Monitored Anesthesia Care
Monitored anesthesia care does not describe the continuum of depth of sedation; rather, it describes a specific anesthesia service in which an anesthesiologist has been requested to participate in the care of a patient undergoing a diagnostic or therapeutic procedure.2
Monitored anesthesia care was defined by the ASA in 2008 as a specific anesthesia service for a diagnostic or therapeutic procedure.3
Indications for MAC include the nature of the procedure, the patient's clinical condition, or the potential need to convert to a general or regional anesthetic. MAC includes all aspects of anesthesia care: a preprocedure evaluation, intraprocedure care, and postprocedure anesthesia management. During MAC, the anesthesiologist provides or medically directs a number of specific services, including but not limited to:
- Diagnosis and treatment of clinical problems that occur during the procedure
- Support of vital functions
- Administration of sedatives, analgesics, hypnotics, anesthetic agents, or other medications as necessary for patient safety
- Psychologic support and physical comfort
- Provision of other medical services as needed to complete the procedure safely
Monitored anesthesia care may include varying levels of sedation (Table 69-1), analgesia, and anxiolysis as necessary. The provider of MAC must be prepared and qualified to convert to general anesthesia when necessary. For instance, if the patient loses consciousness and the ability to respond purposefully, anesthesia care must then convert to a general anesthetic, regardless of whether airway instrumentation is used or not.
Table 69-1 American Society of Anesthesiologists Definitions of General Anesthesia and Levels of Sedation and Analgesia ||Download (.pdf)
Table 69-1 American Society of Anesthesiologists Definitions of General Anesthesia and Levels of Sedation and Analgesia
|Minimal Sedation (Anxiolysis)||Moderate Sedation/Analgesia (Conscious Sedation)||Deep Sedation/Analgesia||General Anesthesia|
|Responsiveness||Normal response to verbal stimulation||Purposeful response to verbal or tactile stimulation||Purposeful response after repeated or painful stimulation||Unarousable even with painful stimulation|
|Airway||Unaffected||No intervention required||Intervention may be required||Intervention often required|
|Spontaneous ventilation||Unaffected||Adequate||May be inadequate||Frequently inadequate|
|Cardiovascular function||Unaffected||Usually maintained||Usually maintained||May be impaired|
Moderate Sedation (Conscious Sedation)
Moderate sedation is a drug-induced depression of consciousness during which patients respond purposefully to verbal commands, either alone or accompanied by light tactile stimulation. No interventions are required to maintain a patent airway, and spontaneous ventilation is adequate. Cardiovascular function is usually maintained.2
Deep sedation/analgesia is a drug-induced depression of consciousness during which patients cannot be easily aroused but respond purposefully after repeated or painful stimulation. The ability to independently maintain ventilatory function may be impaired. Patients may require assistance in maintaining a patent airway, and spontaneous ventilation may be inadequate. Cardiovascular function is usually maintained.2
General anesthesia is a drug-induced loss of consciousness during which patients are not arousable, even by painful stimulation. The ability to independently maintain ventilatory function is often impaired. Patients often require assistance in maintaining a patent airway, and positive-pressure ventilation may be required because of depressed spontaneous ventilation or drug-induced depression of neuromuscular function. Cardiovascular function may be impaired.2
Guidelines and Statements
The ASA issued a statement on granting privileges to a non-anesthesiologist practitioner for personally administering deep sedation or for supervising deep sedation by individuals who are not anesthesia professionals.4 It should be noted that because of the significant risk that patients who receive deep sedation may enter a state of general anesthesia, privileges to administer deep sedation should be granted only to practitioners who are qualified to administer general anesthesia or to appropriately supervised anesthesia professionals.
Another important ASA statement issued on granting privileges to a non-anesthesiologist practitioner for administering moderate sedation5 expressed genuine concern that individuals, however well intentioned, who are not anesthesia professionals may not recognize that sedation and general anesthesia lie on a continuum and thus deliver levels of sedation that in fact represent general anesthesia; however, they lack the training and experience to recognize this state and respond appropriately. Because of this concern, the ASA suggested a framework for granting privileges that would help ensure competence of individuals who administer or supervise the administration of moderate sedation. The aim of this statement is to assist health care organizations develop a program for granting privileges for providing moderate sedation.
