The DA is defined as “the clinical situation in which a conventionally trained anesthesiologist experiences difficulty with mask ventilation, difficulty with endotracheal intubation, or both.”^6,⁷ The American Society of Anesthesiologists (ASA) Closed Claims Study in 1990 found that 34% of injury cases in the Closed Claims database involved respiratory events.⁸ The ASA formed the Task Force on Management of the Difficult Airway (TFMDA) in 1992 and sought to establish guidelines to facilitate the management of the DA and reduce the likelihood of adverse outcomes.⁶ The task force constructed an algorithm to assist the anesthesiologist in devising a strategy for management of the DA (see Chapter 10). In this algorithm, one of the primary management choices for the anesthesiologist is the decision whether to perform AI or to attempt intubation after induction of general anesthesia.⁶ In 2002, the practice guidelines were updated and the choice of AI remained an important component of the recommendations.⁷

C Indications for Awake Intubation

The ASA algorithm stresses the concept that formulation of a strategy for intubation should include the feasibility of three basic options: (1) AI versus intubation after induction of general anesthesia, (2) noninvasive versus invasive (surgical) techniques, and (3) preservation versus ablation of spontaneous ventilation.⁷ It is the opinion of most of the consultants of the TFMDA, and has been expressed in the literature,^2,^3,^5–7,9–11 that the safest method for a patient who requires endotracheal intubation and has a DA is for that patient to undergo AI for the following reasons:

1. Patency of the airway is maintained by upper pharyngeal muscle tone.

2. Spontaneous ventilation is maintained.

3. A patient who is awake and well topicalized is easier to intubate, because the larynx moves to a more anterior position after induction of anesthesia compared with the awake state.

4. The patient can still protect his or her airway from aspiration.

5. The patient is able to monitor his or her own neurologic symptoms (e.g., the patient with potential cervical pathology).^2,^5,⁹

General indications for AI are compiled in Box 11-1. There are no absolute contraindications to AI other than patient refusal, a patient who is unable to cooperate (such as a child, a mentally retarded patient, or an intoxicated, combative patient), or a patient with a documented true allergy to all local anesthetics.⁹

Box 11-1

Indications for Awake Intubation

From Kopman AF, Wollman SB, Ross K, et al: Awake endotracheal intubation: A review of 267 cases. Anesth Analg 54:323–327, 1975; Thomas JL: Awake intubation: Indications, techniques and a review of 25 patients. Anaesthesia 24:28–35, 1969; Bailenson G, Turbin J, Berman R: Awake intubation: Indications and technique. Anesth Prog 14:272–278, 1967.

II the Preoperative Visit

In this section, the focus is placed on elective patients, for whom there is time for airway evaluation and meaningful communication. In the setting of an emergency—which in itself should increase the probability of airway difficulty, especially with a patient in extremis ⁹—the physician may not have time, nor can he or she be expected to be able to perform the detailed investigation of the airway described here.

A Reviewing Old Charts

Whenever possible, previous anesthesia records should be examined because they may provide useful information.^7,¹² Obviously, the most important records are those involving intubation, especially the most recent ones. Other records documenting ease of mask ventilation and tolerance of drugs are also valuable. One should be alert for evidence of reactions to local anesthetics and of apnea with minimal doses of opioids. Another reason for checking as many anesthesia records as possible, including the surgical procedure involved, is that the last intubation may have been routine but the three previous ones may have been difficult, or the last intubation may have been routine but the operation then performed may have rendered the airway difficult.

When reading through a chart, one should focus on four important features:

1. Degree of difficulty of the endotracheal intubation (the difficulty encountered and the method used)

2. Positioning of the patient during laryngoscopy (sniffing position, use of a ramp)

3. Equipment used (even if the intubation was performed routinely in one attempt, a Bullard blade or a fiberoptic bronchoscope [FOB], neither of which requires alignment of the three axes, may have been used)

4. Whether the technique that was used previously is a familiar one (one should not attempt to learn a new technique on a DA)

After the medical records have been reviewed, the preoperative interview should address the possibility of events that may have occurred since the last anesthesia, such as weight gain, laryngeal stenosis from previous airway intervention, facial cosmetic surgery (e.g., chin implants), worsening temporomandibular joint disorder, or worsening rheumatoid arthritis.

