The 16th Annual Meeting of the Society for Ambulatory Anesthesia was held from May 3-6, 2001 in Indian Wells, California, with the program attracting more than 500 registrants. The general sessions featured a vast array of timely topics including clinical and regulatory issues pertaining to office-based anesthesia, a presentation of cases from the "real world," regional anesthesia in the outpatient setting, medicolegal issues and their ramifications for the individual practitioner, sedation/analgesia outside of the operating room, and dealing with production and other pressures. The six scheduled workshops offered certification in pediatric advanced life support (PALS) as well as practical advice and/or hands-on experience with technological toys, perioperative medicine challenges, pediatric dental anesthesia, regional anesthesia, and new medical challenges involved with such procedures as ultrarapid opiate detoxification. A special Residents' Conference focused on managing one's career from the perspectives of a male and a female anesthesiologist as well as the thorny topic of managing one's finances. Obviously, temporal and spatial constraints do not allow comprehensive coverage of all these presentations, and we have chosen to highlight the role of outpatient regional anesthesia for both adults and children.
I. Regional Anesthesia for Ambulatory Adults
Michael F. Mulroy, M.D., Staff Anesthesiologist at the Virginia Mason Medical Center in Seattle, Washington, moderated the regional anesthesia session that featured presentations on selective spinal anesthesia, pediatric ambulatory regional anesthesia, and the cost-effectiveness of central neuraxial blocks in the outpatient setting.
Himat Vaghadia, M.B.B.S., F.R.C.A., Clinical Associate Professor of Anesthesia at the University of British Columbia in Vancouver, Canada, discussed "Selective Spinal Anesthesia: Walk-In and Walk-Out Spinals." Owing to the introduction of small gauge, pencil-point spinal needles that have dramatically reduced if not totally eliminated the complication of postdural puncture headache, there has been significantly increased interest in spinal anesthesia for ambulatory patients. However, conventional dose spinal anesthesia is associated with prolonged discharge times and may be a risk factor for readmission owing to difficulty with micturition. Because duration of spinal anesthesia is proportionate to the dose administered, smaller doses of local anesthetic have been advocated to facilitate faster recovery. Selective spinal anesthesia (SSA), defined as the practice of employing minimal doses of intrathecal agents so that only the nerve roots supplying a specific area and only the modalities that require anesthesia are affected, has been suggested to provide both satisfactory operating conditions and earlier discharge times. Because intrathecal opioids act synergistically to enhance the effect of local anesthetics, Dr. Vaghadia and colleagues at Vancouver General Hospital during the past 5 years have studied SSA in approximately 200 patients undergoing outpatient laparoscopy. Comparison of low dose lidocaine-fentanyl spinal anesthesia (25 mg lidocaine plus 25 µg fentanyl plus sterile water to a final volume of 3 ml) with conventional dose spinal anesthesia (75 mg lidocaine) demonstrated that the low dose technique provided comparable operating conditions with a significantly faster recovery and less hypotension. If 10 µg sufentanil is substituted for the 25 µg dose of fentanyl, it is possible to reduce the dose of lidocaine to 10 mg without impairing surgical conditions. Studies of spinal cord function with this low dose lidocaine-sufentanil solution (10 mg of 1% lidocaine + 10 µg sufentanil + sterile water to a final volume of 3 ml) showed that most patients had normal motor power and dorsal column function and were able to ambulate at the end of surgery. Intraoperatively patients were aware of touch and proprioception but not pain. Sympathetic function remained intact, so volume loading to prevent hypotension was not necessary.
Dr. Vaghadia did not encounter problems with either transient neurologic symptoms or pruritis using this low dose lidocaine-sufentanil SSA technique. He believes that lidocaine is probably the drug of choice for procedures lasting up to 1 hr and bupivacaine for longer procedures. Although bupivacaine is often used in longer outpatient procedures, few studies have investigated doses less than 5 mg. Dr. Vaghadia thinks this is unfortunate because the duration of sensory block with 5 mg bupivacaine exceeds 4 hours and typically patients develop some motor block with this dose. Newer drugs such as ropivacaine still have to be evaluated with respect to SSA.
