 |
 |
Anesthesia
The clinically useful local anesthetics are either amino amides or amino esters. These agents can be applied topically, injected subcutaneously, or injected in the area of major peripheral nerves. Local anesthetics cause a blockade in nerve conduction. The local anesthetic diffuses passively through the cell membrane in the nonionic state, becomes charged, and blocks the sodium channel within the neuron. With sodium conductance inhibited, threshold potential is not reached and an action potential is not generated. The most common method of achieving local anesthesia for minor office procedures is infiltration anesthesia, in which the agent is injected into the operative site without selectively blocking a specific nerve. Any local anesthetic can be used for infiltration except cocaine. Injection may be intradermal or subcutaneous, or both. Once again, the duration of action will vary and the addition of epinephrine will prolong the duration of analgesia. Dilute anesthetic solutions are recommended for large areas to avoid toxicity. Infiltration of local anesthetic causes a painful, burning sensation. Injection into the dermis is the most painful and provides the fastest onset faction. Addition of sodium bicarbonate decreases the pain associated with infiltration.
|
|
|
|
The "tumescent technique" has recently been described and casts some doubt on published data regarding maximal local anesthetic dose. This technique involves the infiltration of large volumes of a dilute solution of lidocaine (0.1% or 0.05%) into the subcutaneous adipose tissue producing swelling and firmness of relatively large areas. The technique calls for epinephrine to be added to the solution to a final concentration of 1:1,000,000 and was originally described as a useful anesthetic technique for liposuction procedures. Klein suggests that the tumescent technique produces anesthesia that is so complete that liposuction of even large volumes of fat can be performed using local anesthetic alone in the absence of general anesthesia, IV sedation, or narcotic analgesia. Furthermore, with this technique, Klein demonstrates that doses up to 35 mg/kg lidocaine (5 times the manufacturer's recommended dose) can be given safely. Serial serum lidocaine levels drawn postoperatively appear to verify the safety of this technique, which has been extended to other procedures such as abdominoplasty |
|
Treatment of local anesthetic toxicity
In the patient who is convulsing as a consequence of local anesthetic toxicity, hyperventilation with an Ambu bag and face mask using 100% oxygen is an important first step. Hypercarbia can worsen CNS toxicity. If the patient has a full stomach, an endotracheal tube should be placed as soon as possible to prevent aspiration. Hyperventilation may terminate the seizure, but if it does not, diazepam, 0.1 mg/kg, or thiopental, 2 mg/kg, IV is usually effective.
In the patient who is hypotensive as a result of local anesthetic toxicity, the treatment is IV fluids, peripheral vasoconstrictor (e.g., phenylephrine), and Trendelenburg positioning. An inotropic agent (e.g., dopamine) may also be required. The patient in whom arrhythmias develop as a consequence of toxicity may be refractory to therapy. If the arrhythmia is causing the cardiac output to be significantly compromised, or if cardiac arrest occurs, a prolonged period of resuscitation may be necessary as these conditions are known to resolve over time as redistribution of the local anesthetic occurs.
|
|
Cocaine
The most common clinical use of cocaine is as a topical anesthetic. Plastic surgeons find topical cocaine advantageous primarily because of the improved hemostasis. The addition of epinephrine to the topical cocaine would enhance the vasoconstriction and improve hemostasis; however, this is not generally believed to be a safe practice. When mixed with cocaine and applied topically, epinephrine, can cause arrhythmias. Also, epinephrine may initially decrease absorption of cocaine. It is not even clear that adding epinephrine to topical cocaine enhances the operating conditions. Studies have not demonstrated a consistent benefit from adding epinephrine to either 10% cocaine or lower concentrations of topical cocaine |
|
 |
 |
|
|
 |
Toxicity of local anesthetics
To avoid toxicity, local anesthetics must be administered in a safe dose range and in the correct anatomic location. During local anesthesia, when toxic reactions occur, they are almost always the result of inadvertent intravascular injection, or the administration of an excessively large dose. Many patients report an "allergy" to local anesthesia that was probably actually symptoms related to an intravascular injection. Every effort should be made to avoid intravascular injection. The syringe should always be aspirated before local anesthetic is injected, regardless of the anatomic site of injection. Repeat aspirations should be made after every 2 to 3 mL local anesthetic injected. If blood is seen in the syringe, the needle must be repositioned. Local anesthetics freely cross the blood-brain barrier. The initial result of toxic levels of local anesthetics is depression of cortical inhibitory pathways, which allows excitatory pathway activity to be unopposed. When even higher blood levels are reached, generalized CNS depression occurs. Early signs of CNS toxicity include light-headedness, restlessness, tinnitus and other auditory or visual disturbances, slurred speech, tremors, metallic taste in the mouth, and numbness of the lips or tongue. If more local anesthesia is given, grand mal seizures may result. At even higher blood levels, loss of consciousness, apnea, and cardiovascular collapse are seen.
|
|
 |
|
 |
|
|
|
|
 |
|
|
|
|