How they work
Local anaesthetics block sodium channels inside the nerve membrane, so the nerve can't conduct impulses. The block is reversible. Small, thin fibres (pain and temperature, sympathetic) block first and recover last; large motor fibres need more drug.
Onset is faster with higher concentration and volume, and when the drug is close to the nerve. Duration depends on the drug, the dose, blood supply at the site and adjuvants such as adrenaline or dexamethasone.
Two families
| Family | Examples | Notes |
|---|---|---|
| Amides (two 'i's in the name) | Lidocaine, prilocaine, mepivacaine, bupivacaine, levobupivacaine, ropivacaine | Metabolised in the liver. Used for almost all blocks. True allergy is very rare. |
| Esters | Chloroprocaine, procaine, tetracaine | Broken down quickly in the blood (plasma cholinesterase). Allergy (to PABA) more likely than with amides. |
The common drugs
Typical onset and length of a single-shot peripheral nerve block. Treat these as a guide.
| Drug | Strengths | Onset | Best for | Watch for |
|---|---|---|---|---|
| Lidocaine | 1–2% | Fast (5–15 min) | Short procedures, quick onset, mixing with a long-acting drug | Short (1–2 h; 2–5 h with adrenaline) |
| Prilocaine | 1–2% (0.5% for IVRA) | Fast | Intravenous regional anaesthesia (Bier's block) | Methaemoglobinaemia at high doses |
| Mepivacaine | 1.5% | Fast | Medium-length surgery, e.g. hand and arm | Not for long analgesia |
| Bupivacaine | 0.25–0.5% | Slow (15–30 min) | Long analgesia; spinal (heavy 0.5%) | Most cardiotoxic: arrhythmias, hard to resuscitate |
| Levobupivacaine | 0.25–0.5% | Slow (15–30 min) | Long analgesia | Less cardiotoxic than bupivacaine, same care with dose |
| Ropivacaine | 0.2–0.75% | Moderate (10–20 min) | Long analgesia with less motor block at low strength | Slightly shorter than bupivacaine |
| Chloroprocaine | 2–3% | Very fast (3–5 min) | Very short procedures and day-case spinal (1%) | Very short (30–60 min) |
Maximum doses
Maximum doses are a guide, not a safe limit. Toxicity depends on where you inject (fastest absorption from intercostal and other vascular sites), how fast, and the patient: reduce the dose for the elderly, frail, small, pregnant, or those with heart, liver or kidney disease. Add up every dose given, including the surgeon's infiltration.
| Drug | Maximum (mg/kg) | Usual maximum single dose |
|---|---|---|
| Lidocaine, plain | 3 | 200 mg |
| Lidocaine with adrenaline | 7 | 500 mg |
| Prilocaine | 6 | 400 mg |
| Bupivacaine (with or without adrenaline) | 2 | 150 mg |
| Levobupivacaine | 2 | 150 mg |
| Ropivacaine | 3 | 225 mg |
Use our dose calculator, which uses lean body weight.
Choosing a drug
- How long the surgery lasts, and how long the patient needs pain relief afterwards.
- How quickly the block must work.
- Whether movement must be kept (lower strengths of ropivacaine or levobupivacaine for analgesia).
- The patient's toxicity risk and other conditions.
- Availability and local protocols.
- When in doubt, choose the drug with the safer toxicity profile and adjust volume and concentration to the block you need.
Adjuvants
- Adrenaline (1:200,000–1:400,000): shows an intravascular injection early (heart rate rise), lowers peak blood levels and lengthens the block. Avoid in end-artery areas such as the digits and penis, and take care in heart disease.
- Dexamethasone: prolongs analgesia by several hours. The intravenous route (for example 0.1–0.2 mg/kg, up to 8 mg) works nearly as well as adding it to the injection, and avoids off-label perineural use.
- Dexmedetomidine or clonidine: prolong the block, but can cause sedation, low blood pressure and slow heart rate.
- Sodium bicarbonate: can speed onset of lidocaine and mepivacaine slightly; it makes bupivacaine and ropivacaine precipitate.
Toxicity (LAST)
- Early signs: tingling around the mouth, metallic taste, ringing in the ears, light-headedness, agitation, confusion, twitching.
- Late: seizures, low consciousness, slow or fast arrhythmias, cardiac arrest. Heart problems can come first, especially with bupivacaine.
- If suspected: stop injecting, call for help, oxygen and airway, treat seizures (benzodiazepine), and give lipid emulsion 20% (1.5 mL/kg bolus, then infusion). Follow the LAST protocol.
- Prevention: ultrasound guidance, the lowest effective dose, aspiration and incremental injection, and monitoring for at least 30 minutes.
References
- El-Boghdadly K, Pawa A, Chin KJ. Local anesthetic systemic toxicity: current perspectives. Local Reg Anesth 2018;11:35–44. PubMed
- Neal JM, Neal EJ, Weinberg GL. American Society of Regional Anesthesia and Pain Medicine Local Anesthetic Systemic Toxicity checklist: 2020 version. Reg Anesth Pain Med 2021;46:81–2. PubMed
- Rosenberg PH, Veering BT, Urmey WF. Maximum recommended doses of local anesthetics: a multifactorial concept. Reg Anesth Pain Med 2004;29:564–75. PubMed