B The Patient Interview

Dorland’s Medical Dictionary defines “empathy” as the intellectual and emotional awareness and understanding of another person’s thoughts, feelings, and behaviors, even those that are distressing and disturbing. Although the anesthesiologist may participate in 1000 operations per year, few patients undergo more than five in a lifetime.¹³ The patient’s perception of empathy from the physician is the cornerstone of the patient’s acceptance of an AI. Empathy helps the interviewer establish effective communication, which is important for accurate diagnosis and management.¹⁴ Two facets of medical education limit the clinician’s development of empathy: the traditional format of interviewing training and the social ethos of medical training and medical practice, which stresses clinical detachment.^14,¹⁵ With empathy and the ability to communicate it, the physician can perform the interview in a more patient-oriented rather than disease-oriented fashion, resulting in better data gathering and better patient compliance.¹⁶

After the anesthesia practitioner has made the decision that AI is necessary, communication with the patient and psychological preparation is of the utmost importance to maximize the odds for a successful AI.⁹ One should in a careful, unhurried manner describe to the patient conventional intubation contrasted with AI. Focusing on the fact that the former is easier and less time-consuming but that the latter is safer in light of the patient’s own anatomy or condition, one must communicate to the patient that the knowledgeable, caring physician is willing to take extra measures to ensure the patient’s safety. Recommendations should be presented to the patient with conviction but at the same time allowing the patient the option of the conventional method of intubation as a last resort.¹⁰

Complications of AI should also be presented, including local anesthetic toxicity, airway trauma, discomfort, recall, and failure to secure the airway. Patients’ recall after AI using different methods of sedation, analgesia, or local anesthetics has not been studied in a controlled fashion. Although episodes of explicit awareness during general anesthesia are rare, the incidence of recall during AI with minimal levels of sedation is likely to be higher.¹³ In a review of 443 cases of AI (four studies) in which various combinations of sedation and analgesia were used (Table 11-1), a mean of 17% of the patients had partial recall and 6% had recall with unpleasant memories.^2,^5,^17,¹⁸

TABLE 11-1 Incidence of Recall among Patients Undergoing Awake Intubation (AI)

If a patient refuses AI, the anesthesiologist may discuss the case with the primary care physician or the surgeon, or both, in order to recruit their help to convince the patient of the importance of cooperation. If this and subsequent discussion with the patient are unsuccessful, the anesthesiologist should document these efforts in the chart.

III Preoperative Preparations

The preparation of the patient for AI begins, as discussed, with the patient interview, followed by appropriate premedication (discussed later). Additional preparation for AI includes assembling the necessary equipment and arranging in advance for needed assistance.

A Transport

The acuteness and urgency of the case must be considered when arranging transport to the operating room (OR). In some cases, the airway needs to be secured at once (e.g., the patient in extremis who warrants a bedside emergency airway procedure). In other cases, there is an urgent need for intubation but with sufficient time to transport the patient to the OR with supplemental oxygen (O₂) and appropriate monitors (electrocardiogram [ECG], pulse oximeter, and noninvasive blood pressure monitoring), accompanied by the anesthesiologist, surgeon, or both. Finally, there may be no requirement for immediate attention, and the patient can be transported routinely to the OR. In the elective scenario, supplemental O₂ should be provided, if appropriate (high-dose O₂ may be detrimental in some patients, such as those who rely on hypoxic respiratory drive),¹³ and position should be considered (e.g., the patient who is morbidly obese may experience dramatic physiologic changes when supine and should be transported in a wheelchair or on a gurney in a semirecumbent position.).^19,²⁰

B Staff

According to the TFMDA guidelines, there should be “at least one additional individual who is immediately available to serve as an assistant in difficult airway management.”⁷ Whenever possible, it is preferable to have a second member of the anesthesia care team who can assist in the monitoring, ventilation, and pharmacotherapy of the patient, as well as providing an extra set of hands during fiberoptic intubation (FOI). For patients in extremis and those who refuse AI, a surgeon trained in performing a surgical airway should be available with a tracheostomy/cricothyrotomy tray, ready to perform an emergency surgical airway, if necessary.

