In this chapter, we will discuss the following anesthetic techniques: general anesthesia, monitored anesthesia care (MAC), and local anesthesia, specifically tailored to ambulatory surgery.
The emphasis will be on the goals of the ideal ambulatory anesthetic, quick recovery with minimal adverse effects, and short times to discharge readiness. (Also see Chapter 7.)
The techniques and agents discussed in this chapter will therefore focus on short-acting agents, agents with minimal or easily treatable side effects, and the use of a combination of agents (multimodal) to minimize adverse effects and maximize therapeutic effects.
General anesthesia is a medically induced reversible coma accomplished by administering a combination of anesthetic drugs and agents for achieving amnesia, analgesia, hypnosis, and skeletal muscle relaxation. General anesthesia consists of the following phases.
This could be inhalational or intravenous.
Inhalational induction may be achieved with a mixture of oxygen/nitrous oxide (30/70) with added sevoflurane, all administered via a face mask to the patient until loss of consciousness. The airway and ventilation are supported as needed. Sevoflurane is non-irritating and has a low solubility which makes it the anesthetic of choice for inhalational induction. One option is to start with a nitrous oxide induction, wait for 1–2 min and then introduce sevoflurane 8%. Another option is to start both simultaneously, or eventually sevoflurane 8% may be used as the only agent. There are no major documented benefits or drawbacks to any of these options, and the choice of one may depend on local tradition and experience. Inhalational induction is typically used in infants and children, or in the needle-phobic adult who will not allow the placement of an intravenous line preoperatively. After loss of consciousness and placement of an IV, the airway device of choice is inserted. At this point, the patient may be given IV propofol (1 mg/kg) to achieve an adequate depth of anesthetic for insertion of a laryngeal mask airway (LMA).
The advantages of inhalational induction are ease of administration and widespread acceptance in the pediatric population. Disadvantages include unpleasant odor, claustrophobia, and prolonged induction in larger and older patients with the potential for complications such as breath-holding or laryngospasm and increased incidence of PONV. Therefore, inhalational induction is reserved for patients in whom obtaining preoperative IV access would be difficult or unduly traumatic. Some short procedures (such as myringotomies) may not require IV access or airway instrumentation, and can be completed with inhalational induction and maintenance with a facemask.
In most patients, IV access is established preoperatively. For ambulatory procedures, propofol is the induction agent of choice due to its speed of onset, adequate depth for LMA insertion, and favorable recovery profile with decreased PONV and very low emergence delirium rates. It is administered in the dose of 1.5–2 mg/kg and usually combined with an analgesic (short-acting opioid) for induction. Some will also use a small dose of midazolam as a part of the induction, but this has no documented benefit in the stable, elective ambulatory patient. (Please also see Chapter 7 for details on premedication.)
After induction, the airway device of choice is inserted (LMA or ETT, see Chapter 7). Maintenance of anesthesia is achieved by inhalational agents or intravenous agents or a combination of the two. In either technique of maintenance, other IV adjuncts are used, primarily opioids, muscle relaxants, and analgesics to supplement the IV or inhalational agents.
Maintenance of anesthesia is by inhalation of a mixture of air, oxygen, and sevoflurane or desflurane (at 1 MAC concentration). The use of nitrous oxide is controversial. The advantages of using nitrous oxide are that it is non-irritating, reduces the amount of halogenated agent required for maintenance, and due to reduced solubility, allows for quicker emergence. The disadvantages to using nitrous oxide are an increased incidence of PONV and relative contraindications in patients undergoing certain surgeries (tympanomastoidectomy) or coexisting medical illnesses such as pulmonary hypertension or anemia (secondary to vitamin B12 cyanocobalmin deficiency).
For a healthy individual undergoing intermediate extensive surgery, who is at a low risk for PONV, nitrous oxide could be a useful adjunct in the maintenance of general anesthesia.
IV induction with propofol is followed by maintenance of GA using either air–oxygen with inhaled agent (sevoflurane or desflurane) or with nitrous oxide–oxygen with an inhaled agent at half a MAC plus a propofol infusion at 50–100 µg/kg/min. The advantage of a combination of using half a MAC of each agent is one of smooth onset during induction, PONV prophylaxis, and a smooth emergence.
IV induction is achieved with propofol 1.5–2 mg/kg followed by maintenance of GA with air–oxygen and an IV propofol infusion at 150–200 µg/kg/min for the first 10–15 min, and then titrated to surgical anesthesia, often in the range of 100–150 µg/kg/min. This technique of Total Intravenous Anesthesia (TIVA) completely avoids inhalational anesthetics and is useful in patients at high risk for postoperative nausea and vomiting (PONV) or patients who are susceptible to malignant hyperthermia (MH). TIVA is also very practical in areas or rooms not equipped with a full inhalational setup, anesthetic machine, vaporizer or scavenger.
