Lidocaine is a widely used and highly effective local anesthetic, valued for its ability to block nerve signals and numb specific areas of the body. Despite its general safety, complications can arise from improper administration, with the most significant being Local Anesthetic Systemic Toxicity, or LAST. A wrong injection—whether due to an excessive dose, rapid absorption from highly vascularized tissue, or inadvertent intravascular injection—can send the drug into the bloodstream, triggering systemic effects that can become severe and life-threatening if not managed immediately. Understanding what happens when lidocaine is injected incorrectly is crucial for both healthcare providers and patients.
The Pathophysiology of Lidocaine Toxicity
Lidocaine works by blocking sodium channels in nerve cells, preventing them from sending pain signals to the brain. When injected properly into the intended tissue, the drug remains localized and provides effective, temporary numbness. However, if an injection enters a vein or artery by mistake, or if too much is absorbed too quickly, the blood plasma concentration of lidocaine can rise to toxic levels.
The systemic toxicity primarily impacts the central nervous system (CNS) and the cardiovascular system (CVS). This occurs because lidocaine affects sodium channels in all excitable tissues, not just the nerves at the injection site. The initial CNS symptoms are often due to the drug blocking inhibitory pathways in the brain, leading to paradoxical excitation before progressing to overall CNS depression. The cardiovascular effects, which can be even more severe, involve slowed cardiac electrical conduction and reduced heart contractility.
Local Anesthetic Systemic Toxicity (LAST): What to Look For
Symptoms of LAST typically appear within minutes of an accidental intravascular injection, though they can be delayed for up to an hour. The presentation can vary from patient to patient, but it often follows a progressive pattern, with initial CNS effects followed by more serious cardiovascular complications.
Central Nervous System (CNS) Symptoms
Early signs of CNS toxicity can be subtle but are vital warning signals for clinicians.
- Circumoral numbness: A tingling or numb sensation around the mouth is a classic early symptom.
- Metallic taste: Patients may report a metallic or strange taste in their mouth.
- Tinnitus and blurred vision: Auditory and visual disturbances, such as ringing in the ears or difficulty focusing, are common.
- Dizziness and lightheadedness: Feeling unsteady or faint can be an indicator of toxicity.
- Restlessness and confusion: Early CNS excitation can manifest as nervousness, agitation, or confusion.
As toxicity progresses, these symptoms can escalate rapidly.
- Muscle twitching and tremors: Uncontrolled muscle movements can develop and coalesce into more severe signs.
- Seizures: These are a hallmark of significant lidocaine toxicity and require immediate medical intervention.
- CNS depression: With very high doses, the initial excitation is followed by depression, leading to drowsiness, unconsciousness, and coma.
Cardiovascular System (CVS) Symptoms
Cardiovascular effects typically occur at higher plasma concentrations than CNS symptoms and can be more immediately life-threatening, often without premonitory CNS signs in some patients.
- Hypotension and bradycardia: A drop in blood pressure and a slow heart rate are common cardiovascular signs.
- Cardiac arrhythmias: Abnormal heart rhythms, including ventricular tachycardia and fibrillation, can occur and lead to cardiac arrest.
- Atrioventricular (AV) heart block: The electrical conduction within the heart can be blocked, disrupting the heart's normal rhythm.
- Cardiovascular collapse: Severe toxicity can result in complete heart failure and collapse.
Comparison of Injection Scenarios
| Feature | Intended Subcutaneous/Local Injection | Accidental Intravascular Injection |
|---|---|---|
| Drug Absorption | Slow and localized, with gradual metabolism. | Rapid, systemic delivery of the full dose. |
| Onset of Symptoms | Unlikely to cause systemic toxicity unless overdose is administered or absorbed rapidly from a highly vascular area. | Rapid onset, often within minutes, leading to immediate systemic effects. |
| Primary Risk | Minimal systemic risk when administered correctly within safe dose limits. | Local Anesthetic Systemic Toxicity (LAST) affecting CNS and CVS. |
| Clinical Signs | Local numbness, rare systemic side effects. | Tinnitus, circumoral numbness, dizziness, confusion, seizures, and cardiac arrhythmias. |
Treatment and Prevention of LAST
The management of LAST is a critical medical emergency requiring immediate and decisive action. For healthcare providers, the process involves:
- Immediate cessation of the injection.
- Calling for help and initiating basic life support (ABCs: airway, breathing, circulation).
- Managing the airway: Administering 100% oxygen is crucial.
- Controlling seizures: Benzodiazepines are the preferred treatment.
- Administering lipid emulsion therapy: In severe cases, especially those with cardiovascular instability, a 20% lipid emulsion is given intravenously. It acts by sequestering the lidocaine molecules in the bloodstream, preventing them from reaching the CNS and heart.
- Cardiovascular support: IV fluids and vasopressors may be needed for hypotension.
Preventing LAST is the priority. Medical professionals use several techniques to minimize risk, including:
- Aspiration before injection: Pulling back on the plunger before injecting helps check for blood in the syringe, indicating intravascular placement.
- Incremental injection: Delivering the total dose in small, repeated increments allows for continuous monitoring of the patient for adverse reactions.
- Using the lowest effective dose: Dosing should be based on the patient's ideal body weight and limited to the minimum necessary.
- Ultrasound guidance: Using ultrasound to visualize nerves and blood vessels is a highly effective way to prevent accidental intravascular injection, especially for regional nerve blocks.
Conclusion
While generally safe, improper lidocaine injection carries the significant risk of Local Anesthetic Systemic Toxicity (LAST). Accidental intravascular injection can lead to a cascade of CNS and CVS effects, ranging from mild sensory changes to life-threatening seizures and cardiac arrest. For this reason, healthcare professionals must adhere to strict safety protocols, including aspiration, incremental dosing, and, when appropriate, ultrasound guidance, to minimize the risk of complications. Early recognition and prompt management are key to ensuring a positive outcome for the patient.
Disclaimer: This article is for informational purposes only and is not a substitute for professional medical advice. Always consult with a qualified healthcare provider for any medical concerns or before making any decisions related to your treatment.
