What happens if you inject local anesthetic into a vein?

October 11, 2025 •

4 min read

Local Anesthetic Systemic Toxicity (LAST) is a rare but life-threatening complication, with an estimated mortality rate of 0.4% in reported cases. This occurs when you accidentally inject local anesthetic into a vein, leading to a cascade of effects on the central nervous and cardiovascular systems.

Quick Summary

An unintentional injection of a local anesthetic into a vein can cause severe systemic toxicity, affecting the brain and heart with symptoms ranging from dizziness and seizures to cardiac arrest, requiring immediate medical intervention.

Key Points

Immediate Risk: Inadvertent injection can lead to Local Anesthetic Systemic Toxicity (LAST), a medical emergency affecting the central nervous system (CNS) and cardiovascular (CV) systems.
CNS Symptoms: Early signs of toxicity include perioral numbness, a metallic taste, tinnitus, and agitation, which can progress to seizures, unconsciousness, and coma.
Cardiovascular Collapse: Severe cases can cause dangerous heart arrhythmias, low blood pressure, and cardiac arrest, particularly with potent anesthetics like bupivacaine.
Lipid Emulsion Rescue: Treatment protocols include administering intravenous lipid emulsion, which helps sequester the anesthetic from the bloodstream and mitigate its toxic effects.
Prevention is Key: Healthcare providers use techniques like aspiration, incremental dosing, and ultrasound guidance to prevent accidental intravascular injections.
Modified CPR: Cardiac arrest caused by LAST requires modified resuscitation protocols, including conservative use of epinephrine and avoiding drugs like lidocaine that can worsen the condition.
Agent-Specific Risks: Bupivacaine is significantly more cardiotoxic than lidocaine, with a narrower safety margin between the dose causing CNS effects and that causing life-threatening cardiac collapse.

Local anesthetics are a critical tool in modern medicine, used to numb specific areas of the body for procedures ranging from dental work to minor surgeries. They work by blocking voltage-gated sodium channels, preventing nerves from transmitting pain signals to the brain. However, the safety and efficacy of these drugs depend on their localized action. When an anesthetic is unintentionally injected directly into a vein, it can lead to a severe medical emergency known as Local Anesthetic Systemic Toxicity, or LAST.

The Mechanism of Local Anesthetic Systemic Toxicity (LAST)

LAST occurs when the concentration of local anesthetic in the bloodstream becomes high enough to disrupt the function of other excitable tissues, primarily those in the central nervous system (CNS) and the cardiovascular system (CVS). By blocking sodium channels in these vital areas, the anesthetic can trigger a range of serious, systemic effects. While LAST can result from overdose or rapid absorption from highly vascularized areas, the most common cause is inadvertent intravascular injection. The onset of symptoms is typically rapid, often occurring within minutes of the injection.

Central Nervous System (CNS) Effects

Symptoms of LAST often follow a predictable pattern of CNS excitation followed by depression, though this progression can be masked in sedated patients or with certain types of anesthetics. Early symptoms can include numbness or tingling around the mouth and tongue, a metallic taste, ringing in the ears, lightheadedness, dizziness, agitation, or confusion. More severe symptoms include muscle twitching, tremors, slurred speech, and generalized seizures. The excitatory phase can progress to CNS depression, leading to unconsciousness, coma, and respiratory arrest.

Cardiovascular (CV) System Effects

The cardiovascular system is more resistant to the effects of local anesthetics than the CNS, but high doses can cause profound cardiac depression. Cardiac toxicity can be resistant to standard resuscitation and may be the first or only sign in severe cases, especially with more potent anesthetics. Initial effects may include an increase in blood pressure and heart rate, which can progress to low blood pressure and slow heart rate. Other serious effects include weakening of the heart's pumping ability (myocardial depression), irregular heartbeats (arrhythmias) like ventricular tachycardia or fibrillation, and ultimately cardiac arrest.

