Local Anaesthetic Systemic Toxicity (LAST)

Local anaesthetics are the most commonly used drugs in orthopaedic surgery, from digital blocks to total joint infiltration. Because they are used daily, complacency can set in. However, Local Anaesthetic Systemic Toxicity (LAST) is a catastrophic event that can lead to seizure and cardiac arrest within minutes.

Every surgeon who injects local anaesthetic must know the "Lipid Rescue" protocol by heart. This guide covers the prevention, recognition, and management of this emergency.

Visual Element: Diagram showing the "Site Absorption Hierarchy" - ranking injection sites from highest absorption (Intercostal) to lowest (Subcutaneous), emphasizing why site matters as much as dose.

Pathophysiology

Local anaesthetics (LAs) work by blocking voltage-gated sodium channels, preventing nerve conduction. Toxicity occurs when plasma levels rise high enough to block sodium channels in the Brain (Seizures) and Heart (Arrhythmias/Arrest).

Mechanism of Toxicity:

1. Direct Intravascular Injection: Rapid onset (seconds).
2. Systemic Absorption: Slower onset (minutes to hours). Depends on vascularity of the site.

The Bupivacaine Problem: Bupivacaine is far more cardiotoxic than Lignocaine. It binds avidly to cardiac sodium channels and dissociates slowly ("Fast-in, Slow-out"). This makes resuscitation difficult, as the drug stays bound to the heart even during CPR.

Clinical Presentation

Classically, symptoms follow a progression from CNS excitation to CNS depression to CVS collapse. However, 20-40% of cases present with seizure or cardiac arrest as the FIRST sign.

1. CNS Toxicity (The Warning Shots)

• Subjective: Metallic taste in mouth, Circumoral numbness (tingling lips), Tinnitus (ringing ears), Lightheadedness.
• Objective: Confusion, Agitation, Slurred speech, Muscle twitching.
• The Event: Generalized tonic-clonic Seizures.

2. CVS Toxicity (The Crash)

• Early: Hypertension and Tachycardia (Sympathetic surge).
• Late: Hypotension, Bradycardia, Conduction blocks.
• Terminal: Ventricular Arrhythmias (VT/VF) -> Asystole.

Prevention: The "Safe" Dose

Calculators are guidelines. Factors like liver failure, low protein (albumin), and site vascularity affect the real threshold.

Safety Checklist:

1. Know the weight: Calculate the max dose before you draw it up.
2. Aspirate: Aspirate before every injection.
3. Fractionate: Inject in 5mL aliquots, pausing to observe.
4. Monitor: Keep ECG/Sats on for major blocks.

Management: The Lipid Rescue Protocol

If LAST occurs: STOP INJECTING. CALL FOR HELP.

Visual Element: A printable "Crisis Checklist" for LAST, designed to be stuck on the theatre wall.

1. Airway & Breathing

• Hypoxia and Acidosis worsen toxicity by increasing cerebral blood flow (delivering more drug to brain) and reducing protein binding (freeing up more drug).
• Action: 100% Oxygen. Intubate early to secure airway and hyperventilate to induce hypocapnia (alkalosis helps).

2. Seizure Control

• Benzodiazepines: Midazolam (0.1 mg/kg).
• Avoid: High dose Propofol (it is a cardiac depressant).

3. Cardiac Support

• CPR: Start ACLS.
• Modifications:
  • Reduce Adrenaline: Use small doses (<1 mcg/kg). High doses worsen acidosis and arrhythmias.
  • Avoid: Vasopressin, Calcium Channel Blockers, Beta Blockers (these depress the heart further).
  • Prolonged CPR: Be prepared to continue for > 1 hour. Recovery is possible. ECMO may be required.

4. Lipid Emulsion Therapy (The Antidote)

Intralipid 20% works by:

1. Lipid Sink: Capturing the lipophilic anaesthetic from the plasma.
2. Metabolic Shuttle: Providing fatty acids to the mitochondria of the stunned heart.

Dosing Protocol (70kg Adult):

• Bolus: 1.5 mL/kg (approx 100 mL) IV over 1 minute.
• Infusion: 0.25 mL/kg/min (approx 1000 mL/hr).
• If unstable: Repeat Bolus (max 3 times) and Double Infusion rate.
• Max Dose: 12 mL/kg.

Summary

LAST is a rare but reversible cause of cardiac arrest. The difference between life and death is the speed of Lipid administration.

• Know your max dose.
• Recognize the metallic taste.
• Know where the Lipid bag is.

Dose Calculator

Use our interactive calculator to determine safe maximum limits for your patient based on weight and comorbidities.