Monitored Anesthesia Care

The combination of local anesthesia with intravenous sedative and analgesic drugs is extremely popular in the ambulatory setting. It has been suggested that up to 50% of all outpatient procedures could be performed with a MAC technique and that the cost of perioperative care can be reduced by up to 80% in comparison to general anesthesia.^[416] The primary objective in providing MAC is to ensure patient comfort, safety, and satisfaction during surgery. In most instances, the patient is undergoing a diagnostic or therapeutic procedure under local anesthesia. This terminology was originally introduced to describe the care of patients requiring sedative or analgesic drugs during dental procedures and was referred to as "conscious sedation."^[417] As the term implies, conscious sedation is a "minimally" depressed level of consciousness in which the patient retains the ability to maintain an unobstructed airway with continuous respiration and is responsive to verbal commands. However, the ASA has

2610

Monitored anesthesia care is the term used when an anesthesiologist monitors a patient receiving local anesthesia or administers supplemental drugs to patients undergoing diagnostic or therapeutic procedures.^[419] The ASA defines MAC as instances in which an anesthesiologist has been requested to provide specific anesthesia services to a particular patient undergoing a planned procedure, in connection with which a patient receives local anesthesia or, in some cases, no anesthesia at all.^[420] In such a case, the anesthesiologist is in control of the patient's vital signs and is available to administer anesthetics and provide other medical care as appropriate. The standard of care for patients receiving MAC should be the same as for patients undergoing general or regional anesthesia; such care includes a complete preoperative assessment, intraoperative monitoring and postoperative recovery care. Vigilant monitoring is required because patients may rapidly progress from a "light" level of sedation to "deep" sedation (or unconsciousness) and thus may be at risk for airway obstruction, oxygen desaturation, and even aspiration.

Anesthetic drugs are administered during procedures under MAC with the goal of providing analgesia, sedation, and anxiolysis and ensuring rapid recovery without side effects. Systemic analgesics are used to reduce the discomfort associated with the injection of local anesthetics and prolonged immobilization, ^[421] as well as pain that is not amendable to local anesthetics (e.g., endoscopy). Sedative-hypnotic drugs are used to make procedures more tolerable for patients by reducing anxiety and providing a degree of intraoperative amnesia while allowing them to rest during the operation.^[422] During longer surgical procedures, patients may become restless, bored, or uncomfortable when forced to remain immobile under local anesthesia alone. Patient anxiety can be reduced with benzodiazepines, as well as by good preoperative communication, keeping the patient warm and covered, and allowing the patient to listen to music during the procedure.^[423]

Many different sedative-hypnotic drugs have been used during MAC (including barbiturates, benzodiazepines, ketamine, and propofol) with a wide variety of delivery systems (e.g., intermittent boluses, variable-rate infusion, target-controlled infusion, patient-controlled sedation).^{[424] [425]} The most commonly used sedation technique is a small dose of midazolam (1 to 2 mg) or propofol (0.5 to 1 mg/kg), or both, followed by a propofol infusion at 25 to 100 µg/kg/min.^{[426] [427] [428]} Methohexital has also been used successfully during MAC as intermittent boluses (10 to 20 mg) or as a variable-rate infusion (1 to 3 mg/min).^[429] Although residual sedation may be greater with methohexital (versus propofol), differences in recovery times were small when comparing infusions of methohexital (40 µg/kg/min) and propofol (50 µg/kg/min) during MAC.^[418] Furthermore, the incidence of pain on injection is higher with propofol than with methohexital. ^{[232] [244]} Although the short-acting barbiturate is a cost-effective alternative to propofol for sedation during MAC,^[429] manufacturing problems have limited the availability of this sedative-hypnotic drug.