Regardless of the local anesthetic used, the literature indicates that for equivalent doses of the same drug, less motor block is seen with dilute solutions. It is also important to appreciate that the extent of spinal block is determined by the lumbosacral CSF volume (approximately 50 ml in adults). When 10 mg of 1% lidocaine with 10 µg sufentanil made up to 3 ml by dilution with sterile water is injected into 50 ml of CSF, the concentration of lidocaine will be 0.018%, consistent with in vitro studies on differential nerve block. Moreover, it is important to appreciate that 25 µg of intrathecal fentanyl will provide analgesia for up to 6 hours, without affecting carbon dioxide ventilatory response curves. Sufentanil in a dose of 10 µg has a more rapid onset and shorter duration (2 hours). Although intrathecal sufentanil alone produces pinprick analgesia and has been used successfully as the sole agent for extracorporeal shock wave lithotripsy (ESWL), it is not recommended for use as the sole agent in patients having laparoscopy and should, therefore, be combined with dilute local anesthesia for outpatient laparoscopy. Clearly, initial studies of SSA in outpatients are encouraging and we await additional studies to better define its role in the ambulatory setting.
II. Regional Anesthesia for Pediatric Outpatients
Lucinda L. Everett, M.D., Associate Professor at the University of Washington and Attending Anesthesiologist at the Children's Hospital and Regional Medical Center in Seattle, lectured on "Unique Aspects of Regional Anesthesia in the Pediatric Patient." Dr. Everett underscored the differences between adults and children undergoing outpatient regional anesthesia, pointing out that pediatric patients are usually not awake when blocks are performed and surgery is conducted; the doses of local anesthetics used may be closer to toxic levels, with a narrow margin of safety; and, frequently, pediatric patients can be carried home by their parents!
During the past two decades, there has been an impressive increase in the use of pediatric regional anesthesia for outpatients, owing in large part to the effective postoperative analgesia it provides while concomitantly obviating opioid administration with associated side effects. Although there is growing interest in greater utilization of peripheral nerve blocks, the most commonly performed blocks for outpatient pediatric surgery include caudal, ilioinguinal/iliohypogastric, and penile blocks. Much of the available data relate to these techniques and, unfortunately, there is a dearth of information about other approaches.
Unlike the situation in adults, the pediatric anesthesia community strongly believes that if regional anesthesia is to be performed in children, "they must be sedated or anesthetized in order to allow the safe performance of a regional block on a still and quiet subject." [1] Nonetheless, concerns do exist regarding the ability to detect intravascular or intraosseous injection, or nerve injury owing to needle trauma or intraneural injection, under these circumstances.
The largest study of complications associated with pediatric regional anesthesia came from a group of French hospitals in 1996.[2] The series included 15,013 central blocks, most of which were caudals, with a complication rate of 1.5 per 1,000 cases. The authors described all of the complications as "minor," because none resulted in injury or death, but these did include 4 dural punctures resulting in total spinal anesthesia, and intravascular injection after negative test dose was reported in 6 children, resulting in 2 seizures, 2 transient cardiac arrhythmias, and 2 subclinical problems. In the 9,396 cases categorized as "peripheral nerve blocks and local anesthesia," there were no complications, causing the authors to suggest that peripheral blocks are safer and should be used more frequently. However, many of these "peripheral blocks" consisted merely of local infiltration.
A recent publication summarized 1,000 cases of pediatric regional anesthesia in Italy [3]. Interestingly, with 715 caudal blocks, the authors reported a 10% failure rate and one dural puncture without sequelae. Block failure rate was slightly less for peripheral blocks, with 12/153 penile blocks failing and 1/52 ilioinguinal/iliohypogastric blocks unsuccessful. There were no significant complications reported in the 285 peripheral blocks.
Concern has been expressed about air embolus occurring with pediatric epidural anesthesia. A large French retrospective study reported good safety overall, but 5 serious "accidents" in 24,000 regional anesthetics (16,100 caudals and 7,200 epidurals) [5]. All 5 involved infants who sustained spinal cord injury thought most likely to be secondary to air embolism (from loss of resistance to injecting with air) or to ischemic injury. Clearly, the size of the air bolus in relation to the size of the patient makes the use of air testing more concerning in children, and this technique should be avoided in pediatrics.
Several studies have compared analgesia and side effects with caudal blocks and other local techniques, with somewhat variable results. Caudal and penile block each appear to provide equivalent analgesia for procedures such as circumcision; there is some suggestion from studies conducted more than 15 years ago that side effects such as nausea/vomiting and delayed micturition are less common in patients receiving penile block compared with those receiving caudal block [5].