C Monitors

During AI, the routine use of ECG, pulse oximetry, noninvasive blood pressure monitoring, and capnography is required as part of standard basic intraoperative monitoring. Depending on the complexity of the surgery and the patient’s condition, invasive hemodynamic monitoring (i.e., arterial line) may need to be placed before AI. Indications for this procedure include hemodynamic instability, severe ischemic or valvular heart disease, and a patient for whom hypertension and tachycardia are potentially dangerous (e.g., the patient with aortic dissection or intracerebral aneurysm).

D Supplemental Oxygen

Administration of supplemental O₂ should be considered throughout the process of DA management.⁷ Arterial hypoxemia has been well documented during bronchoscopy, with an average decrease in arterial oxygen tension (PaO₂) of 20 to 30 mm Hg in patients breathing room air, and has been associated with cardiac dysrhythmias.²¹ In addition, sedation administered to supplement topicalization for AI may result in unintended respiratory depression or apnea.

Studies have shown that either traditional preoxygenation (≥3 minutes of tidal volume [V_T] ventilation) or fast-track preoxygenation (i.e., four vital-capacity breaths in 30 seconds) is effective in delaying arterial desaturation during subsequent apnea.⁷ Daos and colleagues showed that the use of supplemental O₂ delayed circulatory arrest resulting from local anesthetic toxic effects in animals.²²

In the interest of improving patient safety, therefore, adequate preoxygenation and the use of supplemental O₂ throughout airway management (including sedation, topicalization, intubation, and extubation) is encouraged in all patients undergoing AI.⁷

In addition to the standard methods of supplemental O₂ delivery (nasal cannula or face mask), other opportunities include, but are not limited to, delivering O₂ through the suction port of a flexible fiberoptic bronchoscope (FFB),⁹ delivering O₂ through an atomizer or nebulizer during topicalization, and elective transtracheal jet ventilation (TTJV) in a patient in extremis.^9,^23,²⁴

E Airway Equipment

Consultants of the TFMDA strongly agreed that “preparatory efforts enhance success and minimize risk to the patient” (i.e., lead to fewer adverse outcomes).⁷ The concept of preassembled carts for emergency situations is not new; examples include “crash carts” for cardiac arrest and malignant hyperthermia carts. The task force recommends that every anesthetizing location should have readily available a portable storage unit that contains specialized equipment for DA management. Suggested contents of this portable unit are listed in Box 11-2.

Box 11-2

Suggested Contents of the Portable Unit for Difficult Airway Management

1. Rigid laryngoscope blades of alternative designs and sizes from those routinely used; this may include a rigid fiberoptic laryngoscope

2. Endotracheal tubes (ETTs) of assorted sizes and styles, such as the Parker FlexTip tube (Parker Medical, Highlands Ranch, CO) or the Endotrol tube (Covidien-Nellcor, Boulder, CO)

3. ETT guides, such as semirigid stylets, ventilating tube changer, light wands, and forceps designed to manipulate the distal portion of the ETT

4. Laryngeal mask airways of assorted sizes and styles, such as the intubating laryngeal mask airway and the LMA-Proseal (LMA North America, Inc., San Diego, CA)

5. Fiberoptic intubation equipment

6. Retrograde intubation equipment

7. At least one device for emergency nonsurgical airway ventilation, such as a transtracheal jet ventilation stylet or the esophageal-tracheal Combitube (Kendall-Sheridan Catheter Corporation, Argyle, NY)

8. Equipment suitable for emergency surgical airway access (e.g., cricothyrotomy)

9. An exhaled CO₂ detector

10. Pulse oximetry unit

The items listed in this table represent suggestions. The contents of the portable storage unit should be customized to meet the specific needs, preferences, and skills of the practitioner and the health care facility.

