Local anesthetic


A local anesthetic is a medication that causes absence of all sensation in a specific body part without loss of consciousness, providing local anesthesia, as opposed to a general anesthetic, which eliminates all sensation in the entire body and causes unconsciousness. Local anesthetics are most commonly used to eliminate pain during or after surgery. When it is used on specific nerve pathways, paralysis also can be induced.

Classification

LAs are of 2 types:
  • Clinical LAs:
  • *amino amide LAs
  • *amino ester LAs
  • Synthetic LAs
  • *Cocaine derivatives
Synthetic cocaine-derived LAs differ from cocaine because they have a much lower abuse potential and do not cause hypertension vasoconstriction.
The suffix "-caine" at the ends of these medication names is derived from the word "cocaine", because cocaine was formerly used as a local anesthetic.

Examples

Short Duration of Action and Low Potency
  • Benzocaine
  • Procaine
  • Chloroprocaine
  • Cocaine
Medium Duration of Action and Medium Potency
  • Lidocaine
  • Prilocaine
High Duration and High Potency
  • Tetracaine
  • Bupivacaine
  • Cinchocaine
  • Ropivacaine

    Medical uses

Local anesthetics may be used to prevent and/or treat acute pain, to treat chronic pain, and as a supplement to general anesthesia.
They are used in various techniques of local anesthesia such as:
Even though acute pain can be managed using analgesics, conduction anesthesia may be preferable because of superior pain control and fewer side effects. For purposes of pain therapy, LA drugs are often given by repeated injection or continuous infusion through a catheter. LA drugs are also often combined with other agents such as opioids for synergistic analgesic action. Low doses of LA drugs can be sufficient so that muscle weakness does not occur and patients may be mobilized.
Some typical uses of conduction anesthesia for acute pain are:

Chronic pain

is a complex and often serious condition that requires diagnosis and treatment by an expert in pain medicine. LAs can be applied repeatedly or continuously for prolonged periods to relieve chronic pain, usually in combination with medication such as opioids, NSAIDs, and anticonvulsants. Though it can be easily performed, repeated local anesthetic blocks in chronic pain conditions are not recommended as there is no evidence of long-term benefits.

Surgery

Virtually every part of the body can be anesthetized using conduction anesthesia. However, only a limited number of techniques are in common clinical use. Sometimes, conduction anesthesia is combined with general anesthesia or sedation for the patient's comfort and ease of surgery. However, many anesthetists, surgeons, patients and nurses believe that it is safer to perform major surgeries under local anesthesia than general anesthesia. Typical operations performed under conduction anesthesia include:

Diagnostic tests

Diagnostic tests such as bone marrow aspiration, lumbar puncture and aspiration of cysts or other structures are made to be less painful upon administration of local anesthetic before insertion of larger needles.

Other uses

Local anesthesia is also used during insertion of IV devices, such as pacemakers and implantable defibrillators, ports used for giving chemotherapy medications and hemodialysis access catheters.
Topical anesthesia, in the form of lidocaine/prilocaine is most commonly used to enable relatively painless venipuncture and placement of intravenous cannulae. It may also be suitable for other kinds of punctures such as ascites drainage and amniocentesis.
Surface anesthesia also facilitates some endoscopic procedures such as bronchoscopy or cystoscopy

Side effects

Localized side effects

Edema of tongue, pharynx and larynx may develop as a side effect of local anesthesia. This could be caused by a variety of reasons including trauma during injection, infection, an allergic reaction, haematoma or injection of irritating solutions such as cold-sterilization solutions. Usually there is tissue swelling at the point of injection. This is due to puncturing of the vein which allows the blood to flow into loose tissues in the surrounding area. Blanching of the tissues in the area where the local anesthetic is deposited is also common. This gives the area a white appearance as the blood flow is prevented due to vasoconstriction of arteries in the area. The vasoconstriction stimulus gradually wears off and subsequently the tissue returns to normal in less than two hours.
The side effects of inferior alveolar nerve block include feeling tense, clenching of the fists and moaning.
The duration of soft tissue anesthesia is longer than pulpal anesthesia and is often associated with difficulty eating, drinking and speaking.

