Understanding Local Anesthetic Systemic Toxicity (LAST)
Local Anesthetic Systemic Toxicity (LAST) occurs when a local anesthetic reaches toxic levels in the bloodstream, a rare but life-threatening event. Causes include accidental intravascular injection, excessive dosage, or rapid absorption. High systemic levels of local anesthetics, which normally block nerve signals, can disrupt the central nervous system (CNS) and cardiovascular system (CVS). Recognizing and managing LAST promptly is crucial to prevent severe outcomes like cardiac arrest.
Central Nervous System (CNS) Signs of Toxicity
The CNS often shows the first signs of local anesthetic toxicity, particularly in awake patients. Symptoms typically progress from initial excitation to depression.
Early/Excitatory CNS Symptoms
These initial signs warn of rising toxic levels:
- Perioral numbness and tongue paresthesia: Tingling or numbness around the mouth and on the tongue.
- Metallic taste: A distinct metallic taste in the mouth.
- Auditory and visual disturbances: Tinnitus, blurred or double vision.
- Agitation and restlessness: Confusion, nervousness, or a feeling of impending doom.
- Slurred speech (Dysarthria): Difficulty speaking clearly.
- Muscle twitching: Involuntary twitches, especially in the face and extremities.
Late/Depressive CNS Symptoms
If unmanaged, the excitatory phase leads to CNS depression:
- Seizures: Generalized tonic-clonic seizures are the most common major symptom, occurring in 60-70% of cases.
- Drowsiness and loss of consciousness: Increasing drowsiness leading to unconsciousness or coma.
- Respiratory depression and arrest: Deepening CNS depression can suppress breathing.
Cardiovascular System (CVS) Signs of Toxicity
Cardiovascular signs typically require higher blood concentrations of the anesthetic than CNS signs, though they can appear first or simultaneously, especially with potent agents like bupivacaine. Atypical presentations with cardiovascular symptoms but no prior CNS signs occur in about 20-40% of cases.
Early CVS Symptoms
Initial cardiovascular signs can be excitatory:
- Hypertension: An initial increase in blood pressure.
- Tachycardia: Increased heart rate.
- Ventricular Arrhythmias: Irregular heart rhythms.
Late CVS Symptoms
This phase is often followed by profound cardiovascular depression:
- Hypotension: A significant drop in blood pressure.
- Bradycardia: A significantly slowed heart rate.
- Conduction Blocks: Disrupted electrical signaling in the heart.
- Cardiac Arrest: In severe cases, cardiovascular collapse occurs.
Comparison of CNS vs. Cardiovascular Signs
| Feature | Central Nervous System (CNS) Signs | Cardiovascular System (CVS) Signs |
|---|---|---|
| Onset | Often appear first, at lower drug concentrations | Typically appear at higher drug concentrations, but can be the first sign (especially with bupivacaine) |
| Early Symptoms | Perioral numbness, metallic taste, tinnitus, agitation, muscle twitching | Hypertension, tachycardia, ventricular ectopy |
| Severe Symptoms | Seizures, coma, respiratory arrest | Profound hypotension, bradycardia, conduction blocks, asystole, cardiac arrest |
| Primary Agent | More common with less potent agents like lidocaine | More severe and common with highly lipophilic agents like bupivacaine |
Management and Prevention
If LAST is suspected, immediately stop the anesthetic injection and call for help. Management involves airway support, seizure control with benzodiazepines, and cardiovascular support. The primary treatment for severe LAST is intravenous 20% lipid emulsion solution, which acts as a "lipid sink" to sequester anesthetic molecules.
Prevention is crucial and includes:
- Using the lowest effective dose.
- Injecting incrementally (3-5 mL) with aspiration.
- Using ultrasound guidance.
- Using an intravascular marker like epinephrine.
- Monitoring the patient for at least 30 minutes post-injection.
Conclusion
Recognizing the signs of toxicity from local anaesthetic agents is vital for healthcare providers. Symptoms range from subtle sensory changes to seizures and cardiac arrest. The classic presentation is a biphasic response starting with CNS excitation followed by depression and cardiovascular collapse. However, atypical presentations are common, requiring high vigilance. Understanding risk factors, employing preventative techniques, and being prepared to manage LAST with airway support and lipid emulsion therapy significantly improve patient outcomes.
