risk)
• Regular blood pressure assessment (biphasic response)
• ECG monitoring for conduction changes
• Blood glucose monitoring during prolonged infusions

Regional Anaesthesia Augmentation

While unlicensed, evidence supports perineural or neuraxial clonidineuse:

Neuraxial Dosing:

• Epidural: Up to 150 mcg
• Intrathecal: ~30 mcg

Perineural Dosing: 1-3 mcg/kg

Mechanism of Block Prolongation: Local alpha-2 receptor stimulation in dorsal horn neurons and peripheral nerves reduces C-fibre and A-delta fibre activity.

Controversial Consideration: Systemically administered clonidine (IV) may provide similar block prolongation as perineural administration, as systemic absorption occurs regardless of injection route. Although the literature is mixed….. and not the focus of this episode.

The trade-off: accepting systemic side effects (bradycardia, hypotension) for reliable regional effect. Some practitioners prefer IV administration for simplicity and predictability.

Paediatric Applications

Sevoflurane Emergence Delirium:

• Problem: Paediatric patients frequently experience agitated emergence after sevoflurane
• Solution: Clonidine 1-2 mcg/kg reduces incidence and severity
• Timing: Administered during surgery or as premedication
• It probably helps…. go have a read and drop me a comment if you think I should flesh this out?

Paediatric Pain Adjunct Protocol:

• 1 mcg/kg at 0800 and 1400
• 2 mcg/kg at bedtime
• Provides round-the-clock mild sedation with analgesia augmentation

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5. Safety Considerations

Contraindications

Absolute:

• Severe bradycardia (HR <50 bpm)
• Second or third-degree AV block (without pacemaker)
• Sick sinus syndrome
• Severe hypotension

Relative:

• Recent myocardial infarction (may have conducting system issues)
• Severe renal impairment (dose adjustment required)
• Concurrent beta-blocker therapy (enhanced bradycardia risk)
• Raynaud’s phenomenon (may worsen peripheral vasoconstriction)

Emergency Management of Overdose/Toxicity

Clinical Presentation:

• Profound bradycardia
• Severe hypotension
• Respiratory depression (high doses)
• CNS depression ranging from somnolence to coma
• Miosis (pinpoint pupils)
• Hypothermia

Management Strategy will always be ABCDE

1. Airway/Breathing/Circulation:
   • Secure airway if GCS compromised
   • Supplemental oxygen
   • IV access and fluid resuscitation
2. Bradycardia Management:
   • Atropine 500-600 mcg IV (may have limited efficacy)
   • Consider temporary cardiac pacing if refractory
   • Isoprenaline/adrenaline infusion will hopefully temporise
3. Hypotension Management:
   • IV fluid boluses (crystalloid)
   • Consider vasopressors
   • Direct-acting agents may be more effective than indirect sympathomimetics
4. Specific Antidote: None available BUT! Yohimbine wakes them up but doesn’t seem to help the BP situation as much and there are veterinary agents that are used in animals being sedated with medetomidine….. call poisons helpline if it is an abject disaster of monumental proportions.
5. Supportive Care:
   • Continuous cardiac monitoring
   • Temperature management
   • Blood glucose monitoring
   • Not dialysable—supportive care until hepatic clearance occurs
6. Avoid Abrupt Cessation: Taper gradually once stabilised to prevent rebound hypertension

Duration of Monitoring: Given elimination half-life of 9-18 hours, observe for minimum 24 hours, longer if severe renal impairment present.

Rebound Hypertension Prevention

Risk Factors:

• Doses >1.2 mg/day
• Duration >48-72 hours
• Abrupt discontinuation

Prevention Strategy:

1. Plan discontinuation prospectively (not emergency stop)
2. Reduce infusion rate by 50% every 12 hours
3. Consider oral conversion when at lower infusion rates
4. Continue oral clonidine with stepwise reduction every 24-36 hours
5. Monitor blood pressure for 48 hours post-complete cessation

Treatment if Occurs:

• Reinstate clonidine at previous effective dose
• Restart gradual wean once blood pressure controlled

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For Examination Purposes:

Candidates should be familiar with the concept that clonidine’s neuraxial use remains unlicensed in the UK despite evidence base, and that local hospital policies should guide practice. When discussing off-label use in examination settings, acknowledge this status and the importance of informed consent and local governance approval.