Risks

The risk of temporary or permanent nerve damage varies between different locations and types of nerve blocks.
There is risk of accidental damage to local blood vessels during injection of the local anesthetic solution. This is referred to as haematoma and could result in pain, trismus, swelling and/or discoloration of the region. The density of tissues surrounding the injured vessels is an important factor for hematoma. There is greatest chance of this occurring in a posterior superior alveolar nerve block or in a pterygomandibular block.
Giving local anesthesia to patients with liver disease can have significant consequences. Thorough evaluation of the disease should be carried out to assess potential risk to the patient as in significant liver dysfunction, the half-life of amide local anesthetic agents may be drastically increased thus increasing the risk of overdose.
Local anesthetics and vasoconstrictors may be administered to pregnant patients however it is very important to be extra cautious when giving a pregnant patient any type of drug. Lidocaine can be safely used but bupivacaine and mepivacaine should be avoided.  Consultation with the obstetrician is vital before administering any type of local anesthetic to a pregnant patient.

Recovery

Permanent nerve damage after a peripheral nerve block is rare. Symptoms are likely to resolve within a few weeks. The vast majority of those affected recover within four to six weeks; 99% of these people have recovered within a year. An estimated one in 5,000 to 30,000 nerve blocks results in some degree of permanent persistent nerve damage.
Symptoms may continue to improve for up to 18 months following injury.

Potential side effects

General systemic adverse effects are due to the pharmacological effects of the anesthetic agents used. The conduction of electric impulses follows a similar mechanism in peripheral nerves, the central nervous system, and the heart. The effects of local anesthetics are, therefore, not specific for the signal conduction in peripheral nerves. Side effects on the central nervous system and the heart may be severe and potentially fatal. However, toxicity usually occurs only at plasma levels which are rarely reached if proper anesthetic techniques are adhered to. High plasma levels might arise, for example, when doses intended for epidural or intrasupport tissue administration are accidentally delivered as intravascular injection.

Emotional reactions

When patients are emotionally affected in the form of nervousness or fear, it can lead to vasovagal collapse. This is the anticipation of pain during administration that activates the parasympathetic nervous system while inhibiting the orthosympathetic nervous system. What results is a dilation of arteries in muscles which can lead to a reduction in circulating blood volume inducing a temporary shortness of blood flow to the brain. Notable symptoms include restlessness, visibly looking pale, perspiration and possible loss of consciousness. In severe cases, clonic cramps resembling an epileptic insult may occur.
On the other hand, fear of administration can also result in accelerated, shallow breathing, or hyperventilation. The patient may feel a tingling sensation in hands and feet or a sense of light-headedness and increased chest pressure.
Hence, it is crucial for the medical professional administrating the local anesthesia, especially in the form of an injection, to ensure that the patient is in a comfortable setting and has any potential fears alleviated in order to avoid these possible complications.

Central nervous system

Depending on local tissue concentrations of local anesthetics, excitatory or depressant effects on the central nervous system may occur.
Initial symptoms of systemic toxicity include ringing in the ears, a metallic taste in the mouth, tingling or numbness of the mouth, dizziness and/or disorientation.
At higher concentrations, a relatively selective depression of inhibitory neurons results in cerebral excitation, which may lead to more advanced symptoms include motor twitching in the periphery followed by grand mal seizures. It is reported that seizures are more likely to occur when bupivacaine is used, particularly in combination with chloroprocaine.
A profound depression of brain functions may occur at even higher concentrations which may lead to coma, respiratory arrest, and death. Such tissue concentrations may be due to very high plasma levels after intravenous injection of a large dose.
Another possibility is direct exposure of the central nervous system through the cerebrospinal fluid, i.e., overdose in spinal anesthesia or accidental injection into the subarachnoid space in epidural anesthesia.