Getting On Our Nerves: Basics of Local Anesthetic
"Into whatsoever houses I enter, I will enter to help the sick, and I will abstain from all intentional wrong-doing and harm, especially from abusing the bodies of man or woman, bound or free." -The Hippocratic Oath
Adequate analgesia is vital in the acute management of patients in the emergency department; today I intend to review the basics of the local anesthetics we use day-in and day-out in the emergency department.
1. Basic Principals of Local Anesthesia: -Local anesthetics reversibly bind to voltage-gated sodium channels, preventing the propagation of action potentials.
-Nerve fibers of smaller diameter are preferentially blocked. Nociceptors have the smallest diameter (.2μm-5μm), and are therefore the first to be affected by local anesthetics.
-Larger nerve fibers (6μm-20μm) for things like proprioception or touch can be blocked with higher doses of local anesthetic, with myelinated large nerve fibers being affected before their unmyelinated companions.
2. Choice of Hydrophilic Group: Esters Verses Amides
-Each name contains two "I"'s ("two eyes for amides").
-Examples: Lidocaine, Prilocaine, Bupivacaine
-Processed by the liver
-Decreased risk of systemic toxicity when compared to...
-Each name contains one "I"
-Examples: procaine, tetracaine
-Processed in the serum via esterases
-Increased risk of allergic reaction or systemic toxicity when compared to Amides, generally reserved for patients with previous reaction to Amide based local anesthetic.
3. "Fingers, Toes, Penis, Nose": When Not to Use Epinephrine
-Vasoconstrictors like epinephrine are thought to decrease risk of systemic toxicity, as well as the risk of bleeding, when used locally.
-They increase the duration of action of the anesthetic by keeping it localized.
-Previously thought to result in ischemia in areas with poor collateral blood flow (fingers, toes, penis, nose), which has now come into question. Multiple studies of accidental injection of anaphylaxis dose epinephrine (0.3mg) into small compartments (thumb) have not been shown to cause ischemic necrosis. -Still, it is a good rule of thumb to use local anesthetic without epinephrine in these sites.
4. Maximum Allowable Volume of Local Anesthetics: How Much is Too Much
-If you're unsure of the maximum dose, this is something that should be looked up prior to starting a procedure, but below are some common ones:
-Lidocaine without Epinephrine: 4mg/kg
-Lidocaine with Epinephrine: 7mg/kg
-Prilocaine without epinephrine 8mg/kg
-Bupivacaine without epinephrine 2mg/kg
-Bupivacaine with epinephrine 4mg/kg
-Calculating maximum allowable dose (mg/kg) x (weight in kg/10) x (1/concentration of anesthetic)
-For example, if we had a 100kg patient that we were doing a nerve block for with 1% lidocaine without epinephrine: 4mg/kg x 100kg/10 x (1/1%) = 40ml
-In the same patient with 1% lidocaine with epinephrine it would be 7 x 10 x 1/1 =70ml
-The percentage concentration indicates the grams per 100mL in the vial (ie. 1% = 1g/100mL)
-Keep in mind that these are the maximum doses, and that decreasing the amount of local anesthetic, as well as proper administration technique, decreases the risk of systemic effects.
5. Systemic effects:
-Central Nervous System:
-Methemoglobinemia: cyanosis, fatigue, grey skin color
6. Antidote: How to Treat Local Anesthetic Systemic Toxicity (LAST)
-Stop injecting local anesthetic
-Consider lipid emulsion therapy at the first sign of systemic symptoms
-If patient weighs greater than 70kg: Bolus 100mL of 20% lipid emulsion, followed by continuous infusion of 200-250ml over the next 20 minutes.
-If patient weighs less than 70kg: Bolus 1.5mg/kg of 20% lipid emulsion, followed by continuous infusion of 0.25mg/kg/ideal body weight/minute.
-When in doubt, go to this website for immediate help: http://www.lipidrescue.org/
-Alert the nearest cardiopulmonary bypass team — resuscitation may be prolonged
-Ventilate with 100% oxygen, avoid hyperventilation, and consider an advanced airway device if necessary
-Benzodiazepines are preferred
-Avoid large doses of propofol, especially in hemodynamically unstable patients
-Treat hypotension and bradycardia
-This is a perfect role for push-dose epinephrine
-If pulseless, start CPR and follow the ACLS algorithm
Editor's Commentary (by Dr. Newmyer):
A good, succinct review of the common anesthetics we use every day. Much of the literature on digit necrosis with anesthetics containing epinephrine go back to the days of using procaine and cocaine (also a potent vasoconstrictor!), and the more recent studies in people who have accidentally stuck themselves in the finger with an epi-pen (significantly more epinephrine than you will inject doing a digital block or lac repair) show no long-term effects like digit necrosis.
If you are concerned about systemic toxicity, get your whole team involved, including your pharmacist. Find out if your institution has easy access to intralipid. It had been reported (erroneously) that propofol, because it comes in a medium/large chain triglyceride lipid emulsion, could be used like intralipid if you didn’t have access to intralipid. This is false, and can actually harm your resuscitation efforts by causing worsening hypotension. (Propofol As Intralipid Therapy?? via CMC COMPENDIUM)
Here are our favorite regional anesthesia websites:
New York School of Regional Anesthesia (NYSORA.com)
Highland EM Ultrasound Block Section (http://highlandultrasound.com/med-guide)
Fortuna TJ. In: Tintinalli JE. eds. Tintinalli’s Emergency Medicine Online Updates. New York, NY: McGraw-Hill; 2011.
Thomson CJ et al. Plast Reconstr Surg. 2007;119(1):260-6. PMID: 17255681
Häfner HM et al. J Dtsch Dermatol Ges. 2005;3(3):195-9. PMID: 16372813
Ilicki J. The Journal of Emergency Medicine 2015;49:799–809. PMID: 26254284
Post edited by:
Aileen Newmyer, MD; Assistant Program Director, Mercy Health St. Vincent Medical Center Emergency Medicine Residency
Shyam Murali, MD, PGY-2; Academic Chief Resident 2020-2021, Editor-in-Chief