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.

Test yourself…