Modified from Practice guidelines for management of the difficult airway: an updated report by the American Society of Anesthesiologists Task Force on Management of the Difficult Airway: Practice guidelines for management of the difficult airway. Anesthesiology 98:1269–1277, 2003.

There are many techniques that can be used to secure the airway in the awake patient. Direct laryngoscopy, video laryngoscopy, intubating laryngeal mask airways (iLMAs), FFB, rigid fiberoptic laryngoscopy, retrograde intubation, lighted stylets, and blind nasal intubation have all been used successfully to perform AI.^2,²⁵^–³⁰ No matter which technique is selected, all of the necessary equipment should be prepared ahead of time and readily available when needed. The practitioner should also have several backup modalities in mind, and the required equipment available, in case the initial technique used is ineffective.

IV Premedication and Sedation

Once the preoperative visit is concluded and the patient has agreed to proceed with AI, premedication is commonly employed to alleviate anxiety, provide a clear and dry airway, protect against the risk of aspiration, and enable adequate topicalization of the airway. The agents most commonly used in the preparation for AI include antisialagogues, mucosal vasoconstrictors, aspiration prophylaxis agents, and sedatives/hypnotics.

A Antisialagogues

One of the most important goals of premedication for AI is drying of the airway. Secretions can obscure the view of the glottis, especially when FFB is used. In addition, secretions can prevent local anesthetics from reaching intended areas, resulting in failed sensory blockade, or they can wash away and dilute local anesthetics, diminishing their potency and duration of action.

The medications most often used for their antisialagogic properties are the anticholinergics.³¹ These drugs inhibit salivary and bronchial secretions by their antimuscarinic effects. Because they only prevent formation of new secretions and do not eliminate secretions that are already present, anticholinergics should be administered at least 30 minutes before topicalization. The anticholinergics used in clinical practice are atropine, glycopyrrolate, and scopolamine. A summary of their pharmacologic properties is presented in Table 11-2.

TABLE 11-2 Pharmacologic Characteristics of Anticholinergic Drugs

B Nasal Mucosal Vasoconstrictors

The nasal mucosa and nasopharynx are highly vascular. When a patient requires nasal AI, adequate vasoconstriction is essential, because bleeding can make FOI extremely difficult. One agent commonly used for this purpose is 4% cocaine, which has vasoconstrictive as well as local anesthetic effects (see later discussion). An alternative to cocaine is a 3 : 1 mixture of 4% lidocaine and 1% phenylephrine, which yields a solution of 3% lidocaine with 0.25% phenylephrine.³² When these agents are applied with cotton swabs or pledgets to the nasal area, adequate anesthesia and vasoconstriction can be achieved in 10 to 15 minutes, facilitating nasal AI. Alternatively, vasoconstrictive nose sprays may be administered to the patient before local anesthetic topicalization. Phenylephrine 0.5% and oxymetazoline 0.05% sprays are available. They should be administered 15 minutes before attempted nasal intubation.

C Aspiration Prophylaxis

A percentage of patients requiring AI may need prophylaxis against aspiration pneumonitis because of the presence of risk factors such as a full stomach, symptomatic gastroesophageal reflux disease, hiatal hernia, presence of a nasogastric tube, morbid obesity, diabetic gastroparesis, or pregnancy.^33,³⁴

Preoperative administration of a nonparticulate antacid, such as sodium citrate, provides effective buffering of gastric acid pH.³⁵ Total gastric volume is increased, but this effect is offset by an increase in the pH of gastric fluid so that, if aspiration occurs, morbidity and mortality are significantly lower.³⁶ One disadvantage of sodium citrate is the potential to cause emesis due to its unpleasant taste.

