What is Local Anesthetic Toxicity?
Local anesthetics, which are often used in most medical and dental procedures, are generally safe. However, with their growing use, the occurrence of side effects, although still rare, has increased. These side effects due to local anesthetics are known as local anesthetic systemic toxicity (LAST). LAST can range from mild symptoms to severe effects on the heart or brain.
In the past, systemic toxicity was mainly linked to seizures and breathing problems. But in the 1970s, it was discovered that bupivacaine, a local anesthetic, could cause dangerous heart issues in healthy adults. This piece will discuss how LAST happens, how often it occurs, its symptoms, how to treat it, and how to prevent it.
What Causes Local Anesthetic Toxicity?
Accidentally injecting local anesthetics into a blood vessel is the most common reason for a situation known as LAST (Local Anesthetic Systemic Toxicity), which is like an overdose of local anesthetic. However, having certain other health problems can also increase the chances of getting an overdose. These health problems can be something like liver issues, heart disease, pregnancy, or conditions that change the body’s normal metabolism.
Apart from this, age also plays a role in your risk of toxicity. Younger and older people have a harder time getting rid of the anesthetics from their bodies, which increases their risk. Particularly, infants, who are less than four months old, have a low amount of a protein called a-acid glycoprotein in their blood. This protein helps clear a commonly used local anesthetic called bupivacaine from the body. So, low levels of this protein can lead to a low clearance rate of bupivacaine, thereby increasing the risk of overdose.
Risk Factors and Frequency for Local Anesthetic Toxicity
The number of cases of a complication known as LAST which occurs after nerve block procedures is not consistent across different studies. Some research shows no instances even after 12,000 procedures, while others report up to 25 cases for every 10,000 procedures. One specific study found that seizures occurred in 79 out of 10,000 procedures that worked on the brachial plexus, a network of nerves in the upper body. This was due to the injection of a local anesthetic into the blood vessels causing LAST CNS toxicity. Despite the varying data, one fact is crucial: managing LAST as effectively as possible is incredibly important for the future.
Signs and Symptoms of Local Anesthetic Toxicity
Recognizing the symptoms of Local Anesthetic Systemic Toxicity (LAST) quickly can significantly improve the patient’s condition. Typically, half of all cases show symptoms within 50 seconds after an injection. In 75% of instances, symptoms show within 5 minutes. If there is suspicion that a potentially toxic dose has been given, it is recommended to monitor the patient for at least 30 minutes.
LAST typically begins with changes in the central nervous system (CNS). Early symptoms can include:
- Being restless or confused
- Feeling dizzy or sleepy
- Hearing changes or a ringing sound in ears
- Numbness around the mouth
- A metallic taste
- Difficulty in speaking properly
If not treated promptly, these symptoms can escalate to seizures, respiratory failure, or even coma. Seizures are the most common severe symptom. Also, if a local anesthetic is injected into a blood vessel, seizures might be the first sign of LAST.
In the past, it was believed that problems with the heart (cardiac toxicity) usually occurred after symptoms in the CNS. However, with more potent anesthetics, heart issues can occur simultaneously with seizures or even before them. Initially, low blood pressure and slow heart rate might be observed. But heart rhythm disorders account for the majority of cases. These can result in a variety of symptoms:
- Fast or slow heart rate
- Breathing difficulties
- Pain
- Sudden high blood pressure
- Changes in the heart rhythm or unusual heartbeats
Testing for Local Anesthetic Toxicity
In many cases, specific tests may not be necessary in diagnosing local anesthetic toxicity, as it can often be identified based on symptoms and a doctor’s examination. However, depending on the particular situation, your doctor might recommend certain tests such as a complete blood count, a chemistry panel, and an electrocardiogram. A complete blood count is a blood test that checks the number of different cells in your blood. A chemistry panel is another type of blood test that measures your blood’s chemistry and metabolic functions. An electrocardiogram is a test that records the electrical activity of your heart. These tests can help the doctor understand your overall health status.
