LASTing Anesthetic

/ myemresidency

Author:  William Secor, DO, Emergency Medicine Resident, PGY I
Faculty:  Alexis Cates, DO, Medical Toxicology/Emergency Medicine Attending

The Case.

You are on your anesthesia rotation watching an anesthesiologist perform a femoral nerve block.  The patient is a 56-year-old male who is undergoing a knee arthroplasty.  The patient is somewhat nervous about the operation, but his vitals are within normal limits.  The area is sterilized and draped for the regional anesthesia nerve block.  Under ultrasound guidance, the anesthesiologist begins to inject 20 mL of lidocaine 2% between the fascial layers.  Within a few minutes, you notice the patient appears increasingly anxious and his heart rate is increasing.  

Learning Point 1: Local Anesthetic Systemic Toxicity (LAST)

The differential diagnosis for this presentation could include pulmonary embolism, acute coronary syndromes, panic attack or anxiety and even local anesthetic systemic toxicity.

The patient in our case is experiencing local anesthetic systemic toxicity, likely due to injection of lidocaine directly into the vasculature.  Local anesthetics operate by blocking voltage-gated sodium channels resulting in inhibition of nerve transmission.  Toxicity has a reported incidence of 1:1000 in regional nerve blocks but can also occur with inadvertent administration in a blood vessel when applying the local anesthetic subcutaneously or via incorrect dosing.  Cardiac toxicity can result in conduction disturbances, contractile dysfunction and ventricular arrhythmias.  Neurologic toxicity can present as seizures and is possibly due to membrane depolarization and increased neuronal excitability2.  Bupivacaine is more likely to cause LAST than lidocaine.  There are numerous patient-dependent risk factors that may make LAST more significant, including end-organ dysfunction, low muscle mass, pregnancy, mitochondrial disease and carnitine deficiencies1.  

The presentation of these patients will vary depending on how the anesthetic is administered.  If incorrectly administered intravenously, adverse effects can rapidly occur within minutes.  Delayed effects due to general systemic absorption will occur in the range of hours.  

In the case of lidocaine, central nervous system (CNS) symptoms tend to predominate initially: sensory abnormalities (auditory, metallic taste, circumoral numbness, visual disturbances, and dizziness), delirium, dysarthria, anxiety, somnolence, coma, respiratory suppression and seizures

With bupivacaine, cardiovascular effects present initially: sympathetic activation (tachycardia, hypertension, diaphoresis).  Later effects include bradycardia/AV block, widened QRS, reduced contractility and hypotension.  

Eventually, toxicity with either agent  may ultimately result in cardiac arrest in the form of ventricular tachycardia, ventricular fibrillation or asystole.  Lidocaine typically tends to reduce contractility more, while bupivacaine and ropivacaine more frequently cause dysrhythmias1. 

Maximum safe doses of local anesthetics

AnestheticWithout epinephrineWith epinephrine
Lidocaine5 mg/kg7 mg/kg
Bupivacaine2.5 mg/kg3 mg/kg
Mepivacaine7 mg/kg8 mg/kg
Ropivacaine3 mg/kg

The case continued…

You stop administration of the lidocaine and check the patient’s cardiac monitor.  Vital signs: heart rate 123 bpm, blood pressure 110/64 mmHg, and the patient breathing around 25 breaths per minute, saturating well on room air.  He appears anxious and confused, and he is clutching his chest.  You reach for the code cart and luckily the known antidote to what you suspect is close at hand…

Learning Point 2: Interventions

If you suspect LAST, stop the administration of the anesthetic.  Check the patient’s airway, breathing, circulation and perform any immediate resuscitation.

Intravenous crystalloid can be given as needed for volume resuscitation.

Next, lipid emulsion should be administered intravenously if there is concern for significant toxicity (particularly if there is seizure activity or cardiovascular toxicity manifesting as dysrhythmias, hypotension or cardiac arrest).  This acts as a lipid sink for the lipophilic xenobiotic causing the poisoning. 

Lipid emulsion administered intravenously is dosed initially with a bolus.  Repeat boluses can be considered.  Infusions are less common.

Initial bolus dosing by ideal body weight (IBW): 
>70 kg IBW: ~ 100 mL is generally acceptable
<70 kg IBW: 1.5 mL/kg 

*Repeat bolus dosing up to 10 mL/kg

Infusion dosing: 
>70 kg IBW: 600-1000mL/hr
<70 kg IBW: 15 mL/kg (0.25 mL/kg/min) 
Infuse for at least ten minutes after hemodynamic stability (30-60 min usually)

And don’t forget to call toxicology!  If you have a toxicology service at your facility, call the consult directly.  If not, call your local poison control center (1-800-222-1222).

The case concluded..

The patient received an initial bolus of of lipid emulsion, as well as 1 L of isotonic crystalloid.  His cardiovascular and neurological symptoms resolved and his surgery was rescheduled for the next day while he was closely monitored for recurrence of symptoms.

Learning Point 3: Application to the Emergency Department

This patient recovered fully following administration of lipid emulsion to treat what was suspected to be LAST.  Though LAST is rare, it is associated with high mortality.  It is important for the emergency physician to recall toxic doses of local anesthetics, and to understand the risks of potential infiltration into local vasculature.  It is important to recognize the signs and symptoms of LAST, and to intervene quickly when suspecting this diagnosis.  We suggest having lipid emulsion in your ED medication machines or nearby inpatient pharmacy to expedite administration when LAST is suspected.  This is especially important in EDs that treat injured pediatric patients and performs frequent regional nerve blocks with large amounts of local anesthetic.

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