Which local anesthetics are appropriate for the various regional anesthetic procedures?

Which local anesthetics are appropriate for the various regional anesthetic procedures?

Regional anesthesia may be divided into infiltration, intravenous regional, peripheral nerve blocks, central neural blocks, and topical. Widely utilized by plastic surgeons, tumescent anesthesia is another form of local anesthetic injection.

Infiltration Anesthesia Any local anesthetic may be utilized for infiltration anesthesia. The choice of drug depends primarily on the desired duration of action, as the onset of action for most of the local anesthetics is almost immediate following injection. The addition of vasoconstrictors, such as epinephrine, markedly prolongs the effect of the drug, especially with lidocaine.

Intravenous Regional Anesthesia The Bier block involves intravenous injection of a large volume of local anesthetic solution into a tourniquet-occluded limb. This technique can be used for a variety of short surgical proce-dures, primarily involving the hand and forearm. It has also been used for foot procedures with a calf tourniquet. All the common local anesthetic agents have been used for intravenous regional anesthesia. However, lidocaine 0.5%, preservative free and without epinephrine, and prilocaine are the drugs most frequently used. Chloroprocaine is no longer used because of its association with thrombo-phlebitis. Bupivacaine is also not being utilized because of its cardiac toxicity profile. However, ropivacaine has been safely used for this technique.

Peripheral Nerve Blocks These blocks, whether involving blocking a single nerve entity (e.g., ulnar or radial nerve) or blocking two or more distinct nerves or a nerve plexus (e.g., brachial plexus), are used for anesthesia, postopera-tive analgesia, and diagnosis and treatment of chronic pain syndromes. Most local anesthetics can be used for periph-eral nerve blocks. The choice of drug to be utilized depends primarily on the desired duration of anesthesia, as the onset of block is rapid with most of the drugs. Lower concentrations of local anesthetics (e.g., lidocaine 1% or bupivacaine 0.25–0.5%) are commonly used because of concerns regarding local and systemic toxicity, since large volumes of anesthetic solutions are often required to achieve adequate anesthesia. The addition of a vasocon-strictor to most local anesthetic solutions prolongs their duration of action. However, it has not been shown to predictably prolong the duration of action produced by bupivacaine or ropivacaine.

Central Neural Blockade (Spinal or Epidural Anesthesia) Local anesthetic is injected into the subarachnoid space for spinal anesthesia and into the epidural space for epidural anesthesia.

Spinal Anesthesia: Drugs commonly used for spinal anesthesia include tetracaine, bupivacaine, ropivacaine, lidocaine, and procaine.

·        The baricity of the local anesthetic plays an integral part in choosing the drug appropriate for spinal anesthesia. Local anesthetics are characterized as hyperbaric, hypo-baric, or isobaric compared with cerebrospinal fluid. Hyperbaric solutions such as tetracaine 0.5% (obtained by mixing equal volumes of 1% tetracaine and 10% glucose), lidocaine 5%, and bupivacaine 0.75% (both commercially premixed with glucose) settle to the most dependent aspect of the subarachnoid space, which is dependent on the patient’s position from the time of and immediately after spinal anesthesia placement. The solution gravitates to the thoracic kyphosis in supine patients, providing adequate spinal anesthetic levels for intra-abdominal surgery. However, in the sitting posi-tion, hyperbaric solutions provide “saddle block” anes-thesia or low sensory levels, appropriate for vaginal or anorectal surgeries. Hypobaric solutions, on the other hand, will tend to move away from the dependent area. Tetracaine is the most commonly used agent for the hypobaric technique and is obtained by mixing it with sterile water. Patients undergoing anorectal surgery, such as hemorrhoidectomy, may be positioned in the jackknife position during spinal anesthesia placement. The injected anesthetic solution will float to the non-dependent area, in this case, the sacral area, resulting in the blockade of the sacral dermatomes. Isobaric solutions will stay at about the same level where they are injected, irrespective of the patient’s position. They are produced by mixing the local anesthetic with cerebrospinal fluid or are commonly formulated with sodium chloride. Tetracaine, bupivacaine, ropivacaine, and lidocaine are commonly used as isobaric local anesthetics.

·    The desired duration of spinal anesthesia is another determinant factor in choosing which local anesthetic is appropriate. Local anesthetics differ in their length of action. Tetracaine, bupivacaine, and ropivacaine provide a long duration of spinal anesthesia, whereas lidocaine and procaine provide a short duration. However, onset of action is more rapid with lidocaine relative to the other agents.

Epidural Anesthesia: The choice of drug for epidural anesthesia depends on the desired onset and duration of blockade, degree of sensory or motor blockade, and post-operative analgesia requirements. Chloroprocaine is charac-terized as a rapid-onset and short-acting agent providing up to 90 minutes of surgical anesthesia, and is suitable for outpatient surgical procedures. Lidocaine and mepivacaine are considered to have an intermediate onset of action and duration. However, mepivacaine lasts from 15 to 30 minutes longer than lidocaine at equivalent dosages. Lidocaine provides surgical anesthesia that lasts from 60 to 100 min-utes. The addition of epinephrine to any of these three agents significantly prolongs the duration of blockade. Bupivacaine is the most widely used long-acting anesthetic (with slow onset of action) for epidural anesthesia, provid-ing between 120 and 240 minutes of epidural anesthesia. The addition of epinephrine, however, does not reliably prolong its duration of action. Ropivacaine is another long-acting anesthetic being used for epidural anesthesia. Its duration of action and intensity of motor block are slightly less than those of bupivacaine. However, it does have a more favorable cardiac toxicity profile than bupivacaine.

Topical Anesthesia Anesthesia of the mucous membranes of the nose, oral cavity, tracheobronchial tree, esophagus, or genitourinary tract can be achieved by topical placement of local anesthetic on these areas. Lidocaine, either applied topically or nebulized, is commonly used to provide topical anesthesia to the upper or lower respiratory tract prior to fiberoptic tracheal intubation. Tetracaine is also an effective topical anesthetic used during bronchoscopy. Clinicians must be aware, however, that both drugs undergo significant systemic absorption after topical placement on the tracheo-bronchial mucosa.

Tumescent Anesthesia This method is commonly employed by plastic surgeons during liposuction procedures and involves subcutaneous injection of large volumes of dilute local anesthetic in combination with epinephrine and other agents. The most commonly used drug, lidocaine, with total doses ranging from 35 to 55 mg/kg, has been reported to produce a safe plasma concentration.