Additives (adrenaline, clonidine, dexamethasone)

Surgical approach (if appropriate)

Not an operation: these are pharmacological adjuncts used by anaesthetists to modify local anaesthetic (LA) block characteristics (onset/duration/quality) and perioperative analgesia/antiemesis.
- Typical contexts: peripheral nerve blocks, wound infiltration, IV regional anaesthesia (Bier’s), neuraxial (spinal/epidural), local infiltration with adrenaline for haemostasis (e.g., ENT, plastics).

Anaesthetic management (if appropriate)

Type of anaesthesia: depends on procedure; additives commonly used with regional anaesthesia (PNB/neuraxial) and local infiltration; may be combined with GA.
Airway: not dictated by additives; GA cases may use ETT or SGA as appropriate; regional-only cases often no airway device.
Duration: additives aim to extend block duration (hours) and/or improve block density; dexamethasone often adds the longest prolongation for PNB.
Painfulness: additives are used to improve analgesia and reduce opioid requirements; neuraxial clonidine can improve analgesia but may cause hypotension/sedation.
Monitoring/precautions: standard monitoring; consider haemodynamics (clonidine), arrhythmia risk (adrenaline), glucose/infection risk (systemic dexamethasone), and off-label perineural use (dexamethasone).

Why add adjuncts to local anaesthetic?

Aims: prolong duration, improve block quality/density, speed onset (less consistent), reduce systemic absorption/toxicity, provide haemostasis (infiltration), reduce postoperative pain/opioid use, reduce PONV (dexamethasone).
Trade-offs: more drugs = more side effects; some uses are off-label (notably perineural dexamethasone).

Adrenaline (epinephrine) as an LA additive

Mechanism: α1 vasoconstriction → reduced local blood flow → slower LA uptake → prolonged duration + reduced peak plasma levels; β effects may increase HR/contractility if absorbed.
Clinical effects: prolongs duration most for short/intermediate-acting LAs (e.g., lidocaine); less additional benefit with long-acting LAs (bupivacaine/ropivacaine) but still reduces systemic absorption and provides haemostasis in infiltration.
Common concentrations: 1:200,000 (5 micrograms/mL) and 1:100,000 (10 micrograms/mL).
- Example: 20 mL of 1:200,000 contains ~100 micrograms adrenaline.
Uses: infiltration for haemostasis; PNB to prolong lidocaine/mepivacaine blocks; epidural test dose (detect intravascular injection via HR rise) and to reduce systemic uptake.
Epidural test dose (typical): lidocaine 1.5% 3 mL + adrenaline 1:200,000 (15 micrograms). Interpretation depends on patient factors and concurrent drugs.
- False negatives: beta-blockade, labour analgesia (pain/stress), high spinal sympathectomy, elderly, sedation/GA.
- False positives: anxiety, pain, uterine contraction, hypovolaemia, arrhythmias.
Benefits: reduced systemic absorption → may reduce risk of LAST (but does not eliminate it); marker for intravascular injection when included in test dose.
Risks/contraindications/avoid: tachycardia, hypertension, arrhythmias, myocardial ischaemia; caution in severe IHD, uncontrolled hypertension, significant arrhythmias, hyperthyroidism, pheochromocytoma.
End-artery/ischemia concerns: avoid high concentrations in areas with tenuous blood supply (traditional teaching: fingers, toes, penis, pinna, nose). Modern evidence suggests dilute adrenaline with lidocaine may be safe in digits in selected settings, but exam-safe answer: avoid in end-artery territories and in compromised perfusion (Raynaud’s, vasculitis, severe PVD).
Drug interactions: non-selective beta-blockers → unopposed α vasoconstriction (hypertension/bradycardia); halogenated volatile agents increase arrhythmogenicity (historically with halothane; still consider risk with catecholamines).

