Espresso: Local Anesthetic Systemic Toxicity (LAST)

/

In follow up to last week’s episode, this week we have a quick episode on local anesthetic systemic toxicity (LAST), a classic CREOG question.

First, a bit about how local anesthetics work:

  • Medications like lidocaine reversibly block sodium channels → these meds vary in lipid solubility, potency, time to onset, and duration of activity.

  • You can imagine that some of these medications will be absorbed systemically, especially if you inject it directly into a blood vessel, and can go to other places in the body and block sodium channels in far away places.

    • Organs that we generally care about in this sense are the CNS and the heart.

    • The CNS is more sensitive than the heart to effects of local anesthetics, so will generally manifest signs/symptoms of toxicity first.

To prevent systemic toxicity, there is a max dose for various forms of local anesthetics. If injected with epinephrine, these doses are higher due to epinephrine’s vasoconstriction activity (thus preventing systemic absorption by constricting local blood vessels):

ACOG PB 209

  • Effect on CNS system 

    • Initial = Tinnitus, blurred vision, dizziness, circumoral numbness

    • After can have nervousness, agitation, muscle twitching due to blockage of inhibitory pathways → seizures 

    • You can also have CNS depression like slurred speech, drowsiness, unconsciousness, and even respiratory arrest  

  • Effect on CV system

    • Local anesthetics can block fast sodium channels in the Purkinje fibers of the heart → decreased rate of depolarization 

    • Can lead to prolonged PR intervals and widened QRS complexes 

    • Can lead to sinus bradycardia or even ventricular arrhythmias, especially with bupivacaine 

  • Treatment

    • Stop injecting the local anesthetic!

    • Call for help - definitely call your anesthesia colleagues.

      • Also alert cardiopulmonary bypass team because resuscitation may be prolonged.

      • Some hospitals may have a LAST rescue kit.

    • Airway management - ventilate, and get advanced airway device if necessary 

    • Control seizures 

      • Benzodiazepines preferred 

      • Avoid large doses of propofol 

    • Treat hypotension and bradycardia - if pulseless, start CPR 

    • Give lipid emulsion therapy - lipid emulsion 20% 

      • If >70kg, bolus 100 mL Lipid emulsion over 2-3 min, then 200-250mL over 15-20 minutes 

      • If <70kg, bolus 1.5 ml/kg lipid emulsion over 2-3 min, then 0.25ml/kg/min of ideal body weight 

    • Continue monitoring - at least 4-6 hours after a cardiovascular event or at least 2 hours after a limited CNS event.