Local anesthetics are commonly used medications that temporarily block pain signals in a specific area of the body, allowing for various medical and dental procedures without the need for general anesthesia. While generally considered safe and effective, in rare instances, these medications can lead to a serious condition known as Local Anesthetic Systemic Toxicity, or LAST. LAST occurs when too much local anesthetic enters the bloodstream, leading to adverse effects on the body’s major systems.
What is Local Anesthetic Systemic Toxicity?
Local anesthetics work by temporarily interrupting the transmission of pain signals along nerves to the brain, providing a numbing effect in a targeted region. This is achieved by blocking sodium channels within nerve cells, preventing the electrical impulses that carry pain sensations. When local anesthetics are administered, they are typically confined to the injection site, where they perform their intended function.
This can happen due to an accidental injection directly into a blood vessel or rapid absorption from highly vascularized tissues, which allow the medication to enter the bloodstream quickly. Once in the systemic circulation, these compounds can then affect other organs beyond the intended site.
The primary systems affected by LAST are the central nervous system (CNS) and the cardiovascular system. These systems are particularly vulnerable because local anesthetics can interfere with their normal electrical and metabolic functions. In the CNS, this interference can disrupt nerve cell communication, while in the cardiovascular system, it can impair heart muscle function and electrical conduction.
Recognizing the Signs
Symptoms of LAST can vary and may not always follow a predictable pattern. Central nervous system (CNS) effects often appear first, but cardiovascular signs can occur simultaneously or even precede CNS symptoms, especially with certain anesthetic types or rapid intravascular injection. The onset of symptoms can be immediate, within minutes of injection, or delayed up to an hour or more.
Early CNS symptoms can be subtle, including perioral numbness (numbness around the mouth), a metallic taste, ringing in the ears (tinnitus), lightheadedness, and visual disturbances like blurred vision. As toxicity progresses, more pronounced CNS effects may emerge, such as muscle twitching, tremors, confusion, agitation, and disorientation. In more severe cases, seizures can occur, often as a common and serious initial symptom.
Cardiovascular symptoms can range from mild to severe, affecting heart rate, blood pressure, and heart rhythm. Initial changes might include an increase in heart rate (tachycardia) and blood pressure (hypertension), reflecting the body’s compensatory response. However, as the toxicity deepens, more serious issues can arise, such as a slow heart rate (bradycardia), low blood pressure (hypotension), and various irregular heartbeats (arrhythmias).
Ultimately, severe cardiovascular compromise can lead to complete heart block or cardiac arrest, where the heart stops effectively pumping blood. In some instances, particularly severe toxicity can also cause respiratory depression, leading to shallow breathing or a complete cessation of breathing (apnea).
Managing Local Anesthetic Systemic Toxicity
Managing Local Anesthetic Systemic Toxicity requires immediate, coordinated actions to stabilize the patient. Upon suspicion of LAST, immediately stop local anesthetic administration and call for medical help. Ensuring an open airway and providing supplemental oxygen are immediate priorities, as hypoxia and acidosis can worsen toxicity.
If seizures occur, benzodiazepines are administered to control them, preventing further injury and metabolic disturbances. Support for the cardiovascular system includes addressing low blood pressure with intravenous fluids or other medications, and managing abnormal heart rhythms. In cases of cardiac arrest, advanced cardiac life support protocols are initiated, though certain medications, like calcium channel blockers or beta-blockers, may be avoided as they may worsen the condition.
A key treatment for severe LAST, especially for cardiovascular collapse, is intravenous lipid emulsion (ILE). This therapy works by creating a “lipid sink” in the bloodstream, which attracts and sequesters lipid-soluble local anesthetic molecules, drawing them away from the heart and brain. This reduces the concentration of the toxic drug at sensitive sites, allowing affected organs to recover function.
Lipid emulsion therapy may have other benefits, such as providing metabolic support to the heart and improving its contractility. The initial dose of lipid emulsion is given as a rapid bolus, followed by a continuous infusion, and continued until the patient’s condition stabilizes. In severe and refractory cases, advanced life support measures, such as extracorporeal membrane oxygenation (ECMO), may be considered to support heart and lung function.
Minimizing the Risk
Healthcare providers employ several measures to minimize the risk of Local Anesthetic Systemic Toxicity through careful administration techniques and patient monitoring. A key strategy is using the lowest effective dose of local anesthetic for pain relief. This reduces the total amount of medication in the body, lowering the potential for toxic systemic absorption.
A technique called aspiration is performed before injecting the anesthetic. This involves pulling back on the syringe plunger to check for blood, indicating intravascular placement. If blood is aspirated, the needle is repositioned to avoid direct intravascular injection.
Injecting the local anesthetic slowly and in small increments is another preventive measure. This enables continuous observation for immediate adverse reactions and prevents a rapid surge in systemic drug concentration. Continuous monitoring of vital signs (heart rate, blood pressure) and level of consciousness during and after the procedure is standard practice to detect early signs of toxicity.
Certain patient characteristics can increase the risk of LAST, requiring caution from providers. These include individuals at the extremes of age (very young or elderly) and patients with pre-existing heart, liver, or kidney conditions. These conditions can affect how the body processes and eliminates local anesthetics, leading to higher drug levels. Medical facilities are also equipped with rescue medications and trained personnel prepared to manage LAST.