‘It could be used like this’ not ‘I use it like this’ its the primary FRCA after all, and this approach is more likely to be needed in the Final FRCA…

References

Kloke M, Hoff P, Schmidt P, Schilsky R. Review of clinical pharmacokinetics and pharmacodynamics of clonidine as an adjunct to opioids in palliative care. Basic Clin Pharmacol Toxicol. 2023;133(5):512-23. https://doi.org/10.1111/bcpt.13979

World Federation of Societies of Anaesthesiologists. Clonidine in anaesthetic practice [Internet]. WFSA Anaesthesia Tutorial of the Week; [cited 2025 Oct 3]. Available from: https://resources.wfsahq.org/wp-content/uploads/415_english.pdf

Neil MJ. Clonidine: clinical pharmacology and therapeutic use in pain management. Curr Clin Pharmacol. 2011;6(4):280-7.

Hayashi Y, Maze M. Alpha2 adrenoceptor agonists and anaesthesia. Br J Anaesth. 1993;71(1):108-18.

Amna S, Øhlenschlæger T, Sædder EA, Sigaard JV, Bergmann TK. Review of clinical pharmacokinetics and pharmacodynamics of clonidine as an adjunct to opioids in palliative care. Basic Clin Pharmacol Toxicol. 2024;134(4):485-97.

Fizzicu. The five -tropies of the heart [Internet]. Fizzicu; [cited 2025 Oct 3]. Available from: https://www.fizzicu.com/post/the-five-tropies-of-the-heart

Engelmann TW. Ueber den myogenen Ursprung der Herzthätigkeit und über automatische Erregbarkeit als normale Eigenschaft peripherischer Nervenfasern. Pflüger Arch Gesamte Physiol Menschen Tiere. 1897;65:535-78. https://doi.org/10.1007/BF01795562

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Practice Points and Clinical Pearls

Key Takeaways for Clinical Practice

1. Biphasic Blood Pressure Response – oft will settle into a lower BP state
2. Bradycardia is Common, buzzword, sympatholysis!
3. Don’t Stop Abruptly: Plan your descent from clonidine stupor!
4. IV = Oral (mostly): The 100% bioavailability makes conversion straightforward, but remember the delayed oral onset (60-90 minutes) and the subsequent peak/trough that may lead to withdrawal in the troughs…
5. Regional Blocks: If using for block prolongation, consider just giving it IV—you’ll get systemic side effects either way, and at least IV dosing is familiar licensed territory.
6. Paediatric Emergence: May transform a screaming delirious sobbing child into a calm, comfortable one—consider it for your sevoflurane cases – (dexmed, or a dose of propofol as you turn off the sevoflurane (also a choice here).
7. Not Dialysable: In renal failure patients, you’re relying on 50% hepatic clearance—dose carefully and monitor longer
8. The 220:1 Rule: Remember clonidine’s alpha-2:alpha-1 ratio compared to dexmedetomidine’s 1620:1—this explains why clonidine has more cardiovascular side effects and a ceiling on MAC reduction

Common Pitfalls to Avoid

1. Forgetting the Wean: Abrupt cessation after 48+ hours of use, especially at high doses
2. Ignoring Enzyme Inhibitors: Patient on fluoxetine? They’re a CYP2D6 poor metaboliser now
3. Combining with Multiple Bradycardic Agents: Beta-blockers + digoxin + clonidine = very slow heart rate oops
4. Using in CVS Conduction Disease: Second-degree heart block and clonidine are not friends

Exam Strategy Tips

1. Structure Your Answer: Always use “absorption, distribution, metabolism, elimination” for kinetics
2. Use the Tropies or variants thereof: Demonstrates sophisticated understanding of cardiovascular effects
3. Acknowledge Off-Label Use: Shows awareness of real-world practice vs licensing

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Acknowledgments

Disclaimer: These notes are for educational purposes only and should not replace clinical guidelines, local protocols, or senior advice in clinical practice. Always consult current BNF, local formularies, and hospital policies before prescribing. Off-label uses discussed represent common practice supported by evidence but require appropriate governance approval and informed consent.