Clonidine as an LA additive

Class/mechanism: α2-agonist (central + peripheral). Analgesia via spinal dorsal horn (inhibits nociceptive transmission), hyperpolarisation of neurons; sympatholysis → sedation, bradycardia, hypotension.
Where used: neuraxial (spinal/epidural) and perineural adjunct for PNB (less common than dexamethasone/dexmedetomidine).
Effects: prolongs sensory (and sometimes motor) block; improves analgesia; may reduce shivering; may reduce opioid requirement.
Typical dosing (exam ranges; local policy varies):
- Intrathecal: often 15–75 micrograms (higher doses increase hypotension/sedation).
- Epidural: commonly 75–150 micrograms bolus or infusion adjunct (dose depends on setting).
- Perineural (PNB): commonly 0.5–1 microgram/kg or fixed doses ~50–150 micrograms; balance against haemodynamic effects.
Adverse effects: hypotension, bradycardia, sedation, dry mouth; potential for delayed discharge; additive hypotension with neuraxial sympathetic blockade.
Cautions: hypovolaemia, severe aortic stenosis/fixed cardiac output states, conduction disease, patients on beta-blockers, frail elderly.
Withdrawal: abrupt cessation after prolonged systemic use can cause rebound hypertension (less relevant to single-shot additive but relevant in viva if patient is on chronic clonidine).

Dexamethasone as an LA additive

Class/mechanism: glucocorticoid. Analgesic prolongation likely via anti-inflammatory effects, reduced ectopic neuronal discharge, modulation of potassium channels; systemic effects also contribute (especially if absorbed).
Routes: IV (antiemetic + analgesic-sparing) and perineural (to prolong PNB). Perineural use is often off-label; consider local governance/consent.
Effect on PNB: reliably prolongs duration of analgesia (often several hours); tends to prolong sensory more than motor (variable by block/LA).
Typical dosing:
- IV: 4–8 mg commonly for PONV prophylaxis; up to 8–10 mg used in some analgesic protocols (balance against hyperglycaemia).
- Perineural: commonly 4 mg (sometimes 8 mg); use preservative-free preparation where possible; avoid particulate/contaminants.
IV vs perineural: evidence suggests IV dexamethasone provides meaningful prolongation of analgesia and may be similar to perineural in many settings; perineural may add small incremental benefit but with off-label considerations.
Adverse effects (single dose): transient hyperglycaemia (important in diabetes), possible perineal burning/itching with rapid IV injection (reduce by slow injection/dilution), theoretical infection/wound healing concerns (minimal with single dose), mood changes (rare).
Neurotoxicity: clinical data have not shown clear neurotoxicity with perineural dexamethasone at common doses, but long-term safety is not fully established; avoid intraneural injection; adhere to ultrasound safety principles.
Contraindications/cautions: uncontrolled diabetes, active systemic infection (relative), severe immunosuppression (context-specific), history of steroid psychosis (rare).

Comparative summary (high-yield)

Adrenaline: best for reducing systemic absorption, haemostasis, prolonging short-acting LAs; risks are cardiovascular and local ischaemia in compromised territories.
Clonidine: analgesia + prolongation with neuraxial/PNB; key limitation is hypotension/bradycardia/sedation.
Dexamethasone: strong prolongation of PNB analgesia; IV also gives PONV prophylaxis; watch hyperglycaemia and off-label perineural use.

Practical prescribing/administration points

Check compatibility: avoid mixing multiple adjuncts without evidence/policy; label syringes clearly; document total doses (LA + additive).
Use lowest effective dose; consider patient-specific risks (IHD/arrhythmia for adrenaline; haemodynamics for clonidine; diabetes for dexamethasone).
For perineural dexamethasone: consider consent for off-label use; use preservative-free; avoid intraneural injection; follow ultrasound best practice.
Remember: additives do not make an unsafe LA dose safe—calculate maximum LA dose and monitor for LAST.

You are asked: “Why do we add adrenaline to local anaesthetic solutions?” Give a structured answer.

Cover mechanism, benefits, and limitations.

Mechanism: α1 vasoconstriction → reduced blood flow → reduced LA uptake → lower peak plasma level and longer duration.
Benefits: prolongs block (especially lidocaine), improves haemostasis in infiltration, may reduce systemic toxicity risk, can act as intravascular marker (test dose concept).
Limitations/risks: tachycardia, hypertension, arrhythmias, ischaemia in compromised vascular territories; less incremental prolongation with long-acting LAs.

