Local Anesthetic Systemic Toxicity: Recognizing and Treating LAST

Local Anesthetic Systemic Toxicity – internationally known as LAST – is one of those complications that occurs rarely but can lead to cardiac arrest within minutes when it does. Precisely because local anesthetics are used in virtually every clinical setting – from the emergency department to the operating room to the dental office – every professional who administers these substances must be able to recognize the early symptoms and master the treatment algorithm. Survival rates in LAST depend critically on how quickly the diagnosis is made and specific therapy with intravenous lipid emulsion is initiated.

Pathophysiology: Why Local Anesthetics Cause Systemic Toxicity

Local anesthetics block voltage-gated sodium channels – that is their intended mechanism of action on peripheral nerves. However, when systemically absorbed or accidentally injected intravascularly, they also reach sodium channels in the central nervous system and the myocardium. The pathophysiological sequence follows a clear hierarchy:

• CNS toxicity occurs first because the brain receives greater blood flow than the myocardium and inhibitory neurons are more sensitive than excitatory ones.
• Cardiotoxicity follows as plasma levels rise, with sodium channel blockade in the heart causing conduction disturbances and additional inhibition of potassium and calcium channels.

The potency of cardiotoxicity correlates with the lipophilicity of the substance. Bupivacaine is particularly feared because it has a high affinity for cardiac sodium channels and exhibits so-called "fast-in, slow-out" kinetics: it binds rapidly to the channel but dissociates only slowly – a circumstance that makes resuscitation in bupivacaine-induced cardiac arrest particularly difficult. Ropivacaine and levobupivacaine are somewhat more favorable in this regard but are by no means without risk.

Risk Factors for LAST

Not every overdose leads to intoxication, and not every intoxication requires an absolute overdose. The following factors increase the risk:

• Injection site: High vascularity means rapid absorption. Order of decreasing absorption rate: intercostal block > caudal block > epidural space > brachial plexus > subcutaneous
• Absence of vasoconstrictor additive: Epinephrine addition (e.g., 1:200,000) slows absorption by up to 30%
• Patient factors: Hepatic insufficiency, heart failure, hypoproteinemia, pregnancy (increased cardiac output), age > 70 years, children < 6 months
• Pharmacokinetics: Repeated injections with accumulation, simultaneous administration of multiple local anesthetics (additive toxicity)
• Accidental intravascular injection: The most common cause – not entirely avoidable even with ultrasound-guided regional techniques

Clinical Presentation: Recognizing the Symptom Cascade

The classic textbook picture describes a progressive course from CNS to cardiovascular symptoms. In reality, however, the presentation is more variable than one might expect. Studies show that in approximately 40% of documented LAST cases, cardiovascular symptoms occur without preceding CNS prodromes – particularly with slow absorption or when the patient is sedated.

CNS Symptoms (Early Signs)

The early signs are often subtle and easily overlooked, especially under sedation:

• Prodromes: Metallic taste, perioral numbness, tinnitus, visual disturbances (diplopia, blurred vision)
• Excitatory phase: Restlessness, agitation, muscle twitching, tremor, confusion
• Seizure: Generalized tonic-clonic – often the first moment when LAST becomes clinically obvious
• Depression: Loss of consciousness, respiratory depression progressing to apnea

Cardiovascular Symptoms

• Early: Hypertension, tachycardia (sympathetic stimulation)
• Progression: Hypotension, bradycardia
• Conduction disturbances: PR prolongation, QRS widening, AV blocks
• Terminal arrhythmias: Ventricular tachycardia, ventricular fibrillation, asystole
• Typical for bupivacaine: Treatment-refractory wide-complex tachycardia progressing to pulseless electrical activity (PEA) or asystole

Atypical Presentations

You should be aware that LAST does not always follow the textbook algorithm:

• Delayed onset: Symptoms can occur up to 30 minutes (in individual case reports even up to 60 minutes) after injection, particularly with slowly absorbing depots
• Isolated cardiotoxicity: Without any CNS symptoms, especially in sedated patients
• Non-specific symptoms: Nausea, agitation, or confusion as the only early signs

The key message: Any hemodynamic or neurological event occurring in temporal association with local anesthetic administration must prompt consideration of LAST as a differential diagnosis.

