CLINICAL CONTROVERSIES AND NEURAXIAL BLOCKS

Despite the many years neuraxial blocks have been used, there remain controversies about the appropriate use of these blocks. Some continue to suggest that spinal anesthesia is inappropriate for outpatients because of the occurrence of postdural puncture headache, but there are data that the occasional headache can be successfully treated even in the outpatient,^[186] and considerable other data support the use of the technique in outpatients.^[187] Similarly, spinal anesthesia seems well suited to use for cesarean section because of the short interval from injection to surgical anesthesia. If epidural blood patch therapy is used in a timely fashion, the increased incidence of headache in this patient population should not necessarily limit the technique's use for cesarean section. An outgrowth of the management of postdural puncture headache was the development of the "micro" or small-bore spinal catheter.^[188] This technique's appropriate place in anesthesia practice has not been established because there must be a point at which the risk-benefit ratio of a decreased incidence of spinal headache compared with difficulties in technique with the microcatheter becomes unacceptable. These catheters (i.e., 27-gauge or smaller) are not available because the FDA withdrew the 510k approval in 1992, after concerns over a possible association with the development of cauda equina syndrome.^[7]

One reason that epidural anesthesia is not used as widely as may be indicated is that controversies surrounding its use create indecision. One of the most controversial adjuncts to epidural anesthesia has been the use of the epinephrine-containing test dose.^[189] The original description used 15 µg of epinephrine in 3 mL of local anesthetic to indicate intravascular placement of an epidural catheter or needle. Most controversy surrounds the use of epinephrine in obstetric patients, in whom uterine blood flow may be decreased by the intravascular injection, thereby putting the fetus at risk. Some experimental data suggest that epinephrine may place the fetus at risk^[190] ; nevertheless, no clinical data suggest that any fetus has been harmed by a test-dose use, and many anesthesiologists believe epinephrine-containing epidural test doses are useful in obstetric anesthesia.^[191] Patients receiving β-adrenergic blockers also confound the use of a test

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Related to systemic toxicity with neuraxial blocks is information about benzodiazepines. Despite a widely held belief that benzodiazepines should be used with regional anesthesia to minimize systemic toxicity by elevating seizure threshold, available data indicate that this concept requires rethinking.^[195] Resuscitation may be more difficult after cardiovascular collapse due to bupivacaine when diazepam is used as a premedication. It has also been demonstrated that animals premedicated with benzodiazepines may not evidence seizures before cardiovascular collapse.^[195] The benzodiazepines may cover up one of the early signs of systemic toxicity, potentially delaying definitive therapy.

The clinical arena has not been immune to controversy about epidural anesthesia. Yeager and colleagues^[10] demonstrated a significant decrease in morbidity and mortality in high-risk patients undergoing epidural anesthesia and postoperative analgesia compared with patients receiving high-dose narcotic anesthesia and parenteral narcotic analgesia. This information provided impetus to many investigators to design studies to answer whether the anesthetic technique, especially postoperative analgesic technique, may be able to modify patient outcomes. In the case of Yeager and coworkers, it remains unclear whether their data support the benefits of the epidural techniques or condemn the use of high-dose narcotics and postoperative mechanical ventilation. Tuman and associates^[11] addressed many of the criticisms directed at the work of Yeager and colleagues^[10] and documented fewer complications in vascular surgery patients when epidural analgesia was used in an integrated perioperative regimen. Christopherson and coworkers^[12] and Rosenfeld^[129] and associates linked the analgesic regimens with possible mechanisms of improved patient outcomes. They identified that using epidural anesthesia and analgesia into the postoperative period limited the adverse effects of surgery on the fibrinolytic system. Further investigations into the appropriate place of neuraxial blocks in anesthetic practice will have to be as concerned with the issue of cost in relation to outcome as with risk in relation to benefit to the patient.

Another advance in neuraxial block technique that demands our attention is the development of effective methods of combining epidural and spinal anesthesia in a single-needle system.^{[196] [197] [198] [199]} These methods use epidural needle placement, followed by insertion of the spinal needle through a side lumen on the epidural needle or directly through the epidural needle lumen. The combined technique allows flexibility in a number of clinical settings; some of the possible combinations and advantages

TABLE 43-9 -- Possible clinical advantages of using combined spinal-epidural anesthesia

Initial epidural needle placement allows the spinal needle to be guided near the dura, minimizing the number of times the spinal needle tip impacts bone and potentially becomes dulled.

Lower local anesthetic blood levels are possible when an initial spinal anesthetic is used for the operation and the epidural catheter is used for analgesia.

More rapid onset of spinal block allows the operative procedure to begin earlier, and the epidural catheter allows effective analgesia to be provided.

During labor, an opioid may be injected through a small spinal needle, and epidural analgesia can then be added if needed.

Lower initial mass of drug may be used during spinal anesthesia, minimizing the physiologic perturbations, while the epidural catheter is available to provide a higher level if needed.

are listed in Table 43-9 . There is considerable evidence that combined spinal and epidural techniques can be effectively used in community practices and academic institutions.^[200]