Immediate Assessment for Suspected Local Anesthetic Systemic Toxicity (LAST)
Stop any ongoing injection immediately and assess for signs of local anesthetic systemic toxicity (LAST), which presents first with neurological symptoms including perioral numbness, tongue tingling, tinnitus, light-headedness, metallic taste, confusion, or agitation, followed by potential cardiovascular collapse. 1, 2
Primary Assessment Steps
Evaluate for Systemic Toxicity (LAST)
The increased pain and numbness after lidocaine injection raises concern for LAST, which requires immediate recognition:
Early CNS manifestations to assess for:
- Perioral numbness or tongue tingling 2
- Tinnitus or metallic taste 2
- Light-headedness, confusion, or agitation 2
- Slurred speech or drowsiness 2
Severe manifestations (if progression occurs):
- Muscle twitching or seizures 2
- Loss of consciousness 2
- Respiratory depression 2
- Bradycardia, hypotension, or cardiovascular collapse (late signs) 2
Assess Hand Perfusion for Ischemia
While less likely without epinephrine, evaluate for potential vascular compromise:
Key perfusion parameters:
- Skin color changes: pallor, mottling, or cyanosis of hand/fingers 1
- Capillary refill time (>2 seconds indicates ischemia) 1
- Temperature: cold fingers compared to contralateral hand 1
- Pulses: radial and ulnar artery pulses, digital pulses if palpable 1
- Motor function: ability to move fingers 1
Immediate Monitoring Protocol
Establish continuous monitoring:
- ECG and pulse oximetry continuously 2
- Blood pressure every 5 minutes during initial period 2
- Assess vital signs for cardiovascular instability 1, 2
Critical pitfall: The correlation between symptoms and plasma lidocaine concentrations is not fully reliable—clinical signs precede dangerous levels, so do not wait for laboratory confirmation before acting. 2
Emergency Preparedness
Ensure lipid emulsion 20% is immediately available wherever lidocaine is used for potential LAST treatment. 1, 2 This is the definitive treatment if systemic toxicity develops, with serious toxicity typically occurring around plasma levels of 9-10 μg/mL. 2
If ischemia is present (unlikely without epinephrine):
- Be prepared for pharmacologic intervention such as phentolamine for vasospasm 1
Documentation and Investigation
- Document all findings thoroughly 1
- If toxicity is suspected, draw blood for lidocaine levels in both EDTA and lithium heparin tubes for later analysis 2
- Preserve any equipment for investigation 1
Context and Clinical Reasoning
The presentation of increased pain and numbness immediately after lidocaine injection without epinephrine is atypical. Normally, lidocaine provides effective anesthesia for carpal tunnel release with minimal pain during and after surgery. 3 The absence of epinephrine makes vascular compromise less likely but does not eliminate the risk of LAST, particularly if inadvertent intravascular injection occurred or if excessive dosing was used. 2
The key distinction: Numbness from successful local anesthesia is expected and benign, but when accompanied by increased pain and systemic symptoms, this suggests either inadequate anesthesia with nerve irritation or early LAST. 2, 4