Local Anaesthetic (LA)
Local Anaesthetic (LA) Agents
Overview of LA Agents
LA agents have a shared MoA of reversible blocking voltage-gated sodium channels (in neuronal membranes)
The commonly used LA agents today are all amides:
- Lignocaine (lidocaine)
- Prilocaine
- Bupivacaine
- Levobupivacaine (isomer of bupivacaine)
Lignocaine and prilocaine have a rapid onset and intermediate duration, making them ideal for minor surgical procedures, dental anaesthesia etc.
Bupivacaine and levobupivacaine have a slower onset and much longer duration, therefore it is favoured for regional nerve blocks, epidural and spinal anaesthesia.
Maximum Doses
It is important to be aware of the maximum doses of lidocaine and bupivacaine. [Ref]
| LA agent | Max dose without adrenaline | Max dose with adrenaline |
|---|---|---|
| Lidocaine | 3 mg/kg | 6 mg/kg |
| Bupivacaine | 2 mg/kg | |
Adrenaline can be added to the local anaesthetic solution to increase the maximum safe dose as it causes local vasoconstriction, which reduces systemic absorption. It also prolongs the duration of the anaesthetic and reduces local bleeding.
However, note that adding adrenaline does NOT increase the maximum safe dose of bupivacaine. This is because bupivacaine inherently has a strong vasoconstrictive effect, which limits its systemic absorption to some extent, unlike lidocaine or prilocaine, which are more vasodilatory.
Avoid adding adrenaline to the LA solution when it is being used in areas supplied by end-arteries or with limited collateral circulation (e.g. finger, toes, nose, ear, penis). The strong vasoconstrictive effect of adrenaline can cause ischaemic injury in these areas.
Adverse Effects / Complications
Use of LA agents has low risks of complications.
Key adverse effects to be aware of:
- Injection site reaction
- Nerve injury
- Allergic reaction (rare with amides)
- Cardiotoxicity (bupivacaine is the most cardiotoxic agent)
- Local anaesthetic systemic toxicity (LAST)
Local Anaesthetic Systemic Toxicity (LAST)
Cause:
- Accidental intravascular injection, or
- Excessively high dose (→ systemic absorption)
Presentation:
- CNS manifestation
- Early symptoms – tinnitus, metallic taste, perioral paraesthesia, agitation, visual disturbances
- Seizures (common)
- Late CNS depression (e.g. respiratory depression, coma, unconsciousness)
- Cardiovascular manifestation
- Initial cardiac excitation – tachyarrhythmia, hypertension
- Late cardiac depression – bradyarrhythmias, asystole, cardiac arrest
Management:
- Stop the LA administration immediately
- The mainstay of management is IV lipid emulsion (20% Intralipid)
- Supportive care (secure airway, 100% oxygen, assist ventilation)
- Provide ALS if needed, treat seizures with benzodiazepines
To prevent LAST:
- Always adhere to the maximum recommended doses of the LA agent (see above)
- Always perform aspiration with the syringe before each injection to ensure the needle is not inside a blood vessel
Local Anaesthetic (LA) Dose Calculation
Calculating LA Dose
Key concepts:
- Local anaesthetic (LA) doses are calculated based on the maximum recommended dose (mg/kg) and the patient’s body weight
- LA drugs are typically supplied as percentage (%) solutions (e.g. 1% lidocaine)
- To safely administer an LA, you must calculate the volume in millilitres (mL) that delivers the maximum safe dose
A 3-step process can be followed to calculate the LA dose:
| Step | Purpose | Formula |
|---|---|---|
| 1 | Calculate total maximum dose for your patient | Patient weight (kg) × max recommended dose (mg/kg) = mg |
| 2 | Convert % solution into mg/mL | % strength × 10 = mg/mL (e.g., 1% = 10 mg/mL) |
| 3 | Calculate max safe volume to administer | Total dose (mg) ÷ concentration (mg/mL) = volume (mL) |
This method is not restricted to LA agents; it applies to any drug dosed by weight and provided as a % solution (e.g. dobutamine, dopamine).
Always double-check the units when working with drug concentrations. % stands for how many g is in 100 mL, so 1% is 1g in 100 mL.
Worked Example 1
Question:
- Calculate the maximum safe volume of 2% lidocaine (without adrenaline) that can be administered to a 60 kg adult. The maximum dose of lidocaine without adrenaline is 4.5 mg/kg.
Working (mapped to the earlier mentioned 3-step process):
- Total maximum dose = 60 kg × 4.5 mg/kg = 270 mg
- Concentration of 2% lidocaine = 2% × 10 = 20 mg/mL
- Volume to administer = 270 mg ÷ 20 mg/mL = 13.5 mL
Answer:
- The maximum safe volume of 2% lidocaine without adrenaline for a 60 kg adult is 13.5 mL.
Worked Example 2
Question:
- Calculate the maximum safe volume of 0.5% bupivacaine with adrenaline that can be administered to a 70 kg adult. The maximum dose of bupivacaine with adrenaline is 3 mg/kg
Working:
- Total maximum dose = 70 kg × 3 mg/kg = 210 mg
- Concentration of 0.5% bupivacaine = 0.5% × 10 = 5 mg/mL
- Volume to administer = 210 mg ÷ 5 mg/mL = 42 mL
Answer:
- The maximum safe volume of 0.5% bupivacaine with adrenaline for a 70 kg adult is 42 mL.