TCI refers to target controlled infusion, where a microprocessor-controlled syringe pump controls and varies automatically the rate of infusion of a drug, to attain a target level (set by the operator) in the patient’s blood (see also Chapter 4 on Pharmacology). These devices are so far not FDA-approved in the United States.
The LMA is the mainstay of airway devices commonly used for GA and is discussed in detail in Chapter 7. There are several other supraglottic airway devices available that could be used as alternatives. Keep in mind that the LMA is a supraglottic airway device and therefore does not completely isolate the upper airway tract. An endotracheal tube is preferred in patients where there is a potential risk for aspiration, such as patients with severe untreated/symptomatic gastroesophageal reflux disease, massive trauma, pregnancy, and other conditions associated with delayed gastric emptying. However, some of these concerns regarding gastric fluid and reflux may be resolved with an LMA equipped with a gastric tube line (e.g. LMA-Proseal®). An endotracheal tube may otherwise be needed in case of consistent inspiratory leakage with the LMA, as well as in cases with high airway pressure and/or resistance, such as in some morbidly obese patients.
The patient may be allowed to breathe spontaneously or controlled ventilation may be used depending on the requirement of the procedure. Both spontaneous ventilation and controlled ventilation are possible with an LMA, depending on the requirements of the procedure. Newer modes of ventilation available on current anesthesia machines include Pressure Support mode where the patient initiates the breath but the tidal volume is increased by additional positive pressure provided by the ventilator. The PRVC™ mode of ventilation (Pressure Regulated Volume Control), called other names by the machine manufacturers, ensures an adequate (preset) tidal volume while staying within preset pressure parameters and adapts to varying chest and abdominal wall compliance of the patient.
For procedures requiring skeletal muscle relaxation to improve surgical exposure, or procedures requiring isolation of the upper respiratory tract, endotracheal intubation is the preferred technique of airway management. However, if appropriate ventilation and a good LMA seal are present, there is no contraindication for using LMA even during moderate muscle relaxation.
Endotracheal intubation may be facilitated by muscle relaxation, but it is not always required. The choice of muscle relaxant, if used, depends on the type and duration of the procedure. For a short procedure, where continuous muscle relaxation is not required, the lowest effective dose of a short-acting agent should be used. The degree of muscle relaxation and adequacy of reversal should be monitored with a neuromuscular monitor that records muscle response to a peripheral nerve using acceleromyography. Muscle relaxation should be completely reversed using neostigmine at the end of the procedure, before extubation. If the extent of spontaneous recovery of muscle function is greater than 90%, neostigmine need not be given. Given the mixed evidence of the effect of neostigmine on PONV, reversal of residual skeletal muscle paralysis using neostigmine should take precedence over the potential risk of PONV associated with its use.
Endotracheal intubation may also be achieved with a combination of propofol induction and high-dose remifentanil without paralysis. Usually, a remifentanil dose of 3–4 µg/kg given within 1–3 min (bolus or infusion) will render the vocal cords fairly relaxed and non-responsive for endotracheal intubation. This method works well, although there have been rare reports of mucus membrane damage in healthy patients, and it may also result in bradycardia and significant hypotension, the latter being move common in an elderly patient.
As the surgical procedure approaches an end, the agents used for maintenance of the anesthetic are decreased in dose and then discontinued. Neuromuscular blockade is reversed with appropriate doses of neostigmine and glycopyrrolate as described above. Sugammadex is theoretically a better alternative of reversing rocuronium or vecuronium (see page 50, Chapter 4), but the high cost of this drug should be weighed against very marginal everyday clinical benefits. The patient is allowed to breathe spontaneously. The airway device (LMA or ETT) is removed once ventilatory parameters are satisfactory and protective airway reflexes have returned.
MAC refers to a type of anesthesia service rather than an anesthetic technique. MAC is a service where an anesthesiologist has been requested to participate in the care of a patient undergoing a procedure. MAC does not refer to the depth of anesthesia required for safe and successful completion of the procedure. Patients receiving MAC, depending on the type of procedure and depth of stimulation, may be at different points along the continuum of depth of sedation. This continuum has been defined by the ASA Committee on Quality Management and Departmental Administration.
Patients receiving minimal sedation have usually received an anxiolytic. They are able to respond appropriately to verbal commands. They may have mild impairment of cognition and coordination but airway reflexes, respiratory, and cardiovascular functions are unaffected. Examples include a patient undergoing surgery for cataract extraction who has received intravenous midazolam.