Think about the number of times a month you use a local anaesthetic; maybe not every day, but I know there are a lot of emergency department shifts when I use a local anaesthetic. The uses and applications for local anaesthesia abound: wound care and laceration closure, pain control with painful procedures like a paracentesis or lumbar puncture, and targeted regional anaesthesia blocks after a broken hip. It is important to know and understand a bit more about this commonly used class of drug given how often we use them in emergency medicine, including the recommended dosing, signs of toxicity, and treatment of toxicity.
Local anaesthetics fall into two divisions, based on their chemical structure:
- the Esters (have one i): procaine, cocaine, tetracaine, chloroprocaine, etc
- the Amides (have two i’s): lidocaine, bupivacaine, mepivacaine, prilocaine, ropivacaine, etc
Effect
These drugs have their effect as sodium-channel blocking medications with variable durations of action. Interestingly, 1% diphenhydramine has also been used as a local anaesthetic since the 1930s, given its sodium channel blocking mechanism. Local anaesthetics can be administered with other drugs, namely epinephrine, to help increase the duration of action and minimize the spread of the anaesthetic from the site of injection.
Maximum Dose
The safe maximal dose for the local anaesthetics is based on patient weight and correlates to the risk of systemic toxicity. The maximally safe dose of two common local anaesthetics is detailed below, and as you can see, the use of epinephrine allows for an increased dose of local anaesthetic injection.
| Max dose without Epi | Max dose with Epi | Duration of Action | |
|---|---|---|---|
|
Lidocaine |
4.5 mg/kg |
7 mg/kg |
0.5 - 1.5 hours |
|
Bupivacaine |
3 mg/kg |
3 mg/kg |
6-8 hours |
Usage abd Absorbtion
Absorption into the bloodstream of a local anaesthetic can occur when the drug is injected directly into the bloodstream. Still, it can also occur in highly vascular areas or near neurovascular bundles in locations such as intracostal, epidural, and the brachial plexus. Local anaesthetic systemic toxicity (LAST) occurs when there are elevated circulating levels of local anaesthetic and occurs within minutes of injection. As you may know, lidocaine is used intravenously as an antiarrhythmic drug, and cocaine when used (or abused) systemically can cause numerous systemic effects and a sympathomimetic toxidrome. Bupivacaine is the most commonly discussed cause of LAST, and extra care should be taken when utilizing this for local anaesthesia.
Sign and Symptoms of LAST
Signs and symptoms of LAST predominate in the central nervous system and the cardiovascular system. CNS symptoms can include oral/perioral numbness, paresthesia, restlessness, tinnitus, fasciculations/tremors, seizures, decreased level of consciousness, and/or apnea. Cardiovascular symptoms can include: hypertension and tachycardia though more commonly vasodilation and hypotension, sinus bradycardia, AV blocs, conduction defects (notably: long PR and QRS), ventricular dysrhythmias, cardiovascular collapse, and/or cardiac arrest.
The differential diagnosis for LAST includes anaphylaxis (rare with amides), other sodium channel blockers (antihistamines, TCAs, cocaine, antimalarials), and anxiety. However, the timing nearly immediately following local anaesthetic administration should help one to hone in on the diagnosis.
Management
If a patient develops LAST, ACLS protocols should be followed. Furthermore, lipid emulsion (Intralipid) is the treatment that will help bind the anaesthetic in the bloodstream. While this medication is not on the WHO essential medication list, in a patient with LAST, Intralipid should be administered if available. Dosing is a 1.5 mL/kg bolus (standard dose of 100mL for 70kg patient), followed by a 0.25-0.5 mL/kg/min infusion until the patient is hemodynamically stable (and for at least 10 minutes).
How To Decrease Risk of LAST
- know the maximum dose your patient can receive
- know the dose you’re giving by dose (milligrams) and how that correlates to drug volume (mg/mL)
- aspirate prior to injection(s) to ensure you are not in a blood vessel
- consider using point of care ultrasound to ensure needle location
References and Further Reading
- https://www.ncbi.nlm.nih.gov/books/NBK499964/
- http://www.emdocs.net/toxcard-diphenhydramine-rescue-useful-hives
- https://coreem.net/core/last/
- https://www.openanesthesia.org/local_anesthetics_systemic_toxicity
- https://www.aliem.com/2015/04/lipid-rescue-why-arent-we-using-it
- https://emedicine.medscape.com/article/873879-overview
- https://rebelem.com/local-anesthetic-systemic-toxicity-last/