1 Histamine Receptor Blockers

Histamine (H₂) receptor blockers are selective and competitive antagonists that block secretion of hydrogen ion (H⁺) by gastric parietal cells and also decrease the secretion of gastric fluid. With IV administration of cimetidine 300 mg, famotidine 20 mg, or ranitidine 50 mg, peak effects are achieved within 30 to 60 minutes, increasing gastric pH and decreasing gastric volume.^37,³⁸ Of the three, ranitidine is probably the drug of choice because it has fewer adverse effects, greater efficacy, and a longer duration of action.^39,⁴⁰

2 Proton Pump Inhibitors

Proton pump inhibitors (PPIs), such as pantoprazole and omeprazole, have not been shown to be as effective as H₂ blockers at increasing gastric pH and decreasing gastric volume.^41,⁴² PPIs may have a role in aspiration prophylaxis for the patient on chronic H₂ blocker therapy.⁴³

3 Metoclopramide

Metoclopramide is a dopamine antagonist that stimulates motility of the upper gastrointestinal tract and increases lower esophageal sphincter tone. The net effect is accelerated gastric emptying with no effect on gastric pH. The standard adult dose is 10 mg IV. Metoclopramide can precipitate extrapyramidal symptoms and should be avoided in patients with Parkinson’s disease.³⁴ For patients at high risk for aspiration, antacids, H₂ blockers, and metoclopramide may be used alone or in combination.

D Sedatives/Hypnotics

Depending on the clinical circumstance, IV sedation may be useful in allowing the patient to tolerate AI by providing anxiolysis, amnesia, and analgesia. Benzodiazepines, opioids, hypnotics, α₂-agonists, and neuroleptics can be used alone or in combination.²⁵ It is important that these agents be carefully titrated to effect, because oversedation can render a patient uncooperative and make AI more difficult.⁹ Spontaneous respiration with adequate oxygenation and ventilation should always be maintained.²⁵ Care should be taken in the presence of critical airway obstruction, because awake muscle tone is sometimes necessary in these patients to maintain airway patency.⁴⁴ Avoidance of oversedation is also important in the patient with a full stomach, because an awake patient can protect his or her own airway in the chance of regurgitation.⁹

1 Benzodiazepines

Benzodiazepines, via their action at the γ-aminobutyric acid (GABA)–benzodiazepine receptor complex, have hypnotic, sedative, anxiolytic, and amnestic properties.⁴⁵ They have also been shown to depress upper airway reflex sensitivity,⁴⁶ a property that is desirable for AI. Benzodiazepines are frequently used to achieve sedation for AI in combination with opioids,⁴⁷ and they are used for their amnestic and anxiolytic effects when other sedatives (e.g., dexmedetomidine, ketamine, remifentanil) are chosen as the primary agent.^48,⁴⁹ Three benzodiazepine receptor agonists are commonly used in anesthesia practice: midazolam, diazepam, and lorazepam.⁴⁵

a Midazolam

Because of its more rapid onset and relatively short duration, midazolam is the most commonly used agent. Sedation with midazolam is achieved with doses of 0.5 to 1 mg IV repeated until the desired level of sedation is achieved. The IM dose is 0.07 to 0.1 mg/kg. Onset is rapid, with peak effect usually achieved within 2 to 3 minutes of IV administration. The duration of action is 20 to 30 minutes, with termination of effect primarily a result of redistribution. Although recovery is rapid, the elimination half-life is 1.7 to 3.6 hours, with increases noted in patients with cirrhosis, congestive heart failure, or morbid obesity; in the elderly; and in patients with renal failure. It is extensively metabolized by the liver and renally eliminated as glucuronide conjugates.^45,⁵⁰

b Diazepam and Lorazepam

Diazepam has a slightly slower onset and longer duration of action than midazolam and has been shown to be a less potent amnestic.^45,^50,⁵¹ It can cause pain on IV injection and has the added risk of thrombophlebitis.²⁵ Lorazepam provides the most profound sedating and amnestic properties; however, it is more difficult to use, because these effects are slower in onset and longer lasting than with either midazolam or diazepam.^25,⁴⁵

c Precautions

Care must be used when using benzodiazepines in combination with other sedative drugs. The pharmacologic effects of benzodiazepines are augmented synergistically by other medications used for sedation, including opioids and α₂-agonists.⁵² Propofol has been shown to increase the plasma concentration of midazolam by decreasing distribution and clearance.⁵³ Systemic absorption of local anesthetics used for airway topicalization may also lead to potentiation of the sedative/hypnotic effects of midazolam.⁵⁴