Additional important points from the statement:
Only physicians, dentists, or pediatrists who are qualified by education, training, and licensure to administer moderate sedation should supervise the administration of moderate sedation. These practitioners should have completed formal training in the safe administration of sedative and analgesic drugs and in rescuing patients who exhibit adverse physiologic consequences of a deeper-than-intended level of sedation. Additional stipulations are that practitioners:
- Have the skills necessary for obtaining the patient's medical history and performing a physical examination to assess risks and comorbidities.
- Be able to assess the patient's risk for aspiration of gastric contents.
- Understand the pharmacology of all sedative and analgesic drugs the practitioner requests privileges to administer and their pharmacologic antagonists, vasoactive drugs, and antiarrhythmics.
- Understand the benefits and risks of supplemental oxygen.
- Be proficient in airway management with facemask and positive-pressure ventilation.
- Monitor physiologic variables.
- Document the drugs administered and at regular intervals monitor the patient's level of sedation and physiologic condition.
Because it is frequently not easy to predict the level of a patient's responsiveness to sedatives and because the sedation is a continuum, in which deeper levels of sedation might end up with a state of general anesthesia, it is recommended that the same NPO (nothing per os) guidelines for general anesthesia be followed in MAC cases. In urgent or emergent cases, the risk of pulmonary aspiration should be considered when determining the required level of sedation.
Preprocedure evaluation of the patient is crucial in determining the risk factors related to the patient's condition and also to anesthesia.
Some MAC cases are treated outside ORs; therefore, preprocedural evaluation of the place where the planned procedure will be performed is also of great importance. In many instances, patients undergoing interventional procedures are deemed to be at high risk for surgery because of multiple comorbid conditions or a specific life-threatening condition. Nevertheless, regardless of the anesthetic technique planned, these patients should receive the same level of attention given to patients being prepared for a surgical procedure in the main OR (MOR) pavilion. Focused history (major illnesses, medication allergies, NPO status, and previous anesthesia complications) and physical examination (mainly cardiovascular, respiratory, and airway) are mandatory. Relevant laboratory studies and other investigations (eg, electrocardiography [ECG], chest radiographs) should be reviewed. Patient counseling before the start of the procedure is vital to the success of the planned procedure and the anesthesia care. A patient who is well informed of the entire plan regarding MAC and who knows what to expect regarding the level of consciousness and awareness during the procedure is a more satisfied patient. Not all patients are candidates for MAC. The preprocedure visit is also important to determine candidacy for MAC.
Candidacy for Monitored Anesthesia Care
Procedures performed under MAC primarily depend on the patient's cooperation and motivation, as well as on the nature of the procedure being performed.
Mild to moderate sedation can actually disinhibit a patient's response to painful stimulation; therefore, if adequate local or regional anesthesia is unavailable for a patient who must undergo a painful procedure, MAC cannot be implemented successfully.
Although MAC carries no absolute contraindications, it may be unsuitable for the following patients and under the following conditions:
- Pediatric patients
- Patients without full mental capacity
- Intoxicated patients
- Patients with a condition that inhibits them from lying still for the period of time needed for the procedure
- Language barrier between the patient and the provider
- Psychotic or uncooperative patients
- Medically unstable patient, such as a patient with congestive heart failure, who is scheduled for an emergency procedure that requires supine position and the patient has orthopnea and cannot lie supine, although the procedure requires a supine position
- Patient with a suspected or known difficult airway; for instance, the airway may be difficult to monitor because of the position required or the nature of the procedure
- Excessively long procedures
- Procedures performed in an uncomfortable position. (eg, prone, lithotomy, kidney rest, and kneeling positions)
- Procedures in which a large volume blood loss or cardiorespiratory instability is expected
- Procedures in which even minor movement could be hazardous to the patient.
Standards for intraoperative monitoring of cases performed under MAC should be identical to those performed under general or regional anesthesia. These standards would naturally extend to providing anesthesia in settings outside the ORs as well.