Another important topic in pediatric regional analgesia is the issue of test doses administered to detect inadvertent injection of local anesthetic into either the subarachnoid or intravascular space. Obviously, in the awake patient, subarachnoid injection of a small dose of local anesthetic intended for the epidural space will result in spinal anesthesia and is easily detected. However, in the anesthetized patient, detection is typically more difficult, and children may not show signs of hemodynamic instability, even in the situation of a total spinal. Loss of respiratory effort in a previously spontaneously breathing patient may be the only clue. Similarly, detection of intravascular (or interosseous) injection of local anesthetic, with the potential for cardiotoxicity, may also be difficult in the anesthetized patient. Aspiration of blood may not reliably occur, and CNS signs, which might occur earlier in an awake patient, are usually not present in the anesthetized child. Additionally, heart rate changes to epinephrine-containing test doses may be less sensitive in the pediatric population, particularly in conjunction with volatile anesthetics, and choice of inhalational anesthetic may also influence the sensitivity to test doses.
Current suggested criteria for a positive test dose in anesthetized children include HR increase > 10 beats per minute, systolic blood pressure increase > 15 mmHg, and an increase in T wave amplitude > 25% in lead II. With a simulated intravascular test dose of 0.5 µg/kg epinephrine, all patients met T wave criteria, while 85% had an increase in HR exceeding 10 beats per minute in sevoflurane-anesthetized children given atropine pretreatment [6]. In summary, recommendations to minimize the risk of intravascular injection include: use of a short-beveled needle or angiocath; slow injection of local anesthetic in small incremental amounts; assessment for passive flashback in addition to aspiration, because small veins may be collapsed with aspiration; and use of epinephrine-containing test doses, with attention to T-wave morphology as well as heart rate and blood pressure changes. That said, it should be pointed out that toxicity has occurred typically in the setting of ongoing infusion for postoperative pain management, at doses higher than the ones currently administered . Recommendations for infusion doses have recently been revised, with a maximum dose of 0.4 - 0.5 mg/kg/hr bupivacaine suggested [7]. Moreover, infants may be at greater risk of toxicity owing to lower levels of plasma binding proteins and/or reduced clearance. Again, this is a particular hazard with re-dosing or infusion techniques.
Another safety consideration focuses on the fact that spinal epidermoid tumors have been linked to lumbar puncture performed with a nonstyletted needle. This has raised concern about tissue coring when caudal blocks are performed. Although spinal epidermoid tumors are extremely rare and associated with spinal rather than epidural puncture, Goldschneider and Brandom [8] recommend the use of either a styletted needle or an intravenous catheter to avoid introduction of epidermoid tissue into the epidural space. Additionally, it is probably reasonable to make a small skin nick in an attempt to eliminate tissue coring [9].
Pharmacologic considerations center around the selection of local anesthetics, opioids, and other adjuncts. Bupivacaine has been the traditional mainstay of pediatric regional analgesia, but opinions have differed concerning the optimal concentration to use. A comparison of 0.125% versus 0.25% bupivacaine with epinephrine for caudal block at the end of surgery showed no difference in the analgesia provided or in time to first urination. However, the children given 0.125% bupivacaine had better return of motor function [10]. Moreover, analgesia produced by 0.0625% bupivacaine was rated inadequate. Ropivacaine offers the potential advantages of less cardiovascular toxicity and less motor block at equianalgesic doses, but the toxicity issue has been rather controversial. Only preliminary work on the safety and efficacy in children of levobupivacaine that is hoped to be less toxic, has been conducted.
Various additives, such as opioids, clonidine, midazolam, and ketamine, have been used to enhance the analgesic effect of local anesthetics given caudally, allowing a lower concentration of local anesthetic with less risk of motor block. Although long-acting neuraxial opioids are not recommended for outpatients, fentanyl is safe and effective, although nausea and vomiting may be side effects. Clonidine has been shown to prolong and enhance the efficacy when combined with local anesthetic for pediatric caudal block [11] and it also prolongs peripheral blocks. For caudal anesthesia, a dose of 1-2 µg/kg seems most appropriate for outpatients because this dose is unlikely to produce significant sedation. Indeed, a combination of dilute local anesthetics and clonidine may be the ideal choice for caudal block in pediatric outpatients, producing excellent and long-lasting analgesia with minimal side effects [12]. Unfortunately, however, the package insert specifies that clonidine is not to be used for acute postoperative pain, although the epidural preparation is considered acceptable for chronic pain.
In summary, pediatric regional anesthesia has an excellent track record in terms of safety and efficacy. Comprehensive knowledge of germane anatomy and meticulous incremental dosing of local anesthetics and adjuncts, with frequent aspiration and attention to electrocardiographic indicators of intravascular injection, may reduce the incidence of complications. Further studies should better define the optimal techniques and agents to provide excellent analgesia with minimal side effects.
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Constant I. Gall O, Gouyet L, et al. Addition of clonidine or fentanyl to local anaesthetics prolongs the duration of surgical analgesia after single shot caudal block in children. Br J Anaesth 1998;80:294-8.
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