Factors Influencing Systemic Toxicity

Several factors can influence the severity and presentation of LAST:

Type of Local Anesthetic: More potent and lipid-soluble anesthetics, such as bupivacaine, pose a greater risk of severe cardiotoxicity compared to less potent ones like lidocaine.
Dosage and Concentration: Higher doses and concentrations increase the risk of elevated plasma levels and toxicity.
Injection Site: Highly vascularized areas lead to faster absorption and higher risk.
Addition of Epinephrine: Epinephrine can slow systemic absorption, reducing toxicity risk.
Patient Factors: Pre-existing heart or liver conditions, acidosis, hypercapnia, and hypoxia increase susceptibility.

Comparison of Lidocaine and Bupivacaine Systemic Toxicity

Bupivacaine has a narrower therapeutic window than lidocaine, increasing the risk of cardiac collapse relative to CNS effects.


Feature	Lidocaine	Bupivacaine
Potency	Moderate	High
Primary Toxicity	CNS effects are typically the first to appear in conscious patients.	Cardiovascular toxicity is a more prominent and dangerous risk, sometimes occurring without preceding CNS signs.
CNS:CV Toxicity Ratio	Relatively high safety margin; a toxic dose for CNS is often well below a cardio-toxic dose.	Lower safety margin; much closer in dosage between CNS and cardio-toxic effects.
Resuscitation	More responsive to standard resuscitation protocols.	More resistant to resuscitation; requires specific interventions.

Managing Local Anesthetic Systemic Toxicity (LAST)

LAST is a medical emergency requiring a rapid and structured response. The management protocol focuses on supportive care and the use of specific antidotes.

Immediate Actions

Stop the injection and call for help.
Secure the airway and provide high-flow oxygen.
Control seizures with benzodiazepines.

Lipid Emulsion Therapy

Administer 20% lipid emulsion, the standard of care for LAST.
It creates a "lipid sink" to absorb the anesthetic.
Dosing follows specific guidelines.

Cardiovascular Support

Use modified CPR for cardiac arrest.
Administer small, conservative doses of epinephrine for hypotension.
Avoid certain drugs like vasopressin, calcium channel blockers, beta-blockers, and other local anesthetics.
Consider advanced options like ECMO in refractory cases.

Prevention is Key

Preventive strategies are crucial:

Aspiration: Check for blood before injecting.
Incremental Injection: Administer in small doses with repeated aspiration.
Ultrasound Guidance: Use ultrasound for real-time visualization during regional blocks.
Patient Monitoring: Maintain verbal contact to detect early CNS signs.

Conclusion

Accidental injection of local anesthetic into a vein is a serious event. However, risks are minimized through proper technique, vigilance, and tools like ultrasound guidance. Prompt recognition and adherence to specific management protocols, including lipid emulsion therapy, are crucial for a positive outcome. All local anesthetics can cause systemic toxicity if injected intravascularly, with potent agents like bupivacaine presenting a greater cardiotoxic risk. Awareness of the signs, symptoms, and treatment is essential for healthcare professionals.

Authoritative Outbound Link

For the latest guidelines and protocols for managing Local Anesthetic Systemic Toxicity, refer to the resources provided by the American Society of Regional Anesthesia and Pain Medicine (ASRA). ASRA guidelines

Frequently Asked Questions

The most common severe sign of LAST in an awake patient is the onset of seizures.

Symptoms typically appear within 1 to 5 minutes after injection but can sometimes be delayed for up to an hour or more, particularly if the anesthetic is absorbed slowly from a vascular site.

Yes, severe LAST can lead to cardiac arrest and death if not recognized and treated promptly with appropriate resuscitative measures.

Intravenous lipid emulsion therapy involves administering a fat-based solution that acts as a 'lipid sink,' absorbing the lipid-soluble local anesthetic from the blood to reduce its toxic concentration in the heart and brain.

Bupivacaine is more cardiotoxic than lidocaine. This means bupivacaine is more likely to cause severe and difficult-to-treat cardiac problems, like arrhythmias and cardiac arrest, at lower relative doses compared to lidocaine.

Preventive techniques include aspirating the syringe before injection to check for blood, administering the anesthetic in small incremental doses, and using ultrasound guidance to visualize the needle tip and surrounding vessels.

The immediate steps are to stop the injection, call for help, manage the patient's airway, provide oxygen, and prepare for cardiopulmonary resuscitation (CPR).