The primary adverse effect of oversedation with benzodiazepines is respiratory depression, which may lead to hypoxemia or apnea.⁴⁵ Flumazenil, a specific benzodiazepine antagonist, may be used to reverse the sedative and respiratory effects of benzodiazepines if a patient becomes too heavily sedated. It is given in incremental IV doses of 0.2 mg, repeated as needed to a maximum dose of 3 mg. Because it has a half-life of 0.7 to 1.8 hours, resedation can be a problem if flumazenil is being used to reverse high doses or longer-acting benzodiazepines, and patients should be monitored carefully in those circumstances. Flumazenil is generally safe and devoid of major side effects.^55,⁵⁶

2 Opioids

Opioids, by way of their agonist effect on opioid receptors in the brain and spinal cord, provide analgesia, depress airway reflexes, and prevent hyperventilation associated with pain or anxiety. These properties make them a useful addition to the sedating regimen for AI. Although any opioid receptor agonist could theoretically be used for this purpose, the synthetic phenylpiperidine class of opioids—fentanyl, sufentanil, alfentanil, and remifentanil—are best suited to the task. These drugs are particularly useful due to their rapid onset, relatively short duration of action, and ease of titration.⁵⁷

a Fentanyl

Analgesic doses range from 0.5 to 2 µg/kg IV. Onset is rapid, within 2 to 3 minutes. Duration of a single bolus dose is roughly 30 to 60 minutes. The duration is relatively short because fentanyl is redistributed to a large peripheral compartment rather than rapidly eliminated. Hence, the duration of effect after cessation of a prolonged infusion is markedly longer, due to redistribution to the central compartment from the peripheral compartment. Fentanyl is widely used in anesthesia practice, and it seems to be the most commonly used opioid for AI.^47,⁵⁸

b Sufentanil

Sufentanil is 7 to 10 times more potent than fentanyl and has a similar pharmacokinetic profile after a single bolus dose. The primary difference is a significantly faster recovery, compared with fentanyl, after prolonged infusion.⁵⁹ Sedative doses are 5 to 20 µg IV in adult patients. Its use in combination with midazolam and droperidol for AI has been reported.⁶⁰

c Alfentanil

Compared with fentanyl and sufentanil, alfentanil has an even quicker onset (1.5 to 2 minutes). It is approximately 1/70 as potent as fentanyl; however, because of rapid plasma to effect-site equilibration, comparatively smaller doses are needed to achieve a similar peak effect. Because smaller doses are needed relative to its potency, recovery from a single bolus of alfentanil is faster than with the other agents of this class, potentially making alfentanil the drug of choice when a transient peak effect after a single bolus is desired, as in AI.⁵⁹ Sedative doses range from 10 to 30 µg/kg IV.⁵⁷ In patients premedicated with oral diazepam, alfentanil 20 µg/kg IV significantly improved fiberoscopic conditions and attenuated the hemodynamic effects of awake nasal FOI. Moderate respiratory depression was noted without overt apnea or hypoxia.⁶¹

d Remifentanil

Remifentanil is an ultrashort-acting opioid that is unique compared with the other short-acting agents in that it is metabolized by nonspecific plasma esterases, with a half-life of 3 minutes. It undergoes no redistribution and therefore has no context sensitivity. Its potency approximates that of fentanyl.⁶² Several studies have shown the effectiveness and safety of remifentanil sedation for AI as a single agent,^48,^49,⁶³ as well as in combination with midazolam or propofol.⁶⁴^–⁶⁸ Dosing is usually weight-based.

