What factors affect the spread of anesthetic within the subarachnoid space?

What factors affect the spread of anesthetic within the subarachnoid space?

Specific gravity greatly influences the local anesthetic’s spread within the subarachnoid space. The specific gravity of CSF ranges between 1.003 and 1.008. Local anesthetics with lower specific gravities, i.e., hypobaric solutions, tend to float toward the nondependent areas. Hypobaric solutions have been used for patients with fractured hips, placing the injured extremity above as the patient lies in the lateral decubitus position. Hypobaric solutions are created by dis-solving local anesthetic in distilled water. Local anesthetic solutions with specific gravities above 1.008 are hyperbaric. Hyperbaric solutions tend to migrate toward dependent areas. They are the local anesthetic preparations most commonly used for spinal anesthesia in the United States. Hyperbaric solutions are readily adaptable to most spinal anesthetic needs and are particularly valuable for anes-thetizing lower thoracic nerve roots and for “selectively” anesthetizing sacral nerve roots (saddle blocks). Hyperbaric solutions are created by mixing a local anesthetic with 10% dextrose in water. Level of the dural puncture and patient position also affect anesthetic spread.

The spinal column contains a natural lumbar lordosis as well as a thoracic and sacral kyphosis. L₄ resides at the peak of the lumbar lordosis, and T₅ lies at the nadir of the thoracic kyphosis (Fig. 56.1). After local anesthetic is injected at the L₄ level and the patient is turned to the supine posi-tion, hyperbaric solutions tend to pool in the sacral and thoracic kyphoses. In other words, hyperbaric solutions run downhill, providing maximal anesthesia of the sacral and midthoracic nerve roots. Middle lumbar spinal roots may be exposed to minimal amounts of local anesthetic, thereby providing poor sensory block to these dermatomes whenever inadequate local anesthetic is administered. Raising the legs into the lithotomy position will flatten the lumbar lordosis and help prevent large unanesthetized windows. Pooling of solution in the thoracic kyphosis tends to prevent anesthesia rising above the T₄ dermatome, preventing a “total spinal”. Using excessively large volumes of anesthetic may overcome the lumbar lordosis problem but it may also spread the anesthetic above T₄. To avoid both excessively high levels and inadequate levels, the process of selecting the proper dose of local anesthetic for subarachnoid injection has, in the past, taken into account innumerable variables. Height, weight, age, increased abdominal pressure, position, site of injection, volume of injectate, concentration of local anesthetic, amount of local anesthetic, and baricity represent only some of the factors that were traditionally thought to significantly affect anesthetic spread. With these criteria in mind, a myriad of guidelines for spinal anesthesia dosing have been presented over the years. Table 56.1 lists one of the many possible dosing schedules for hyperbaric tetracaine spinal anesthet-ics. It has generally been recommended to reduce amounts by 2 mg for patients 5 feet (152 cm) tall and to increase amounts by 2 mg for patients 6 feet (183 cm) tall. Despite rigorous adherence to such dosing schedules, spinal anes-thesia has frequently been inadequate to provide satisfac-tory patient comfort during surgery.

Recognizing the high frequency of inappropriately low anesthesia levels, Norris (1988) conducted a study that has revolutionized spinal anesthesia dosing regimens. Evaluation of term parturients receiving 12 mg of hyperbaric bupivacaine demonstrated no correlation between the spread of anesthesia and the subject’s height, weight, or body mass index. Although peak analgesic levels ranged from T₇–C₈, most patients attained levels between T₄ and T₁. Extrapolating these results to the nonpregnant patient, several authors revised the recommended dosing schedules for subarachnoid block. Adult patients, placed in the supine position, can receive 15 mg of hyperbaric bupiva-caine or hyperbaric tetracaine. These solutions will eventu-ally migrate to the T₅ nerve roots, producing profound and reliable anesthesia below this level. This technique is espe-cially useful for surgery on structures innervated below T₈. The newer recommendations calling for significantly larger local anesthetic doses provide more intense blockade for longer durations without significantly extending the spread of anesthesia. Tuominen and colleagues (1992) have suggested that unpredictable individual anatomic varia-tions may play an important role in subarachnoid spread of local anesthetics.

To prevent local anesthetic spread to thoracic levels when only lumbosacral anesthesia is required, isobaric lidocaine 2.0% or isobaric bupivacaine 0.5% without preservatives represent an excellent choice. Isobaric solu-tions tend to remain in the lumbosacral area into which they are injected. These local anesthetics do not migrate significantly, providing for maintenance of adequate anes-thetic concentrations bathing the lumbosacral nerve roots. The benefits include more intense analgesia, greater dura-tion of action, and reduced potential for hypotension and respiratory depression.

Speed of anesthetic injection should approximate 0.5 mL/sec. Significantly more rapid injection rates tend to push the anesthetic toward higher dermatome levels.