Treatment Options for Local Anesthetic Toxicity
If someone has a reaction to local anesthesia, the immediate concern is to maintain a clear way for them to breathe, support their blood flow, and limit any side effects. The first step is to ensure they’re getting enough air and prevent a state where they lack enough oxygen (hypoxia) and their body’s fluids become too acidic (acidosis). This can help in recovery and may prevent a collapse of the cardiovascular system or seizures.
If seizures do break out, it is advised to quickly give the patient a class of drugs called benzodiazepines to avoid further harm and stop the acidosis. If benzodiazepines are not available, certain other medications can be used, though they might worsen any low blood pressure or negative effects on the heart. If these drugs can’t control seizures, small doses of another medication called succinylcholine may be given occasionally to stop muscle movements and further acidosis.
The focus of managing cardiac arrest caused by local anesthesia is to restore the functions of the heart. This can help to pump blood to the body’s tissues, avoiding and treating acidosis. In this situation, standard guidelines are suggested, with a few modifications. Small doses of a drug called epinephrine are preferred, but some drugs, such as vasopressin, calcium-channel blockers, and B-adrenergic-receptor blockers, are not recommended. If the heart rhythm goes astray, a medication called amiodarone is favored.
Recent research suggests treating these symptoms with a therapy using a lipid emulsion as soon as seizures or heart rhythm irregularities are suspected. This method is thought to improve the heart’s ability to conduct electrical signals, tighten its muscles, and improve blood flow by pulling the lipid-soluble local anesthetic out of the heart tissue. It should be given in a specific volume, then followed by an ongoing infusion. This should continue for 10 minutes after the patient’s condition becomes stable. If the patient does not become stable, the treatment can be repeated. If the patient doesn’t recover even after this treatment and resuscitation efforts, a last resort is usually to put the patient on a heart-lung machine until the local anesthetic is cleared from the body.
What else can Local Anesthetic Toxicity be?
When doctors are trying to diagnose whether a patient is reacting negatively to a local anesthetic, they also consider other possible explanations for the symptoms. These potential diagnoses include:
- Anxiety disorder
- Severe allergic reaction (Anaphylaxis)
- Poisoning from illegal drugs (Illicit drug toxicity)
Possible Complications When Diagnosed with Local Anesthetic Toxicity
Local anesthetic toxicity can lead to several harmful complications including:
- Seizures: This is a sudden, uncontrolled electrical disturbance in the brain. It can cause changes in behavior, movements, feelings, and levels of consciousness.
- Cardiac arrest: This is a sudden loss of blood flow due to the heart’s inability to pump correctly.
- Hypotension: This is abnormally low blood pressure.
- Arrhythmias: These are irregular heartbeats. They can lead to feelings of fluttering in the chest, palpitations, fainting, or, in severe cases, cardiac arrest.
- Death: This is the most severe outcome and can unfortunately occur in some cases.
Preventing Local Anesthetic Toxicity
The best way to deal with any negative health event is often to prevent it from happening in the first place. But it’s important to know that there’s no foolproof way to completely eradicate all risks. When it comes to handling a medical condition known as LAST, which stands for Local Anesthetic Systemic Toxicity, the main aim is to prevent a certain kind of injection that goes into your blood vessels.
One trustworthy method to prevent this is something called “intravascular test dosing” that uses a drug named epinephrine. To know the test has worked, we will observe if your heart rate increases by 10 beats per minute or more, or if your systolic blood pressure (the pressure in your blood vessels when your heart beats) rises by 15 mm Hg or more. If these changes are observed when 10 to 15 micrograms/ml of epinephrine test dose is given, it indicates the test is 80% effective.
But please note, this test may not be reliable in everyone. It can vary with patient characteristics such as those under general or spinal anesthesia, patients who are sedated, older patients, or those taking beta-blockers (heart medications).
Another preventative method involves the use of ultrasound to guide the injections. However, there aren’t any rigorous clinical trials to confirm or refute the effectiveness of this method in reducing the toxic effects.
Additionally, one can prevent LAST by ensuring not to overuse anesthetic drugs. Research shows that patients sometimes receive excessive anesthetic, which can lead to overdose. The dose should be the lowest possible amount that’s still effective. This is particularly important for patients who are at a higher risk for LAST, like the ones mentioned above.
