Local anesthetic nerve block

Local anesthetic nerve block (local anesthetic regional nerve blockade, or often simply nerve block) is a short-term nerve block involving the injection of local anesthetic as close to the nerve as possible for pain relief. The local anesthetic bathes the nerve and numbs the area of the body that is innervated by that nerve. The goal of the nerve block is to prevent pain by blocking the transmission of pain signals from the surgical site. The block provides pain relief during and after the surgery. The advantages of nerve blocks over general anesthesia include faster recovery, monitored anesthesia care vs. intubation with an airway tube, and much less postoperative pain. Local anesthetic nerve block (local anesthetic regional nerve blockade, or often simply nerve block) is a short-term nerve block involving the injection of local anesthetic as close to the nerve as possible for pain relief. The local anesthetic bathes the nerve and numbs the area of the body that is innervated by that nerve. The goal of the nerve block is to prevent pain by blocking the transmission of pain signals from the surgical site. The block provides pain relief during and after the surgery. The advantages of nerve blocks over general anesthesia include faster recovery, monitored anesthesia care vs. intubation with an airway tube, and much less postoperative pain. Local anesthetics act on the voltage-gated sodium channels that conduct electrical impulses and mediate fast depolarization along nerves. Local anesthetics also act on potassium channels, but they block sodium channels more. Lidocaine preferentially binds to the inactivated state of voltage-gated sodium channels, but has also been found to bind potassium channels, G protein-coupled receptors, NMDA receptors, and calcium channels in vitro. The duration of the block is mostly influenced by the amount of time the anesthetic is near the nerve. Lipid solubility, blood flow in the tissue, and presence of vasoconstrictors with the anesthetic all play a role in this. A higher lipid solubility makes the anesthetic more potent and have a longer duration of action; however, it also increases the toxicity of the drug. Local anesthetic toxicity is indicated by numbness and tingling around the mouth, metallic taste, or ringing in the ears. Additionally, this may lead to seizures, arrhythmias, and may progress to cardiac arrest. This reaction may stem from an allergy, excessive dose, or intravascular injection. Other complications include nerve injury which has an extremely low rate of 0.029-0.2%. Some research even suggests that ultrasound lowers the risk to 0.0037%. The use of ultrasound and nerve stimulation has greatly improved practitioners’ ability to safely administer nerve blocks. Nerve injury most often occurs from ischaemia, compression, direct neurotoxicity, needle laceration, and inflammation. Local anesthetics are broken down into two categories: ester-linked and amide-linked. The esters include benzocaine, procaine, tetracaine, and chloroprocaine. The amides include lidocaine, mepivacaine, prilocaine, bupivacaine, ropivacaine, and levobupivacaine. Chloroprocaine is a short-acting drug (45–90 minutes), lidocaine and mepivacaine are intermediate duration (90–180 minutes), and bupivacaine, levobupivacaine, and ropivacaine are long-acting (4–18 hours). Drugs commonly used for peripheral nerve blocks include lidocaine, ropivacaine, bupivacaine, and mepivacaine. These drugs are often combined with adjuvants (additives) with the end goal of increasing the duration of the analgesia or shortening time of onset. Additives may include epinephrine, clonidine, and dexmedetomidine. Vasoconstriction caused by local anesthetic may be further enhanced synergistically with the addition of epinephrine, the most widely used additive. Epinephrine increases the length of analgesic duration and decreases blood flow by acting as an agonist at the α1-adrenoceptor. Dexmedetomidine is not as widely used as epinephrine. Studies in humans indicate improved onset time and increased duration of analgesia. The duration of the nerve block depends on the type of local anesthetics used and the amount injected around the target nerve. There are short acting (45–90 minutes), intermediate duration (90–180 minutes), and long acting anesthetics (4–18 hours). Block duration can be prolonged with use of a vasoconstrictor such as epinephrine, which decreases the diffusion of the anesthetic away from the nerve. There are various types of nerve blocks currently performed. Therapeutic blocks may be used for acute pain patients, diagnostic blocks are used to find pain sources, prognostic blocks are used to determine subsequent pain management options, preemptive blocks minimize postoperative pain, and some blocks can be used in place of surgery. Certain surgeries may benefit from placing a catheter that stays in place for 2–3 days postoperatively. Catheters are indicated for some surgeries where the expected postoperative pain lasts longer than 15–20 hours. Pain medication can be injected through the catheter to prevent a spike in pain when the initial block wears off. Nerve blocks may also reduce the risk of developing persistent postoperative pain several months after surgery. Local anesthetic nerve blocks are sterile procedures that can be performed with the help of anatomical landmarks, ultrasound, fluoroscopy (a live X-ray), or CT. Use of any one of these imaging modalities enables the physician to view the placement of the needle. Electrical stimulation can also provide feedback on the proximity of the needle to the target nerve. The brachial plexus is a bundle of nerves innervating the shoulder and arm and can be blocked at different levels depending on the type of upper extremity surgery being performed. Interscalene brachial plexus blocks can be done before shoulder, arm, and elbow surgery. The interscalene block is done at the neck where the brachial plexus emerges between the anterior and middle scalene muscles. Lidocaine is injected first to numb the skin and then a blunt needle is used to protect the nerves from damage as the physician places the needle very close to the nerves. The needle goes in about 3–4 cm and a single shot of local anesthetic is injected or a catheter is placed. The most common local anesthetics used at the site of the nerves are bupivicaine, mepivicaine, and chloroprocaine. There is a very high chance that the phrenic nerve, which innervates the diaphragm, will be blocked so this block should only be done on patients who have use of their accessory respiratory muscles. The block may not affect the C8 and T1 roots which supply part of the hand, so it is usually not done for hand surgeries.