Monitoring Depth of Sedation during Monitored Anesthesia Care
Accurate assessment of sedation depth is important in minimizing the risks of MAC and procedural sedation both inside and outside the OR. Many patients, particularly the elderly and very ill patients, may rapidly move from a plane of light sedation to obtundation. Practitioners should have the means to effectively monitor depth of sedation. Because the direct effect of sedatives and hypnotics on the brain cannot be measured, clinicians usually rely on indirect measures of the level of sedation, such as frequent patient stimulation (eg, Observer's Assessment of Alertness/Sedation Scale (OAA/S), Table 69-2) to measure the depth of sedation. These techniques, however, require persistent patient cooperation and are subject to testing fatigue. Additionally, there are procedures and cases during which periodical patient movement and speech might preclude the successful completion of the procedure. Also, clinicians traditionally have relied on subjective measures or autonomic signs of patient responsiveness to judge depth of sedation and analgesia. However, changes in autonomic signs (eg, hypertension and tachycardia) do not reliably predict awareness and discomfort during general anesthesia.6-8
Table 69-2 Observer's Assessment of Alertness and Sedation Scale ||Download (.pdf)
Table 69-2 Observer's Assessment of Alertness and Sedation Scale
|Responsiveness||Speech||Facial Expression||Eyes||Composite Score Level|
|Responds readily to name spoken in normal tone||Normal||Normal||Clear, no ptosis||5|
|Lethargic response to name spoken in normal tone||Mild slowing or thickening||Mild relaxation||Glazed or mild ptosis (less than half the eye)||4|
|Responds only after name is called loudly or repeatedly||Slurring or prominent slowing||Marked relaxation||Glazed and marked ptosis (half the eye or more)||3|
|Responds only after mild prodding or shaking||Few recognizable words||2|
|Does not respond to mild prodding or shaking||1|
An objective measure of sedation depth theoretically could decrease the incidence of patients being undersedated and oversedated, reduce anesthetic wastage, and shorten recovery and discharge times. Because the brain is the target of anesthetic action, and electrical activity of the cerebral cortex can be measured via electroencephalography (EEG), most depth-of-anesthesia monitors have focused on measuring changes in the EEG. The Bispectral Index Scale (BIS) was the first clinically available depth-of-anesthesia monitoring device. The BIS is a proprietary algorithm (Aspect Medical Systems, Newton, MA) that generates a linear dimensionless number ranging from 0 to 100 that decreases in proportion to increased anesthetic depth. The BIS does not correlate with movement, heart rate, or blood pressure. Although the BIS has been used more commonly during general anesthesia, it has been evaluated measuring the depth of sedation with propofol,9,10 midazolam,8 and even sevoflurane.11 During propofol-induced sedation, the BIS was shown to correlate with OAA/S scores,10,12 loss of response to verbal commands,9 suppression of learning,13 and propofol blood concentrations.13 Gan and colleagues14 demonstrated faster recovery times and decreased propofol usage with the addition of the BIS monitor, although formal cost-effectiveness studies during MAC or regional anesthesia have not been undertaken. Furthermore, BIS monitoring accurately and objectively measures patient sedation level during endoscopy15 and during procedural sedation in the emergency department (ED).16 In contrast to the targeted BIS readings for patients undergoing general anesthesia (BIS values 40-60), BIS readings of 60 to 80 are targeted during MAC. BIS values approaching 60 are associated with a low probability of recall.9,10,17 However, recall is impaired at much higher BIS values than is response to command.18 It is important to realize that BIS measurements are slightly dependent on the type of anesthetic agents used. Several authors have shown higher BIS values for loss of consciousness when opioids were added to an anesthetic.11,18,19 This phenomenon, which extends to other agents (eg, ketamine, nitrous oxide), exists because noncortical structures underrepresented in the EEG contribute to the mechanism of hypnosis and sedation with opioids.20 It should be noted that ketamine can cause loss of response to verbal command in doses as low as 0.25 mg/kg without altering BIS measurements.21 It also has been suggested that spinal and epidural anesthesia may affect BIS measurements independently of other anesthetics.22,23 Morley and colleagues22 demonstrated a small but detectable EEG suppression in patients receiving spinal anesthesia without concomitant intravenous (IV) sedation. The mechanism appears to be related to decreased afferent stimulation of the reticular activating system and is independent of block level.
A priori, the authors believe there is a selective subset of patients undergoing MAC in whom BIS monitoring is a valuable adjunctive clinical monitor to standard clinical evaluation. BIS monitoring would probably be particularly beneficial during procedures in which patients should not move or speak, procedures of prolonged duration, and procedures involving patients of advanced age or ill health. Whenever possible, patients should be monitored via behavioral methods (eg, continual assessment of response to commands) in addition to the BIS. Because both the general public and many non-anesthesiologists have difficulty distinguishing between deep levels of sedation and general anesthesia, it is advisable to discuss patient concerns about "awareness" or "being awake" during the patient's preanesthetic assessment and interview. Patients insisting on guaranteed amnesia and hypnosis for procedures under MAC should be offered general anesthesia if feasible.