In women undergoing laparoscopic tubal sterilization with a MAC technique, anesthetic drug costs were significantly reduced in comparison to general anesthesia ($21 versus $46, respectively).^[430] The MAC technique was also associated with less time in the operating room, a higher degree of "awakeness" on the evening of the day of surgery, and decreased postoperative pain and sore throat when compared with general anesthesia, thus contributing to improved quality of recovery and reduced perioperative cost. Analogous to earlier studies involving patients undergoing unilateral hernia repair and anorectal surgery, ^{[431] [432]} Patel and coauthors^[433] reported that the use of MAC sedation resulted in a 6- to 7-minute decrease in operating room exit time when compared with general anesthesia, thus contributing to enhanced turnover of cases. This consideration is important in a practice environment that emphasizes "fast-tracking" after ambulatory surgery.

Avramov and White^[434] described the combined use of alfentanil (0.3 to 0.4 µg/kg/min) and propofol (25 to 75 µg/kg/min) infusions for MAC. Propofol produced a dose-related reduction in the opioid dose requirement (25% to 50%) and the incidence of PONV (0% to 17%) when compared with alfentanil alone (33%). Ketamine infusion has been used as an alternative to the opioid analgesic.^[435] An infusion of remifentanil, 0.05 to 0.15 µg/kg/min, can provide adequate sedation and analgesia during minor surgical procedures performed with the patient under local anesthesia.^[436] The addition of midazolam (2 to 4 mg IV) improves the quality of sedation and anxiolysis. Sá Rêgo and colleagues^[437] compared the use of intermittent remifentanil boluses (25 µg) with a continuous variable-rate infusion (0.025 to 0.15 µg/kg/min) in patients undergoing extracorporeal shock wave lithotripsy (ESWL) with a MAC technique consisting of midazolam (2 mg) and propofol (25 to 50 µg/kg/min). Patient comfort was higher during the procedure when remifentanil was administered by a variable-rate infusion. However, these patients also experienced a higher incidence of episodic oxygen desaturation (30% versus 0%) than did those given small intermittent boluses of remifentanil. In a direct comparison of remifentanil and propofol administered by continuous infusion after premedication with midazolam (2 mg IV), a decreased level of sedation and a greater degree of respiratory depression were noted with remifentanil than with propofol.^[438] When used in combination with sedative-hypnotics, remifentanil infusions must be very carefully titrated to avoid excessive respiratory depression.^[437] The use of remifentanil in combination with local anesthetics obviates the disadvantage associated with its minimal residual postoperative analgesia.

Given the increased risk of ventilatory depression when potent opioid analgesics are combined with sedative-hypnotics, a variety of nonopioid analgesics have been evaluated during MAC.^[421] Ketorolac has been used both as a sole analgesic supplement and as an adjunct to propofol sedation during local anesthesia. The use of ketorolac was associated with a lower incidence of pruritus, nausea, and

2611

Subanesthetic concentrations of inhaled anesthetics (e.g., N₂ O, 30% to 50%, or sevoflurane, 0.3% to 0.6%) have also been used to supplement local anesthesia.^[432] The primary concern relates to the ease which the patient can drift into an unconscious state, as well as operating room pollution. Therefore, these techniques are typically reserved for patients not tolerating the operation under MAC because of inadequate analgesia or agitation.

α₂ -Agonists reduce central sympathetic outflow and have been shown to produce anxiolysis and sedation. Kumar and associates^[446] demonstrated that oral clonidine (300 µg) provided effective anxiolysis for elderly patients undergoing ophthalmic surgery under local anesthesia and also decreased the incidence of intraoperative hypertension and tachycardia. Dexmedetomidine (1 µg/kg) significantly decreased anxiety levels and reduced the requirements for supplemental analgesic medications when given before intravenous regional anesthesia for hand surgery.^[447] In comparing dexmedetomidine with midazolam for sedation, Aho and colleagues^[448] reported faster recovery from sedation when dexmedetomidine was followed by reversal with the specific α₂ -antagonist atipamezole. However, administration of dexmedetomidine has been associated with bradycardia, which may limit its usefulness during MAC in the ambulatory setting.^{[114] [447] [448] [449]}