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Final Thoughts

Clonidine represents a drug that has found its niche across multiple areas of anaesthetic practice. While perhaps overshadowed by its newer, more selective cousin dexmedetomidine, clonidine’s long use, lower cost, and established safety profile ensure its continued relevance. Understanding its partial agonist nature, biphasic cardiovascular effects, and the critical importance of gradual withdrawal demonstrates sophisticated pharmacological knowledge essential for FRCA Primary success.

The “five tropies” framework for discussing cardiovascular effects not only applies to clonidine but provides a structured approach applicable to numerous drugs encountered in anaesthesia. This systematic thinking—breaking complex effects into chronotropy, inotropy, lusitropy, dromotropy, and bathmotropy—will serve you well across many examination scenarios.

As always, exam success comes from understanding principles, not just memorising facts. Know why clonidine causes bradycardia (reduced sympathetic outflow plus baroreceptor sensitisation), not just that it causes bradycardia. Know why there’s a ceiling on MAC reduction (alpha-1 agonism counteracting alpha-2 effects at higher doses), not just thatMAC reduction plateaus at 50%.

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“Thanks for listening guys… Every day you are getting better at this. Take it day by day, don’t overcook yourself, don’t freak out, and keep studying!”

Podcast Information

Transcript

00:00–01:19

Hello and welcome to Gas Gas Gas, the best podcast for the FRCA primary exam. Our goal is to fill your brain with highly useful information. You might be in the gym right now, commuting, or ironing your scrubs. Regardless, the revision is eventually going to end, but for now, expect facts, concepts, model answers, and the odd tangent. Make sure to check out gasgasgas.uk – there are show notes there with loads more detail. Make sure to like and subscribe.

This is the 43rd episode of the pharmacology FRCA series. We did about 16 Viva Cast episodes previously, and Tom is coming back for more reciprocal grilling sessions. We covered dexmedetomidine last week, and we’re doing clonidine this week. Let’s get into the depths of clonidine today. I’m sure you’ve seen it in the cupboard, seen it being used, and wondered about it. It seems to be used in some places more than others, and there are a number of preferences as to whether to bolus it, use it for ITU sedation, or use it to prolong regional anaesthesia – not neuromuscular blockade. It doesn’t do anything to neuromuscular blockade.

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Drug Classification and Basic Properties

01:45–03:10

Key Points:

• Partial alpha-2 adrenoceptor agonist (versus dexmedetomidine, which is a full agonist)
• First synthesised in 1962
• Initial use as nasal decongestant, then antihypertensive
• Now used for sedation, analgesia, and as a volatile-sparing agent

Clonidine, like dexmedetomidine, is an alpha adrenoceptor agonist. It is a partial agonist, however, whereas dexmedetomidine is a full agonist. Remember the difference between partial and full agonist: when you have full saturation of all receptors in your system with a full agonist, you will see a maximal response from that system, whereas if you have partial agonists bound to all those receptors, even if every receptor is bound, you will only see a partial response as opposed to maximal response.

Clonidine was first synthesised in 1962 and initially used as a nasal decongestant. It seems that all these drugs found their way into cough remedies – like heroin and cocaine – before making their way to other uses. It was also used as an antihypertensive agent. That’s useful to bear in mind when you’re thinking about using it. It helped people with migraines, but then its utility became a bit more obvious and it started creeping into premeds for anaesthesia and being used as a sedating, volatile-sparing agent.

Presentation:

• Ampoules: clear, colourless solution for injection at 150 micrograms per ml
• Tablets
• Transdermal patches

Physical Properties:

• Molecular weight: 230.093 grams per mole (slightly larger than recently discussed drugs, about 10% more)
• Log octanol-water coefficient: 2.174 (meaning 2.1 orders of magnitude more clonidine in octanol versus water)
• For comparison: dexmedetomidine has a coefficient of 2.8, whilst propofol and fentanyl are both greater than 4

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Pharmacodynamics: Mechanism of Action

04:00–05:10

Mechanism: Acts by stimulating mostly presynaptic alpha-2 adrenoceptors (remember, it’s a partial agonist). This inhibits noradrenaline release across the synaptic cleft, impairing signalling in whichever structures this molecule is binding to.

