The goal of outpatient anesthesia is to provide a rapid, smooth induction of anesthesia, stable hemodynamics with good operating conditions, intraoperative amnesia, and prompt awakening at the conclusion of the surgical procedure. The patients should experience minimal side effects and have a low rate of unanticipated hospital admission. In addition, the time until the patient is judged ready for discharge should be short. This article will review some of the newer anesthetic agents, techniques, monitors, and adjuvant drugs that are enabling us to closely approach these goals.
Administration of an Inhalation Anesthetic is a routine part of general anesthesia for both inpatient and outpatient surgery. Isoflurane is the most commonly used inhalation agent, although halothane is used frequently in children for inhalation induction. Desflurane® and Sevoflurane®, two recently introduced inhalation agents, are especially useful for outpatient anesthesia. They are highly fluorinated, which results in a lower blood solubility than isoflurane or halothane. This leads to faster elimination from the body and quicker recovery from anesthesia.1 Desflurane has a solubility similar to that of nitrous oxide, while sevoflurane is slightly more soluble.
Another result of the lower solubility of desflurane and sevoflurane is that of a more rapid rate of rise of the alveolar concentration toward the inspired concentration. This allows a greater degree of control over the depth of anesthesia. At any given fresh gas flow rate the less soluble anesthetic will have a lower uptake and will therefore be controlled more precisely. When using an agent monitor it will be seen that the difference between the inspired and expired concentration of the agent will be less with the anesthetic that has a lower solubility.2 The lack of airway irritation and low solubility of sevoflurane results in a rapid inhalation induction. It has a low incidence of respiratory complications and provides a quality of induction judged to be similar to or better than that of halothane.3 Desflurane produces airway irritation and is not suitable for inhalation induction.
Sevoflurane and desflurane have not replaced isoflurane because they are expensive. The cost of sevoflurane is increased by the use of the relatively high fresh gas flows of 2 liters/min or greater. These flows are recommended because sevoflurane is degraded by the strong bases in the carbon dioxide adsorbents.1 The breakdown product, compound A, has been shown cause dose dependent nephrotoxicity in rats. Some studies in humans have shown transient elevations in urinary protein, glucose, and renal tubular enzyme excretion,2,4 while others have not.5 ,6 The inconsistent and transient nature of these findings combined with the fact that sevoflurane has not been reported to cause an elevation in blood urea nitrogen or serum creatinine concentration in humans, is evidence that it is not clinically significantly nephrotoxic in surgical patients.
Propofol® is a sedative-hypnotic that was approved by the Food and Drug Administration in 1989. Because it has many desirable characteristics, it has become the drug of choice for induction of anesthesia for outpatient surgery. After a bolus induction dose of propofol there is a rapid decline in plasma levels. This is due to redistribution of propofol from the brain into areas less well perfused, such as muscle. Propofol also has a high metabolic clearance rate, approximately ten times greater than that of thiopental.8 The resultant rapid recovery may lead to an earlier discharge after short outpatient procedures when propofol is used instead of thiopental for induction of anesthesia.
Propofol is also a valuable drug for monitored anesthesia care (MAC). This type of anesthesia care often is used for minor diagnostic procedures or operations in which analgesia is provided by local anesthetics. The patient is monitored for their safety and medications are given to provide sedation, amnesia, and anxiolysis. Propofol may be administered as a variable rate infusion in combination with a small dose of midazolam. This produces reliable amnesia and an easily controlled level of sedation. The patient rapidly regains consciousness after discontinuation of the propofol infusion and may be discharged with minimal residual sedation.
Compared to thiopental, patients that receive propofol for induction of anesthesia experience less postoperative nausea and vomiting. Propofol has been shown to have significant antiemetic properties.9 Propofol, is effective for treatment of postoperative nausea or vomiting. A dose of 10-20 mg was effective in 81% of patients compared to a 35% success rate with placebo. The relapse rate after successful treatment with propofol was unfortunately high (29%), and was similar to the relapse rate after placebo (22%). This is due to the rapid plasma clearance of propofol.
Most patients experience pain during the injection of propofol. This pain can be reduced by the prior administration of fentanyl 100 mcg or by adding lidocaine (40 mg) to 200 mg. of propofol. The lidocaine and propofol should be used within 30 minutes of mixture.8
The laryngeal mask airway (LMA) is a new airway management
device that was invented by Dr. Archie Brain at the London
Hospital, Whitechapel in 1981.10, 11 In the United
States the LMA was approved by the Food and Drug Administration
in August, 1991 and was introduced into clinical practice in late
1992. It is a valuable and important device for airway
management, and is particularly useful for outpatient anesthesia.