Calculate the adrenaline dose: How many micrograms are in 20 mL of 1:200,000 adrenaline?

Know the common conversions.

1:200,000 = 5 micrograms/mL.
20 mL × 5 micrograms/mL = 100 micrograms.

Describe an epidural test dose using adrenaline: what is it, what are you trying to detect, and what are the pitfalls?

Examiners want the concept + limitations rather than a single ‘perfect’ recipe.

Aim: detect intravascular (and sometimes intrathecal) catheter placement before dosing the epidural.
Typical test dose: lidocaine 1.5% 3 mL + adrenaline 1:200,000 (≈ 15 micrograms).
Positive signs: HR rise (e.g., ≥10–20 bpm) and/or SBP rise shortly after injection; interpret in context.
False negatives: beta-blockade, labour analgesia, sedation/GA, high neuraxial block, elderly.
False positives: pain/anxiety, uterine contractions, hypovolaemia, arrhythmias.

Where should you avoid adrenaline-containing local anaesthetic and why?

Give the exam-safe answer and then add nuance if time.

Avoid in end-artery or compromised circulation: digits, nose, pinna, penis (traditional), and in Raynaud’s/PVD/vasculitis or flap compromise.
Rationale: vasoconstriction → risk of ischaemia/necrosis in vulnerable tissues.
Also caution in significant IHD/arrhythmias/uncontrolled hypertension due to systemic absorption effects.

Explain how clonidine improves analgesia when used neuraxially.

Mechanism + clinical effects + side effects.

Clonidine is an α2-agonist acting at the dorsal horn: reduces neurotransmitter release and nociceptive transmission; hyperpolarises neurons.
Produces analgesia and prolongs sensory block; may reduce shivering.
Sympatholysis causes hypotension/bradycardia and sedation—dose limiting.

What doses of clonidine might you use intrathecally and what would stop you using it?

Give a safe range and patient-specific contraindications.

Intrathecal dose commonly 15–75 micrograms (higher doses increase hypotension/sedation).
Avoid/caution: hypovolaemia, severe aortic stenosis/fixed output, significant bradycardia/conduction disease, frail elderly, patients on beta-blockers where bradycardia may be problematic.
Plan mitigation: optimise volume status, incremental dosing, vasopressors ready, close monitoring.

Discuss dexamethasone as an adjunct to peripheral nerve blocks: route, dose, benefits, and concerns.

Examiners like you to mention IV vs perineural and off-label use.

Routes: IV (PONV prophylaxis + analgesic-sparing) and perineural (prolongs block).
Dose: IV 4–8 mg; perineural commonly 4 mg (sometimes 8 mg), ideally preservative-free.
Benefits: prolongs duration of analgesia (often several hours), reduces opioid requirement; IV reduces PONV.
Concerns: hyperglycaemia (diabetes), off-label perineural use/governance/consent, theoretical neurotoxicity (not clearly demonstrated), infection/wound healing concerns minimal with single dose.

A diabetic patient is having a brachial plexus block. How does dexamethasone change your plan?

Focus on risk-benefit and perioperative glucose management.

Recognise risk of steroid-induced hyperglycaemia even after a single dose; discuss with patient/team.
Consider using the lowest effective dose (or omit), consider IV route if using primarily for PONV, and plan perioperative glucose monitoring/insulin adjustment per local protocol.
If perineural use considered: ensure governance/consent and preservative-free preparation.

Which additive is most useful for haemostasis and why?

Adrenaline: α1-mediated vasoconstriction reduces bleeding in the infiltrated field.

How do these additives affect the risk of local anaesthetic systemic toxicity (LAST)?

They modify risk differently; none replaces safe dosing and monitoring.

Adrenaline: reduces systemic absorption and peak plasma levels; may reduce LAST risk and provides marker for intravascular injection (but not foolproof).
Clonidine: does not reliably reduce systemic absorption; main issues are haemodynamic/sedative effects rather than LAST mitigation.
Dexamethasone: not a LAST mitigation strategy; primary effects are prolongation/antiemesis; still calculate max LA dose and use incremental injection/aspiration/US guidance.