Clinical Effects Achieved:

• Analgesia: Either through inhibition of endogenous substance P secretion or stimulation of endogenous opioid release (the jury is out, depending on what you read)
• Sympathetic nervous system depression
• Desensitisation of vasculature to vasoactive substances with prolonged use
• Increased vagal tone

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Clinical Utility and Indications

05:06–07:00

Anaesthetic Uses:

• Blood pressure control (though better drugs exist during anaesthesia specifically for this)
• Analgesic effects
• Sedative effects
• Anxiolytic effects in low doses (high doses actually make people anxious)
• Sympatholysis – mitigates the surgical stress response (fast heart rate, high blood pressure)
• Reduces post-anaesthetic shivering (though seen less frequently now)
• Anti-emetic properties (though not used primarily for this)
• Obtunds tourniquet-induced hypertension
• Decreases post-operative agitation, especially in children with sevoflurane emergence delirium

Non-Anaesthetic Uses:

• Migraine treatment (though not commonly used)
• Menopausal flushing (though not commonly used)
• Opioid and alcohol withdrawal mitigation (this is where it comes into use in ITU – we’re probably indirectly treating people who are withdrawing when we think they just need more sedation)
• Chronic pain adjunct
• Cancer pain adjunct

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Dosing Regimens

06:37–09:47

Licensed Routes:

Oral:

• 50–600 micrograms
• Maximum 1,800 micrograms in a 24-hour period

Intravenous:

• 1–3 micrograms per kilogram
• Onset time: approximately 10 minutes
• Duration: 3–7 hours of slow offset

Unlicensed Routes:

Epidural:

• Up to 150 micrograms
• Prolongs block and provides better pain relief peri/post-operatively

Intrathecal:

• 30 micrograms

Perineural:

• Approximately 1 microgram per kilogram dose regionally
• Note: You end up seeing systemic side effects from perineural dosing, so it’s probably better to give it IV slowly to augment regional blockade

Intensive Care Sedation:

• 0–2 micrograms per kilogram per hour infusion
• Dexmedetomidine may eventually take over this indication, but clonidine has more institutional familiarity due to longer availability and being off-patent (therefore cheaper)
• Prepared in normal saline or 5% dextrose
• Single strength: 750 micrograms in 50 ml
• Double strength: 1,500 micrograms in 50 ml
• For a standard 75 kg patient: 1 microgram/kg/hour = 5 ml/hour on single strength; maximum rate = 10 ml/hour on single strength

Important: Clonidine has almost 100% oral bioavailability, so patients can be stepped down from ITU sedation infusion to oral version. Check local guidelines for step-down protocols. Abrupt cessation causes stark hypertension, sweating, and agitation – particularly problematic in recently extubated patients.

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Cardiovascular Side Effects

10:08–11:40

Blood Pressure Effects:

• Early (IV administration): Initial increase in blood pressure when given quickly due to stimulation of vascular alpha-1 adrenoceptors, causing smooth muscle constriction
• Later phase: Degree of hypotension

Cardiac Effects:

• Negative chronotrope: Causes bradycardia (not through direct cardiac action, but via sympatholysis and sensitisation of carotid baroreceptors – for a given blood pressure, you’ll see a slightly starker heart rate drop)
• Inotropy: Cardiac contractility unaffected
• Negative dromotrope: Slows conducting system conduction, leading to longer PR interval and slowed AV conduction

Coronary Effects:

• Local vasoconstrictive effects occur initially
• Systemic sympatholytic effects follow
• Local vasoconstrictive effects probably wane whilst central sympatholytic effect remains

Rebound Hypertension:

• Occurs with sudden withdrawal
• More prevalent than with dexmedetomidine
• More likely with larger doses

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Central Nervous System and Other Effects

11:46–13:07

CNS Effects:

• Decreases cerebral blood flow
• Reduces MAC by up to 50% at large doses
• Low dose: Anxiolysis (calming effect)
• High dose: Causes anxiety (due to less alpha-2 specificity – clonidine prefers alpha-2 over alpha-1 receptors at 220:1, whilst dexmedetomidine is 1,620:1)
• Central alpha-1 agonism has contrary effects to central alpha-2 agonism, so at higher doses it starts antagonising its own intended effects
• Causes dry mouth
• Lowers intraocular pressure

Respiratory Effects:

• Does very little to minute ventilation at clinical doses
• Supraclinical doses may cause respiratory depression

Renal Effects:

• Mild diuretic effect

Endocrine Effects:

• Decreases rate of insulin release from pancreas
• May cause blood glucose to creep up
• Not really relevant for theatre cases, but consider in ITU (though most patients are on insulin infusions anyway)

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Pharmacokinetics: Absorption and Distribution

13:33–14:04

Absorption:

• Oral bioavailability: 100% (or near enough)
• Can be absorbed transdermally

Distribution:

• Very lipid-soluble (though slightly less than dexmedetomidine)
• Gets into the central nervous system easily
• Volume of distribution: 2.1 litres per kilogram (range: 1.7–2.5 litres/kg)
• Only 20% protein bound

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Metabolism and Elimination

14:04–15:23

Metabolism:

• 50% metabolised by the liver
• Metabolised into one active metabolite: 4-hydroxyclonidine (though less lipid-soluble than parent drug, as the liver generally shifts drugs from lipid-soluble to water-soluble)
• Multiple inactive metabolites also produced
• Chief enzyme: CYP2D6 (about 66% of activity), with some CYP3A4 involvement
• CYP2D6 inhibited by fluoxetine and paroxetine

Elimination:

• 40–60% (approximately 50%) excreted unchanged in the urine
• Elimination half-life: 6–23 hours (or 9–18 hours according to Peck and Hill)
• With severe renal impairment (GFR < 10), clonidine accumulates
• Does not dialyse out of plasma – must rely on hepatic clearance in renal failure patients (<10GFR)

Important Considerations:

• Sudden withdrawal after several days leads to agitated, sweaty, hypertensive patients
• Reduce dose over 36 hours before transitioning to oral dosing
• Unlike dexmedetomidine, clonidine cannot achieve greater than 50% MAC reduction due to alpha-1 receptor activity at higher doses that counters the desired effects

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The Five Tropies of the Heart

16:32–17:55

When approaching cardiovascular effects, we broke it down into systemic vascular resistance and effects on the five tropies of the heart, originally devised by Engelmann in 1897:

1. Chronotropy: Rate (heart rate effects)
2. Inotropy: Contractility (force of contraction)
3. Lusitropy: Relaxation capacity (the heart’s ability to relax)
4. Dromotropy: Conduction velocity (speed through conducting system)
5. Bathmotropy: Excitability (how “ticklish” or prone to arrhythmias the heart is)

Positive Bathmotropic Agents (increase myocardial excitability):

• Adrenaline
• Digoxin
• Dobutamine
• Dopamine

Negative Bathmotropic Agents (decrease myocardial excitability):

• Beta blockers
• Amiodarone
• Anti-arrhythmics
• Severe hypocalcaemia (can lead to VF)

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Summary and Key Takeaways

17:55–19:28

We’ve covered clonidine’s partial agonist effect, explored its side effect profile including early hypertension and later hypotension with bradycardia. It has many useful features in anaesthesia: sedation, analgesia, mild anxiolysis, reduced shivering post-operatively, smoother emergence in adults and children, antihypertensive properties, and ITU sedation applications.

We’ve compared it at numerous points with dexmedetomidine. Alpha-2 agonist – hopefully if I say it enough, it’ll stay in your brain and mine.

Coming up: we’ll either start covering volatile anaesthetic agents or create a quiz-style episode comparing all the opioids (pKa values, protein binding, etc.) so you can answer questions as you drive home.

Thanks for listening. Remember: every day you’re getting better at this. There’s a bucket of content to consume and it’s like drinking from a fire hose. Take it day by day, don’t overcook yourself, don’t freak out, and keep studying.

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