Pharmacology Of Spinalanesthesia

In the U.S., the drugs most commonly used in spinal anesthesia are lidocaine, tetracaine, and bupivacaine. General guidelines are to use lidocaine for short procedures, bupivacaine for intermediate to long procedures, and tetracaine for long procedures. The distribution of local anesthetics determines the height of block. Important pharmacological factors include the amount, and possibly the volume, of drug injected and its baricity. The speed of injection of the local anesthesia solution also may affect the height of the block, just as the position of the patient can influence the rate of distribution of the anesthetic agent and the height of blockade achieved (see below). For a given preparation of local anesthetic, administration of increasing amounts leads to a fairly predictable increase in the level of block attained. For example, 100 mg of lidocaine, 20 mg of bupi-vacaine, or 12 mg of tetracaine usually will result in a T4 sensory block. More complete tables of these relationships can be found in standard anesthesiology texts. Epinephrine often is added to spinal anesthetics to increase the duration or intensity of block, but the mechanism of action of vasoconstrictors in prolonging spinal anesthesia is uncertain.

Spinal anesthesia is a safe and effective technique, especially during surgery involving the lower abdomen, the lower extremities, and the perineum. It often is combined with intravenous medication to provide sedation and amnesia. The physiological perturbations associated with low spinal anesthesia often have less potential harm than those associated with general anesthesia. The same does not apply for high spinal anesthesia. The sympathetic blockade that accompanies levels of spinal anesthesia adequate for mid- or upper-abdominal surgery, coupled with the difficulty in achieving visceral analgesia, is such that equally satisfactory and safer operating conditions can be realized by combining the spinal anesthetic with a "light" general anesthetic or by the administration of a general anesthetic and a neuromuscular blocking agent.

Epidural Anesthesia

Epidural anesthesia is administered by injecting local anesthetic into the epidural space—the space bounded by the ligamentum flavum posteriorly, the spinal periosteum laterally, and the dura anteriorly. Epidural anesthesia can be performed in the sacral hiatus (caudal anesthesia) or in the lumbar, thoracic, or cervical regions of the spine. Its current popularity arises from the development of catheters that can be placed into the epidural space, allowing either continuous infusions or repeated bolus administration of local anesthetics. The primary site of action of epidurally administered local anesthetics is on the spinal nerve roots. However, epidurally administered local anesthetics also may act on the spinal cord and on the paravertebral nerves.

The selection of drugs available for epidural anesthesia is similar to that for major nerve blocks. As for spinal anesthesia, the choice of drugs to be used during epidural anesthesia is dictated primarily by the duration of anesthesia desired. However, when an epidural catheter is placed, short-acting drugs can be administered repeatedly, providing more control over the duration of block. Bupivacaine, 0.5-0.75%, is used when a long duration of surgical block is desired. Due to enhanced cardiotoxicity in pregnant patients, the 0.75% solution is not approved for obstetrical use. Lower concentrations—0.25%, 0.125%, or 0.0625%—of bupivacaine, often with 2 ^g/mL of fentanyl added, frequently are used to provide analgesia during labor. They also are useful preparations for providing postoperative analgesia in certain clinical situations. Lidocaine 2% is the most frequently used intermediate-acting epidural local anesthetic. The duration of action of epidurally administered local anesthetics frequently is prolonged, and systemic toxicity decreased, by addition of epinephrine. Addition of epinephrine also makes inadvertent intravascular injection easier to detect and modifies the effect of sympathetic blockade during epidural anesthesia.

For each anesthetic agent, a relationship exists between the volume of local anesthetic injected epidurally and the segmental level of anesthesia achieved. For example, in 20- to 40-year-old, healthy, nonpregnant patients, each 1-1.5 mL of 2% lidocaine will give an additional segment of anesthesia. The amount needed decreases with increasing age and also decreases during pregnancy and in children.

The concentration of local anesthetic used determines the type of nerve fibers blocked. The highest concentrations are used when sympathetic, somatic sensory, and somatic motor blockade are required. Intermediate concentrations allow somatic sensory anesthesia without muscle relaxation. Low concentrations will block only preganglionic sympathetic fibers. A significant difference between epidural and spinal anesthesia is that the dose of local anesthetic used can produce high concentrations in blood following absorption from the epidural space.

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