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Anestezie(engl)








Ambulatory Anesthesia

CURRENT STATUS OF REGIONAL ANESTHESIA FOR ADULT OUTPATIENTS
Dermot Fitzgibbon MB, BCh, FFARCSI

From the Department of Anesthesiology, University of Washington School of Medicine, Seattle, Washington

Address reprint requests to
Dermot Fitzgibbon, MB, BCh, FFARCSI
Department of Anesthesiology
University of Washington
School of Medicine
1959 N.E. Pacific Street
Seattle, WA 98195

BENEFITS OF REGIONAL ANESTHESIA
The challenge of anesthesia for ambulatory patients is to provide for rapid return to street readiness with the most effective postoperative analgesia and minimal undesirable side effects. Regional anesthesia, with its selective local action and relatively simple equipment, offers an excellent anesthetic choice in an outpatient facility. In addition to limiting the anesthetized area to the surgical site, the common side effects of general anesthesia (e.g., nausea, vomiting, lethargy) are reduced, the risks and side effects of endotracheal intubation are minimized, patient recovery time may be decreased, and improved analgesia is provided in the postoperative period. [7] [46]
A number of studies [51] [82] have evaluated the efficacy of ambulatory regional anesthesia. Urmey et al [82] prospectively recorded data on ambulatory surgery patients at an orthopedic speciality hospital where regional anesthesia was the first-line standard care; the various types of anesthesia administered are listed in Table 1 (Table Not Available) . Only 4.4% of patients who had regional anesthesia required admission compared with 12% of general anesthetics. Discharge times were similar for general, spinal, or epidural anesthesia (average of 3 hours); patients who had peripheral nerve blocks were discharged in approximately 2 hours. Failure of regional anesthesia, necessitating general anesthesia, occurred in only 1% of cases. The authors concluded that regional anesthesia in an ambulatory center is effective in all but a small percentage of patients. Osborne [51] evaluated outcome for 6000 consecutive procedures in a major public teaching hospital day surgery unit. Anesthesia-related complications were more frequent with general anesthesia (1:114) than with regional anesthesia (1:180) or local anesthesia plus sedation

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TABLE 1 -- ANESTHETIC TECHNIQUES FOR AMBULATORY ORTHOPEDIC PROCEDURES
From Urmey WF, Stanton J, Sharrock NE: Initial one-year experience of a 97.3% regional anesthesia ambulatory surgery center. Reg Anesth 18:69, 1993; Churchill Livingstone, with permission.
(Not Available)

(1:780). Recovery with regional or local anesthesia was significantly shorter than after general anesthesia.
Despite the potential advantages cited regional anesthesia should not be considered universally appropriate. Factors that contribute to a successful regional anesthetic include the appropriate selection of patients, anesthetic technique, and local anesthetic, use of sedative and hypnotic agents, and the skill of the anesthesiologist. Prior screening of patients through preanesthesia testing (PAT) clinics is very useful in determining the acceptability of patients for a regional anesthetic. Very young or excessively anxious patients may be poor candidates. Similarly, obese patients may present technical problems, especially for central neuraxial blocks. Patients of American Society of Anesthesiologists (ASA) physical status III or IV may be particularly good candidates for ambulatory regional anesthesia compared to general anesthesia, especially if their systemic diseases are medically stable.

SELECTION OF TECHNIQUE AND LOCAL ANESTHETIC
Outpatient regional techniques require some modification from standard inpatient procedures. Ideally, an outpatient regional technique should be rapid in onset and result in few if any acute or delayed complications (e.g., pneumothorax). The additional time needed to perform many regional blocks, as well as the time needed for the anesthetic to take effect, is a potential drawback when procedures are short and turnover between cases is rapid. Use of blocks that require more time than the procedure itself to perform should be limited to those situations where specifically indicated for medical reasons or the patient expresses a strong preference for a specific technique. Blocks that significantly impair the ability to ambulate and void should be tailored to the anticipated usual duration of surgery by appropriate selection of both local anesthetic agent and technique to minimize both recovery and discharge time. Prolonged analgesia from a block (e.g., foot, arm, or hand blocks) may be beneficial in some instances, particularly if the ability of the patient to perform various activities is not significantly impaired; however, prolonged anesthesia may provoke anxiety or be considered unpleasant or irritating by many patients when

