Infiltration Anaesthesia

Definition

• Pye's Surgical Handicraft, 22nd Ed., p. 102
• Goodman & Gilman's Pharmacological Basis of Therapeutics, 13th Ed.

Types

Drugs Used

1. Lignocaine (Lidocaine, Xylocaine)

• Rapid onset, good tissue diffusion
• Plain (without adrenaline): max dose 200 mg for a fit 70 kg adult
• With adrenaline 1:200,000: max dose 500 mg

2. Prilocaine (Citanest)

• Pharmacologically similar to lignocaine but less toxic
• Slower diffusion; slower absorption reduces need for vasoconstrictor
• Preferred when large volumes are required (e.g. wide head & neck infiltration, Bier's block)
• Max dose: 400 mg (plain), 600 mg with vasoconstrictor
• Caution: causes methaemoglobinaemia in excessive dosage

3. Bupivacaine (Marcain)

• Slower onset, much longer duration (4-6 hours vs. 1.5 hours for lignocaine)
• Used for more prolonged procedures and postoperative pain relief
• Adrenaline provides vasoconstriction but does not greatly prolong its action
• Plain: up to 2 mg/kg; with epinephrine up to ~2.5 mg/kg

4. EMLA Cream (Eutectic Mixture: Lignocaine + Prilocaine)

• Applied topically at least 1 hour (ideally 2 hours) before injection; cover with occlusive dressing
• Used to reduce pain of venepuncture and superficial procedures, especially in children

Safety - Prime Consideration

"Although the effectiveness of a local anaesthetic technique is clearly important, the prime consideration of the operator should be safety. The commonest cause of serious complications is the injection of a local anaesthetic drug in excessive dosage."

• Pye's Surgical Handicraft, 22nd Ed.

Dose Reduction Required In:

• Low body weight (including children)
• Elderly patients
• Debilitated patients
• Hepatic disease
• Renal impairment
• Epilepsy
• Heart block

Technique Points for Safety:

1. Always aspirate before injecting to avoid inadvertent intravascular injection
2. Inject slowly
3. Have resuscitation equipment immediately available
4. Insert an IV cannula before large-volume infiltration
5. Wait 5-10 minutes (up to 15 min for fingers) before commencing surgery

Role of Adrenaline (Epinephrine)

• Reduces systemic absorption, allowing higher safe doses
• Prolongs duration of action (especially for lignocaine)
• Reduces bleeding - creates a dry operative field
• Concentration typically 1:200,000 (safer for large volumes) or 1:100,000 (limits safe volume to 40 ml maximum due to adrenaline toxicity)

Contraindications for Adrenaline:

• End-arterial territories: fingers, toes, ear lobes, nose, penis - risk of ischaemia/gangrene
• Intracutaneous injection (risk of necrosis)
• Patients where adrenergic stimulation is undesirable (e.g. severe hypertension, thyrotoxicosis, certain arrhythmias)

Toxic Effects (Systemic Local Anaesthetic Toxicity - LAST)

Cerebral (occur first):

• Anxiety, excitement, drowsiness
• Convulsions, coma

Cardiovascular:

• Myocardial depression
• Bradycardia, hypotension
• Cardiac arrest (especially bupivacaine)

Management of Toxicity:

1. Stop injection immediately
2. Maintain airway; give oxygen
3. Convulsions: diazepam 5 mg IV (increments) or thiopentone 50 mg IV increments - use minimum dose as these may worsen myocardial toxicity
4. Respiratory arrest: oxygen via bag-mask or cuffed ETT; never use respiratory stimulants
5. Hypotension/bradycardia: atropine 0.6 mg IV; ephedrine 15-20 mg IV for combined vasopressor + cardiac stimulant effect
6. Elevate legs; start saline infusion
7. Consider intralipid (lipid emulsion) therapy for severe LAST (modern standard, per current guidelines)

Advantages of Infiltration Anaesthesia

• Provides anaesthesia without disrupting normal bodily functions
• Simple to perform
• No specialized equipment needed for basic techniques
• Avoids risks of general anaesthesia
• Suitable for day-case/outpatient procedures
• Provides residual postoperative analgesia (especially bupivacaine)

Disadvantages

• Relatively large drug volumes needed for small areas compared to nerve blocks
• Risk of systemic toxicity if large areas require anaesthesia
• Tissue distortion at surgical site (making anatomy less clear)
• Ineffective in infected/inflamed tissues (reduced pH prevents ionization of the drug)
• Not suitable for major surgery

Practical Technique

1. Identify the operative site and calculate the maximum safe drug dose
2. Select appropriate drug, concentration, and volume
3. Raise an intradermal bleb first with a fine (25-26G) needle at the injection point (less painful)
4. Advance needle through the bleb and inject subcutaneously and into deeper layers as needed, always aspirating before each injection
5. Fan the needle to cover the field without multiple skin punctures
6. Wait the appropriate time before incising (minimum 5 min, 10 min preferred)
7. Test anaesthesia before starting surgery

Relationship to Regional Techniques