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Mr. Thompson, 62 years old. Mr. Thompson has just had left knee replacement surgery where he received a spinal block utilizing bupivacaine and an injection of bupivacaine at the surgical site. He was stable throughout the procedure and comes to you in the PACU on 8L simple face mask with an OPA in place. His PMH includes hypertension and osteoarthritis. Your initial assessment reveals no response to stimulation, regular respirations with adequate depth and no accessory muscle use. VS are HR 54, RR 10, SpO2 100%, BP 112/62, Temp 35.8° C.  The dressing is CDI.

What are local anesthetics?

Local anesthetics are medications that block nerve impulses so that pain signals aren’t transmitted to the brain. The result is that the anesthetized area is numb and pain is controlled. A key way to know if a medication is a local anesthetic is that it ends in the suffix -caine. Common local anesthetics are ropivacaine, bupivacaine, and lidocaine. 

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Local anesthetics are used in dentistry, surgery, and for invasive procedures such as central line insertion and placement of stitches. They are also widely used postoperatively and even during labor and delivery (including cesarean delivery). Local anesthetics also have multiple routes of administration, which include:

  • Injection – Ex: When the dentist injects lidocaine prior to filling a cavity.
  • Skin (topical) – Ex: EMLA cream applied prior to IV insertion or a lidocaine patch used for back pain.
  • Skin (airway) – Ex: Lidocaine is utilized prior to bronchoscopy to diminish the gag reflex.
  • Epidural – With an epidural, the medication diffuses through the dura into the CSF to cause analgesia. Ex: A local anesthetic with or without opioid medication is injected into the epidural space during labor.
  • Intrathecal – Also called “spinal” anesthesia or a “spinal block.” Intrathecal anesthetics are injected into the CSF and spinal canal for total pain control, complete loss of sensation, and loss of movement. Ex: A patient undergoing knee surgery may receive a spinal block to control pain during and immediately after the procedure. 
  • Perineural – The medication is injected into the tissue surrounding the nerves. Ex: A patient receives a peripheral nerve block of ropivacaine after knee surgery.
  • Intravenous – Local anesthetics can also be administered intravenously to provide a regional block to an extremity for brief surgical procedures. Note that a tourniquet is utilized above the site of surgery to help prevent the local anesthetic from reaching systemic circulation. Ex: A patient undergoing carpal tunnel surgery receives regional pain control through the IV administration of a local anesthetic.

Just because local anesthetics are “local” doesn’t mean systemic toxicity can’t occur. Local anesthetic systemic toxicity (LAST) occurs when local anesthetics extend into the system at toxic levels. It most typically occurs due to the physician accidentally injecting the medication into the intravascular space. However, it can also be from locally administered overdose, which leads to systemic toxicity. 

This occurs due to an imbalance between absorption and biotransformation of the anesthetic. Increased absorption may be due to injection into highly vascularized tissue or when utilized in a patient with low cardiac output who suddenly has cardiac output restored. Additionally, elderly patients tend to have prolonged absorption due to decreased muscle mass and patients with renal disease simply don’t clear the medication efficiently which can lead to toxic plasma levels.

In general, patients at higher risk for local anesthetic overdose are the very young and the elderly, and patients with comorbidities including heart, lung, liver and renal disease. Pregnancy is also a risk factor for LAST. The increased protein levels in pregnancy can lead to increased unbound levels of the anesthetic and lead to toxicity. Additionally, metabolic syndromes place patients at higher risk for LAST. 

Pathophysiology of local anesthetic systemic toxicity

Two key consequences of LAST are cardiac toxicity and CNS toxicity. Cardiac toxicity occurs most typically due to accidental injection of the local anesthetic into the intravascular space, though it can occur from locally-administered overdose as well. The medication binds to and inhibits sodium channels which affects cardiac cell action potential, leading to cardiac dysfunction and ventricular arrhythmias.

A leading theory suggests that CNS toxicity occurs due to disturbances related to specific potassium channels in neuronal tissue, causing increased neuronal excitability. The effect most often seen in patients is seizures, though other CNS symptoms can occur.

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Mr. Thompson is now awake, the OPA is removed and he’s enjoying some PACU ice chips and waiting for the effects of his spinal block to wear off. The pain team comes to the bedside to insert a catheter for local perineural delivery of ropivacaine. Mr. Thompson receives his first bolus of ropivacaine and a few minutes later complains of a metallic taste in his mouth and numbness in his lips. His speech seems a bit slow and he seems mildly confused. 

Signs and Symptoms of local anesthetic systemic toxicity

Early CNS symptoms include:Signs of cardiac toxicity generally indicate more severe toxicity and include:
Feeling of uneaseDizziness
Numbness/tingling around the mouth
Metallic taste in the mouth
Tachycardia and hypertension may occur initially
Severe hypotension
Shortness of breath
Ventricular ectopy and ventricular tachycardia
Wide QRS complex
Changes to the ST segment

If not caught and managed early, serious manifestations include respiratory arrest, seizures, ventricular fibrillation, cardiovascular collapse, coma, and death.

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How is LAST treated?

Once LAST is suspected or recognized, a key intervention is to stop any local anesthetics that are infusing. For example, a postoperative patient who just had knee surgery may have ropivacaine infusing via an elastomeric pump or a portable pump such as this one here. Other patients may have an epidural infusion utilizing a specific type of pump such as this one (note this will never be the same pump as the one used for standard IV fluids and medications). It’s also vital you immediately notify the MD as the patient will require alternative pain management and potentially aggressive management to prevent complications. 

Initial management of LAST includes supporting the patient’s airway, oxygenation, and circulation. Specific interventions ordered by the MD may include: 

  • Oxygenation to prevent hypoxia and acidosis
  • Ventilation as needed
  • Benzodiazepines to treat seizures and prevent subsequent acidosis
  • If seizures persist, the MD may utilize a paralytic agent to halt muscular activity
  • Address bradycardia with medications that increase the heart rate
  • Treat hypotension with vasopressors
  • Amiodarone may be used for ventricular arrhythmias
  • Lipid emulsion therapy may be utilized in patients with arrhythmias or prolonged seizures (studies suggest it works by drawing the lipid-soluble anesthetic agent out of the cardiac tissue)
  • In severe cases, cardiopulmonary bypass may be needed until the medication is cleared from the system
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You clamp Mr. Thompson’s ropivacaine tubing, stop the infusion and notify the MD. VS are HR 75, BP 122/72, RR 18, SpO2 92% on RA. The MD comes to the bedside and determines Mr. Thompson’s symptoms are mild. She orders supplemental oxygen to maintain SpO2 above 95%, alternative pain medication, and immediate discontinuation of the local anesthetic. He is admitted to the ICU for close observation overnight and he ultimately has a full recovery.

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