The LMA is entirely latex free and is made of soft silicone
rubber that can be sterilized in an autoclave. The manufacturer
recommends that the LMA can be re-used up to 40 times, thus
making it cost effective despite a purchase price of
approximately $250. The LMA has an airway tube that connects to
an elliptical mask with a cuff (Figure 1). When the cuff is
inflated the mask conforms to the hypopharynx with the lumen
facing the glottic opening (Figure 2). After correct insertion
the tip of the LMA is at the base of the hypopharynx against the
upper esophageal sphincter (Figure 3). Since it may act as a
funnel to direct regurgitated fluid into the trachea its use is
not recommended in patients at risk of regurgitation (e.g. full
stomach, pregnancy, hiatal hernia, morbid obesity, intestinal
obstruction). The transparent shaft of the LMA makes it possible
to rapidly detect any aspiration that may occur.
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| Figure 1. The components of the Laryngeal Mask Airway (LMA). Photograph courtesy of LMA North America, Inc. | Figure 2. Dorsal view of the LMA showing position to pharyngeal anatomy. Photograph courtesy of LMA North America, Inc. | Figure 3. Inflation without holding the tube allows the mask to seat itself optimally. Photograph courtesy of LMA North America, Inc. |
The LMA has been shown to protect the trachea from blood and secretions during ENT surgery.12 It may be used as an alternative to tracheal intubation in tonsillectomies, septoplasties, and endoscopic sinus surgery. A study by Ruby et al13 compared children having elective tonsillectomy and adenoidectomy with an LMA to a group in which a standard RAE endotracheal tube was used. They reported that in the LMA group there was a decreased incidence of aspiration of blood, fewer episodes of laryngospasm, less postoperative coughing, and less total blood loss. A flexible LMA with a wire reinforced tube that resists kinking and can be positioned to the side without displacing the cuff is used for these procedures.
One advantage of the LMA is its ease of insertion. Laryngoscopy is not required for insertion which minimizes the stress response that occurs during laryngoscopy. The administration of muscle relaxants is not necessary for placement of the LMA. This avoids the use of succinylcholine, which can cause postoperative myalgias. These myalgias can be particularly problematic in outpatient surgery with its attendant early ambulation. Avoidance of laryngoscopy also reduces the risk of airway trauma and damage to the lips, teeth, gums or pharynx. This leads to a lower frequency of sore throat with the LMA of approximately 10%, compared to 47% of patients who have endotracheal intubation.11 It is also known that tracheal intubation may cause changes in the epithelial lining of the vocal cords which can result in voice changes. Thus the LMA may be advantageous in patients who use their voice professionally.
The LMA is well tolerated at the light levels of anesthesia at which a patient may cough or move in response to an endotracheal tube. This allows a reduction in the amount of anesthesia administered at the end of the procedure for skin closure or cast application. This often leads to a more rapid awakening than can be achieved when using an endotracheal tube. The rate of recovery from anesthesia has been shown to be an important determinant of patient satisfaction with outpatient surgery.14 Faster awakening will also result in cost savings from more efficient utilization of the operating room.
The LMA can be very valuable in patients with a difficult airway. In the anesthetized patient that is difficult to intubate and ventilate, the LMA can be placed successfully in most cases. This quickly establishes an airway for oxygenation and ventilation.15, 16 After ventilation is established the LMA can be used as a conduit for tracheal intubation. The distal opening of the correctly placed LMA is directly across from the laryngeal inlet. This allows an endotracheal tube passed blindly down the LMA to enter the trachea. A well lubricated 6.0mm endotracheal tube will pass through a size 3 or 4 LMA and a 7.0mm endotracheal tube will pass through the shaft of a size 5 LMA. Reported success rates for blind intubation through the LMA vary between 30-93%.17 If a fiberoptic bronchoscope is available and used for intubation through the LMA a success rate greater than 95% can be achieved. A new intubating LMA (ILMA), which was designed to have better insertion and intubation characteristics than the standard LMA,18 has recently become available. It is packaged with silicone, wire reinforced, cuffed endotracheal tube that facilitates blind intubation through the ILMA.18 In a study of 150 patients, Brain et al17 successfully used the ILMA to blindly intubate 69% of the patients on the first or second attempt, and had an overall success rate of 99.3%.
Nausea and vomiting continue to be a major source of morbidity after outpatient anesthesia, and may lead to delayed discharge or unplanned hospital admission. Effective prevention and treatment of nausea and vomiting will not only reduce the cost of outpatient surgery, but increase patient satisfaction as well.
The complex act of vomiting is controlled by the emetic center. It receives neural input from several areas of the body, including the pharynx, gastrointestinal tract, mediastinum, higher cortical centers (visual center and the vestibular portion of the eighth cranial nerve), and the chemoreceptor trigger zone (CTZ). Receptors for serotonin, histamine, acetylcholine (muscarinic), and dopamine are found in the CTZ. Most of the antiemetic drugs affect one or more of these receptors.