Several different remifentanil dosing strategies have been described in the literature for AI, with infusion rates ranging between 0.06 and 0.5 µg/kg/min, with or without an initial bolus of 0.5 to 1.5 µg/kg.^48,^63,^64,⁶⁷ Studies using target-controlled infusions for AI have shown that the mean effect-site concentration of remifentanil needed for AI is 2 to 3 ng/mL.^49,^65,^66,⁶⁸ The dosing strategy described by Atkins and Mirza,⁶² a bolus of 0.5 µg/kg followed by an infusion of 0.1 µg/kg/min, uses the Minto pharmacokinetic model and rapidly achieves a target site concentration of 2 to 2.5 ng/mL. The infusion can subsequently be titrated by 0.025 to 0.05 µg/kg/min in 5-minute intervals to achieve adequate sedation.

e Precautions

The most serious adverse effect of opioids is respiratory depression leading to overt apnea. Opioids reduce the stimulatory effect of carbon dioxide (CO₂) on ventilation while increasing the apneic threshold and the resting end-tidal CO₂. Factors that increase susceptibility to opioid-induced respiratory depression include old age, obstructive sleep apnea, and concomitant administration of central nervous system depressants.

Naloxone, an opioid antagonist, can be used to restore spontaneous ventilation in patients after an opioid overdose. Onset after IV administration is rapid, within 1 to 2 minutes, and the duration of action is 30 to 60 minutes. Naloxone should be administered in boluses of 0.04 to 0.08 µg IV boluses every 2 to 3 minutes. Doses of 1 to 2 µg/kg will restore adequate spontaneous ventilation in most cases while preserving adequate analgesia. Potential complications of naloxone administration are reversal of analgesia, tachycardia, hypertension, and, in severe cases, pulmonary edema or myocardial ischemia. Because of the relatively short duration of action of naloxone, one should carefully monitor for recurrence of respiratory depression, especially when it is used to reverse longer-acting opioids such as morphine or hydromorphone. In those situations, an intramuscular dose of 2 times the required IV dose or a continuous IV infusion (2.5 to 5 µg/kg/hr) should be considered.⁵⁷

Chest wall rigidity leading to ineffectual bag-mask ventilation is commonly cited as a potential adverse effect of opioids, particularly fentanyl, sufentanil, alfentanil, and remifentanil. Opioids do have the potential to cause muscle rigidity, but clinically significant rigidity usually occurs only after an opioid dose sufficient to cause apnea, because the patient loses consciousness.⁵⁷ Studies in intubated patients and patients with tracheostomies have shown that decreases in pulmonary compliance due to chest wall rigidity are not sufficient to explain an inability to maintain bag-mask ventilation after a large dose of opioid,^69,⁷⁰ and fiberoscopic examination of the vocal cords during induction with sufentanil has shown that vocal cord closure is the primary cause of difficult ventilation after opioid-induced anesthesia.⁷¹ Careful titration to prevent overdose is perhaps the best way to prevent rigidity-associated difficult ventilation. Should it occur, treatment with naloxone or neuromuscular blocking agents is effective.^72,⁷³

3 Intravenous Anesthetics

a Propofol

Propofol (2,6-diisopropylphenol) is the most frequently used IV anesthetic today.⁴⁵ Its primary effect is hypnosis, which results from an unclear mechanism; however, there is evidence that a significant portion of this hypnotic effect is mediated by interaction with GABA receptors. Propofol has a rapid onset of approximately 90 seconds, with rapid recovery (4 to 5 minutes after an induction dose) as a result of both elimination and redistribution. It attenuates airway responses in induction doses via an unclear mechanism and provides a smooth induction with few excitatory effects. Although it is frequently used as an induction agent in doses of 1.5 to 2.5 mg/kg IV, intermittent doses of approximately 0.25 mg/kg IV or a continuous IV infusion of 25 to 75 µg/kg/min provides an easily titratable level of sedation with rapid recovery.

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