Treatment: The LAST Management Algorithm

The treatment of LAST follows a clearly defined algorithm that runs in parallel with standard emergency care. Central to this is the early administration of intravenous lipid emulsion (ILE) – the so-called "Lipid Rescue Therapy."

Immediate Measures

1. Stop the injection immediately
2. Call for help – activate the team, request the Lipid Rescue kit
3. Airway management: 100% oxygen, ventilate if respiratory depression occurs, intubate if necessary
4. Treat seizures:
   • Benzodiazepines are the agents of choice: midazolam 0.05–0.1 mg/kg IV or diazepam 0.1–0.2 mg/kg IV
   • No propofol in hemodynamically unstable situations – its cardiodepressant effects can worsen the situation
   • If unsuccessful: consider small doses of succinylcholine to control the muscular component (this does not stop cerebral seizure activity but prevents hyperthermia and acidosis)

Lipid Rescue Therapy: Intralipid 20%

Intravenous lipid emulsion is the specific antidote for LAST. The mechanism of action is multifactorial:

• Lipid sink theory: Lipid particles bind lipophilic local anesthetics in plasma, thereby extracting them from tissues (heart, CNS)
• Direct cardiac effects: Improvement of mitochondrial fatty acid oxidation, positive inotropy, restoration of cardiac energy supply
• Vascular effects: Increase in systemic vascular resistance

Dosing Protocol for Intralipid 20%

Bolus:

• 1.5 ml/kg body weight IV over approximately 1 minute
• For a 70 kg patient: approximately 100 ml as a bolus

Maintenance infusion:

• 0.25 ml/kg/min for at least 10 minutes
• For a 70 kg patient: approximately 18 ml/min = approximately 1,000 ml/h

If cardiovascular instability persists:

• Repeat bolus 1–2 times (at 5-minute intervals)
• Double the infusion rate to 0.5 ml/kg/min

Maximum dose:

• 12 ml/kg body weight in the first 30 minutes (for a 70 kg patient: approximately 840 ml)

Practical Tips

• Intralipid 20% should be available as a ready-to-use kit wherever regional anesthesia is performed – including a laminated dosing card
• Commercially available bottles: 100 ml, 250 ml, 500 ml
• Alternative products: ClinOleic, Lipofundin MCT/LCT – the evidence is strongest for pure soybean oil-based emulsions (Intralipid), but in an emergency, any 20% lipid emulsion is better than none
• The infusion can be administered through any peripheral or central venous access

Cardiovascular Management

If cardiovascular instability persists despite Lipid Rescue or cardiac arrest occurs:

• Start CPR per AHA algorithm, but with important modifications:
  • Epinephrine: Reduced individual doses of ≤ 1 µg/kg (e.g., 10–100 µg in adults instead of 1 mg). High epinephrine doses can worsen ventricular arrhythmias in LAST and reduce the effectiveness of lipid therapy.
  • Amiodarone: Agent of choice for ventricular arrhythmias (300 mg IV bolus)
  • No lidocaine as an antiarrhythmic (it potentiates sodium channel blockade!)
  • No vasopressin
  • No calcium channel blockers, no beta-blockers
• Prolonged CPR: In LAST, the prognosis after prolonged resuscitation is significantly better than with other causes of cardiac arrest – lipid therapy may take effect with a time delay
• ECMO/extracorporeal circulatory support: Consider early if available. LAST is one of the best indications for eCPR because the toxicity is reversible

Prevention: Avoiding LAST Before It Occurs

The best treatment for LAST is prevention. The following measures substantially reduce the risk:

Adhere to Dosing Limits

Knowledge of maximum doses is essential:

Substance	Maximum dose without epinephrine	Maximum dose with epinephrine
Lidocaine	4.5 mg/kg	7 mg/kg
Bupivacaine	2 mg/kg	3 mg/kg
Ropivacaine	3 mg/kg	3.5 mg/kg
Mepivacaine	4.5 mg/kg	7 mg/kg
Prilocaine	6 mg/kg	8 mg/kg

Important: When combining multiple local anesthetics, toxicities are additive. The total dose of all substances used must not be considered independently – as a rule of thumb: the sum of the fractions of the respective maximum doses should not exceed 1.

