Local anesthetic systemic toxicity sits at the crossroads of three domains: pharmacology, regional anesthesia, and crisis management. That is why it shows up so often. The good news is that the content is small and the treatment is a short, fixed sequence.
Why it happens
Every local anesthetic blocks voltage gated sodium channels. When enough drug reaches the circulation, through an intravascular injection or rapid absorption from a vascular site, it blocks sodium channels in the brain and the heart.
Absorption depends on the site. From highest to lowest blood levels:
Intravenous, tracheal, intercostal, caudal, epidural, brachial plexus, sciatic and femoral, subcutaneous.
A common mnemonic is “ICE BP SS” (intercostal, caudal, epidural, brachial plexus, sciatic, subcutaneous). Intercostal blocks give the highest levels of the common regional techniques because the space is so vascular.
The classic progression
Toxicity usually climbs in a predictable order as the blood level rises.
- Circumoral numbness, metallic taste, tinnitus, lightheadedness
- Visual disturbance, slurred speech, muscle twitching
- Seizures
- CNS depression, coma, respiratory arrest
- Cardiovascular collapse: hypotension, bradycardia, arrhythmias, arrest
The exam trap: bupivacaine can skip the warning signs. With bupivacaine, cardiovascular collapse can be the first sign, and it is notoriously hard to resuscitate.
Bupivacaine binds cardiac sodium channels tightly and lets go slowly (“fast in, slow out”), which is why its CC/CNS ratio is low compared with lidocaine. Ropivacaine and levobupivacaine were developed to be less cardiotoxic.
Maximum doses worth memorizing
| Drug | Plain | With epinephrine |
|---|---|---|
| Lidocaine | 4.5 mg/kg | 7 mg/kg |
| Mepivacaine | 4.5 mg/kg | 7 mg/kg |
| Bupivacaine | 2.5 mg/kg | 3 mg/kg |
| Ropivacaine | 3 mg/kg | not applicable |
| Chloroprocaine | 11 mg/kg | 14 mg/kg |
Exact ceilings vary a little by text. Learn the relative order and the common values, and read the question for the number they give you.
The treatment sequence
- Stop injecting. Call for help and get the lipid kit.
- Airway and oxygen. Ventilate with 100 percent oxygen. Hypoxia and acidosis make toxicity worse, so prevent both.
- Stop seizures. Benzodiazepines are first line. Avoid large doses of propofol if the patient is hemodynamically unstable.
- Give 20 percent lipid emulsion early, at the first sign of serious toxicity, not after everything else fails.
- If the heart stops, start ACLS with changes (below), and think early about cardiopulmonary bypass if it is available.
Lipid emulsion doses
Lipid works mainly as a “lipid sink” that pulls the drug out of the tissues, with added effects on cardiac metabolism and contractility.
| Patient | Bolus | Infusion |
|---|---|---|
| Over 70 kg | 100 mL over 2 to 3 minutes | 200 to 250 mL over 15 to 20 minutes |
| 70 kg or less | 1.5 mL/kg (lean body weight) over 2 to 3 minutes | 0.25 mL/kg/min |
If the patient stays unstable, repeat the bolus once or twice and double the infusion to 0.5 mL/kg/min. Continue the infusion for at least 15 minutes after the circulation is stable. The upper limit is about 12 mL/kg of lipid.
Propofol is not a substitute. It is a lipid, but the amount of lipid is tiny and the cardiovascular depression is not.
ACLS changes in LAST
- Epinephrine in small doses, 1 mcg/kg or less, rather than the standard 1 mg.
- Avoid vasopressin, calcium channel blockers, beta blockers, and more local anesthetic (so no lidocaine for the arrhythmias).
- Amiodarone is the antiarrhythmic of choice.
- Expect a long resuscitation. Bupivacaine arrests can need an hour of CPR.
Prevention, which the exam also tests
- Aspirate before every injection, and inject incrementally, 3 to 5 mL at a time.
- Use a test dose with epinephrine. A heart rate rise of 10 or more, or a systolic rise of 15 mmHg or more, suggests an intravascular injection.
- Use ultrasound guidance and the lowest effective dose.
- Monitor for at least 30 minutes after a large block.
- Be more careful at the extremes of age, with heart failure, liver disease, and low muscle mass.
Quick hits for test day
- Earliest signs: perioral numbness, tinnitus, metallic taste.
- Bupivacaine can arrest without a warning.
- Seizures: benzodiazepine first.
- Lipid 20 percent: 1.5 mL/kg bolus, 0.25 mL/kg/min, ceiling about 12 mL/kg.
- Epinephrine 1 mcg/kg or less. No vasopressin, no lidocaine.
The CRNA Dojo local anesthetics lessons cover the pharmacology and the blocks side by side, and every lesson comes with a printable cheat sheet you can tape inside a locker.
Sources
- American Society of Regional Anesthesia and Pain Medicine (ASRA), checklist for treatment of local anesthetic systemic toxicity
- Barash, Clinical Anesthesia
- Nagelhout, Nurse Anesthesia