Patient factors associated with an increased risk of postoperative emesis include age, female gender (highest incidence on 4th and 5th days of menstrual cycle), obesity, previous history of motion sickness or postoperative vomiting, anxiety, gastroparesis, duration of anesthesia, and type of procedure (e.g. laparoscopy, strabismus surgery, tonsillectomy, middle ear procedures).19 Although these factors often are beyond the anesthesiologists control, there are aspects of the anesthetic technique which can influence the incidence of postoperative vomiting. Many studies have shown that propofol instead of thiopental for induction of anesthesia results in a lower incidence of emesis after surgery. Intravenous administration of ketorolac intraoperatively will reduce the amount of narcotic required for postoperative analgesia and thereby decrease narcotic induced vomiting. Neuromuscular blocking drugs often are used in outpatient anesthesia. The choice of one of the new short duration(mivacurium®) or intermediate duration (cis-atracurium,® rocuronium®) non-depolarizing muscle relaxants may allow the patient to have fully recovered from neuromuscular blockade by the end of the procedure. This eliminates the need for anticholinestase drugs to reverse residual neuromuscular blockade. The muscarinic effects of these drugs may increase the incidence of postoperative vomiting.19
Although the incidence of postoperative nausea and vomiting has decreased with the utilization of some of the above techniques, it continues to occur. There are numerous drugs available to prevent or treat postoperative nausea and vomiting. These include droperidol and metoclopramide which block dopaminergic receptors at the CTZ, scopolamine which blocks acetylcholine receptors, hydroxyzine which is a histamine receptor antagonist, and promethazine which has both acetycholine and histamine receptor blocking action. Unfortunately these drugs can have undesirable side effects such as excessive sedation, confusion, dysphoria, hallucinations, and extrapyramidal reactions.
The newest class of antiemetic agents are the serotonin antagonists. Ondansetron was the first of this class to become available and is the most widely studied. It has been shown to be effective in the treatment and prevention of postoperative nausea and vomiting and is especially useful in patients with a history of nausea and vomiting after previous anesthetics. Ondansetron® is most effective when given immediately before the end of surgery.20 Ondansetron does not cause sedation and is not associated with significant side effects. The serotonin antagonists are relatively costly. Most experts recommend the prophylactic use of these agents only in patients at high risk for postoperative nausea and vomiting.
The goal of rapid awakening after anesthesia is facilitated by minimizing the amount of anesthetic agents administered to the patient. A reduction in the quantity of anesthetic agent given will also reduce costs. While these goals are desirable, it is essential to give the patient enough anesthetic medications to ensure amnesia. Awareness under general anesthesia is rare, but continues to occur.
Stability of blood pressure and heart rate are an indication of lack of awareness, but do not guarantee it. Lack of patient movement in response to surgical stimulation is considered a reliable guide of the patient not being aware, but this cannot be used as a monitor of awareness if muscle relaxants have been administered. It is also known that the amount of anesthesia required to prevent movement in response to skin incision is greater than that required to provide amnesia. In addition, a patient may move without being aware, since movement can be mediated by spinal cord mechanisms. To overcome the limitations of monitoring awareness by observing hemodynamic variables or patient movement a direct method of evaluating level of consciousness is needed.
The bispectral index (BIS, Aspect Medical Systems, Natick, MA) is a variable derived from the EEG that has been reported to correlate with the hypnotic/sedation level.21 In bispectral analysis a signal processing technique is used that measures the harmonic and phase relations (interfrequency coupling) of various frequencies in the electroencephalogram to yield the bispectral index.22 BIS has been shown to be a reliable indicator to prevent awareness and facilitate rapid emergence from anesthesia.23- 27
The decreased utilization of anesthetic agents and faster awakening obtained with BIS monitoring has been shown to result in cost savings. In a study by Glass et al28 the use of BIS monitoring for propofol titration resulted in an average savings of $13.77 per case in drug costs alone. Johansen and Sigl29 reported a significantly shorter time to extubation, time to exiting the operating room, and total recovery room time. They calculated potentially significant indirect-cost savings of $80 per patient. The potential cost savings and the fact that BIS monitoring is the only available direct measure of a patients level of consciousness combine to give it the potential of being a very useful clinical monitor. The major limitation of its widespread use is the high capital cost of this technology.
Advances in outpatient anesthesia over the last decade allow patients to awaken faster and be discharged more quickly after outpatient surgery. They experience less postoperative emesis and when nausea occurs, it can be treated with effective antiemetic agents. The LMA, an airway management device often used as an alternative to an endotracheal tube, is useful in patients that are difficult to intubate. Bispectral index (BIS) monitoring can measure the patients level of consciousness and reliably assess the patients risk of awareness.
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