Injection Technique

• Aspiration test: Aspirate before every injection – a positive test is highly specific, but a negative test does not rule out intravascular placement
• Incremental injection: Never inject the entire volume as a single bolus; administer in aliquots of 3–5 ml with pauses of 15–30 seconds
• Ultrasound guidance: Reduces the risk of intravascular injection during peripheral nerve blocks but does not eliminate it
• Test dose: Epinephrine-containing test dose (e.g., 3 ml of lidocaine 1.5% with epinephrine 1:200,000 = 15 µg epinephrine) – a heart rate increase of > 10/min or blood pressure rise within 30–60 seconds suggests intravascular placement

Monitoring and Observation

• Continuous monitoring (ECG, pulse oximetry, blood pressure) during every regional anesthetic procedure
• Verbal contact: In awake patients, actively ask about prodromes: "Do you taste anything metallic?" "Do you hear any ringing?"
• Post-procedure observation: At least 30 minutes after injection; longer for long-acting agents or large volumes
• Sedated patients: Pay particular attention to cardiovascular parameters, as early CNS symptoms may be masked

Organizational Measures

• Lipid Rescue kit with clear dosing instructions at every location where regional anesthesia is performed
• Regular checks of availability and expiration dates
• Team training: Simulation-based training of the LAST scenario at least once per year
• Checklist/Cognitive aid: Laminated LAST algorithm cards at the workplace

Special Patient Populations

Children

In children, the risk of overdose is particularly high because small volumes can already contain toxic doses. Lipid Rescue Therapy is administered identically (same mg/kg dosing). Special considerations:

• Calculate maximum doses strictly by body weight
• Use bupivacaine with particular caution in children under 6 months (reduced protein binding, immature hepatic function)
• In cardiac arrest: standard PALS algorithm with the LAST-specific modifications described above

Pregnant Patients

Pregnant patients have an increased LAST risk due to:

• Higher cardiac output (faster absorption)
• Reduced protein binding (more free active substance)
• Physiological aortocaval compression (hemodynamically more vulnerable)

Lipid Rescue Therapy is safe and lifesaving in pregnancy. In cardiac arrest, the rules of peripartum resuscitation additionally apply (left lateral tilt, perimortem cesarean section from minute 4).

Summary: The 5 Key Points

1. Think of LAST with any neurological or cardiovascular event after local anesthetic administration – even with a time delay of up to 60 minutes.
2. Recognize early symptoms: Metallic taste, tinnitus, perioral numbness – actively ask about them.
3. Start Lipid Rescue immediately: Intralipid 20%, 1.5 ml/kg bolus, then 0.25 ml/kg/min. Do not wait for cardiac arrest.
4. Modified resuscitation: Reduced epinephrine doses, no lidocaine, no vasopressin, prolonged CPR.
5. Prevention: Adhere to maximum doses, inject incrementally, aspirate, use ultrasound, ensure monitoring, keep a Lipid Rescue kit available.

Practical Training

The treatment of LAST requires a well-coordinated team that can reliably execute the algorithm under stress – from recognizing the prodromes to correctly dosing the lipid emulsion and managing the specifics of modified resuscitation. In the emergency training courses offered by Simulation Tirol, you can practice exactly these critical scenarios in a realistic simulation environment, deepening both pharmacological therapy and non-technical skills such as team communication and decision-making under pressure. Because especially with rare but life-threatening complications, the difference between knowing and doing makes all the difference for your patients.