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it persists for many hours after hospital discharge and should be discussed with patients before instituting such a block.
Local anesthetic agents are commonly classified according to their relative potency and duration of action as follows: low potency and short duration (e.g., chloroprocaine), moderate potency and duration (e.g., lidocaine and mepivacaine), high potency and long duration (e. g., tetracaine, bupivacaine, and etidocaine). Selection of specific blocks are discussed later. Selecting the appropriate local anesthetic for a given regional anesthetic requires consideration of a number of factors, including potency, speed of onset, duration of action of local anesthetics, site and duration of surgery, the degree of muscle relaxation required, and the duration of analgesia desired. Duration of anesthesia with a given agent varies with the site of injection and frequently with the total mass of drug injected. [70] Thus, bupivacaine injected into the epidural space lasts approximately 2 to 3 hours whereas the same dose injected into the brachial plexus may last 10 to 11 hours. Vasoconstrictors, such as epinephrine, are added to increase the duration of action, provide an indication of intravascular injection, and reduce peak serum levels of local anesthetic. The extent to which epinephrine prolongs the duration of anesthesia depends on the specific local anesthetic used and the site of injection. Vasoconstrictors do not prolong the duration of action of all local anesthetics in all situations (Table 2) (Table Not Available) . Epinephrine prolongs the duration of action of all agents for peripheral nerve blocks except ropivacaine. [33] It also prolongs the duration of action of epidural chloroprocaine, lidocaine, and mepivacaine. The local anesthetic properties of the intrinsically more potent and longer acting agents (bupivacaine, etidocaine, tetracaine) are influenced less by the addition of epinephrine, particularly when such agents are used epidurally. Epinephrine does not markedly prolong the duration of motor block by epidural bupivacaine or etidocaine; however, it does extend the sensory block by these epidural agents. [71] The effects of epinephrine added to agents used for spinal anesthesia are discussed later.
The optimal dose of epinephrine is one that would produce maximal increase in the duration of a local anesthetic agent and minimal hemodynamic effects. Kennedy et al [36] showed that a supraclavicular brachial plexus block with 30 mL of 1.6% lidocaine has virtually no hemodynamic effects whereas the same agent with epinephrine 1:200,000 produced a dose-related increase in cardiac rate, cardiac output, and stroke volume that persisted for 90 minutes and decreases in peripheral resistance and concomitant changes in mean arterial pressure that persisted for 120 minutes. Absorbed epinephrine produces predominantly beta-adrenergic effects with little evidence of alpha-adrenergic effects at
TABLE 2 -- DURATION OF ACTION OF LOCAL ANESTHETICS AND EFFECT OF EPINEPHRINE
Adapted from Ellis JS: Local anesthetics. In Kirby, Gravenstein: Clinical Anesthesia Practice. Philadelphia, WB Saunders, 1994.
(Not Available)

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doses up to 400 mug. Furthermore, epinephrine produced a dose-related increase in mean duration of anesthesia, but only up to a concentration of 1:200,000, above which the cardiocirculatory changes continued to increase without any further increase in the duration of anesthesia. The optimal dose of epinephrine in the above study was a 1:200,000 solution or 5 mug/ml concentration. If the use of epinephrine is desirable it should be added to the local anesthetic just before the local anesthetic is used. The reason for this is that commercial solutions of epinephrine containing epinephrine are buffered to a lower pH than the standard solution of that agent in an effort to oxidation of the epinephrine. Such acidification moves the pH farther from the pKa of that solution, reducing the availability of the free base and the rate of diffusion of the local anesthetic.
The speed of onset of local anesthetics is primarily related to the agent selected and the site of injection. Thus, agents such as chloroprocaine have a more rapid onset in the epidural space than agents such as lidocaine and bupivacaine, and lidocaine and mepivacaine have a more rapid onset than bupivacaine when used for peripheral nerve blocks. Efforts to increase the speed of onset of local anesthetics by the addition of bicarbonate have yielded contradictory results and appear to be minimally effective.

SEDATION/ANALGESIA
Many patients undergoing surgery with local or regional anesthesia prefer to be sedated. Small doses of short-acting drugs should be carefully titrated. Midazolam is an excellent agent for the moderately-anxious patient. The amnesia it produces does not correlate with the apparent level of sedation; fully conscious patients may have no recall of perianesthetic events. Low-dose propofol infusions also appear to be excellent agents for intraoperative sedation. [84] Adequate intraoperative sedation can usually be achieved with infusion rates of 25 to 100 mug/kg-1 /min-1 . When intraoperative amnesia is desired in addition to a rapid recovery, administration of small titrated doses of midazolam (0.5-3 mg intravenously) prior to the propofol infusion may offer advantages over either drug alone. Many outpatients find the use of local anesthetic techniques acceptable alternatives to both general and regional anesthesia when adequate sedation and anxiolysis are provided. The addition of a short-acting opioid (e.g., fentanyl, alfentanil) is especially useful if paresthesia are sought or nerve stimulation performed during a regional technique and obtundation is undesirable.


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