Chapter 18. Airway Management of a Morbidly Obese Patient Suffering from a Cardiac Arrest

A 67-year-old woman presents to the emergency department (ED) by ambulance with a 3-hour history of increasing dyspnea associated with chest pain. She has a history of coronary artery disease, hypertension, and hyperlipidemia, but no known allergies. Her medications include atenolol, low-dose aspirin, atorvastatin, acetaminophen with codeine, and nitroglycerin spray as needed. Prior to notifying the emergency medical services, the patient had used three sprays of nitroglycerin every 5 to 10 minutes with no relief.

She is placed on 10 L·min−1 oxygen by face mask and is transported to the hospital. Upon arrival at the ED, her vital signs are heart rate (HR) 113 beats per minute (bpm) and irregular, respiration rate (RR) 31 breaths per minute, blood pressure 85/45 mm Hg, and SaO2 86%. On examination, she appears to be in severe respiratory distress and is unable to speak more than three to four words in one breath. She is morbidly obese with an estimated weight of over 137 kg (300 lb) and is 151 cm (5 ft) tall with a body mass index (BMI) of 60 kg·m−2. Chest auscultation reveals faint breath sounds with crackles over the entire lung fields, a significant decrease in air entry in both bases combined with mild wheezing. Other findings include 1+ bilateral ankle edema, S4 heart sound, and a grade III/VI systolic murmur radiating to the axilla. Her jugular venous pressures (JVP) cannot be assessed because of her marked obesity and short neck. Her electrocardiogram reveals a pattern consistent with an acute antero-lateral myocardial infarction. The chest x-ray shows poor inflation and is also consistent with pulmonary edema.

Following the initial assessment, it is noticed that the SaO2 decreases to 81% and her respirations increase to 35 to 40 breaths per minute. Pink froth appears from her mouth.

As you prepare for airway intervention, she loses consciousness. The monitor shows pulseless ventricular tachycardia. What do you do to secure the airway at this time?

18.2.1 Define Obesity

Obesity is the presence of an excess of body fat when compared to average values for age and gender. When the percentage of body fat exceeds 15% to 18% in men, or 20% to 25% in women, the individual is considered obese. Unfortunately, measuring body fat is not practical as it requires sophisticated techniques.

The ideal body weight (IBW) has been used frequently in clinical settings to define obesity:

IBW (kg) = height (cm) - x

where x is 100 for males and 105 for females.

Patients who weigh 20% above IBW are considered overweight, and they are considered morbidly obese if their weight is 200% above the calculated IBW.1

The World Health Organization (WHO) has utilized body mass index (BMI) as the international method of classifying obesity.2 It has become the standard method for defining obesity.

BMI3 = body weight (kg)/height2 (m)

Using the BMI, obesity is categorized as follows4:

• A person is considered overweight with a BMI of 25 to 29.9 kg·m−2.
• Obese individuals have a BMI greater than 30 kg·m−2.
• Morbidly obese individuals have a BMI greater than 35 kg·m−2.
• Super-morbidly obese individuals have a BMI greater than 55 kg·m−2.

It has been well established that obesity is associated with multiple medical issues including hypertension, heart disease, congestive heart failure, diabetes mellitus, stroke, obstructive sleep apnea, an increased incidence of perioperative wound infection, and respiratory complications.

Recently, a more important predictor of long-term outcome has been shown to be the type of fat distribution, rather than BMI. "Male" or "android pattern central obesity" (trunk and abdomen) has been shown to correlate more with negative outcomes and increased risk of cardiac disease and premature death than "female pattern", gynecoid obesity (peripheral).

"Metabolic syndrome" (Syndrome X) describes truncal obesity as being a waist to hip ratio of greater than 0.9 in men, or greater than 0.85 in women, or a waist circumference of greater than 40 in (approximately 100 cm) in men and 35 in (approximately 88 cm) in women. This syndrome is associated with glucose intolerance (type II diabetes), hypertension, dyslipidemia, microalbuminuria, prothrombotic states, and proinflammatory states (eg, elevated C-reactive protein) and represents a particularly high-risk group for the development of cardiovascular and cerebrovascular diseases.5,6

Reducing the degree of obesity has been shown to favorably impact the progression of these disorders.7,8

18.2.2 What Are the Anatomic and Physiologic Factors that Might Contribute to the Difficulty of Airway Management in the Morbidly Obese Patient?

Numerous factors have been implicated as contributing to difficult bag-mask-ventilation, extraglottic device (EGD) use, laryngoscopy and intubation, and the performance of a surgical airway in this population. These include large breasts (male and female), excess adipose tissue in the face and cheeks, short neck, large tongue, redundant palatal and pharyngeal tissue, superior and anterior larynx, limited mouth opening, limited access to the anterior neck, and limited cervical spine mobility.4

Experience and the literature, suggest that, based on weight alone, morbidly obese patients do not represent a difficult airway (see Chapter 1). Even when a morbidly obese patient has favorable airway assessment parameters (ie, Mallampati I, full range of motion of neck, adequate mouth opening, etc), other factors can make airway intervention more challenging.

There is a significant decrease in tolerable apnea time in obese, compared with that of nonobese subjects. This decrease occurs in a linear fashion as obesity increases and relates to both a decreased respiratory reserve and an increase in metabolic requirements.9 This decreased respiratory reserve is the result of a decrease in functional residual capacity combined with a closing capacity that intrudes on tidal volume ventilation.9,10 Furthermore, the high FiO2 employed in all intubations induces absorption atelectasis, further reducing the amount of lung tissue available for gas exchange. Because of these factors, precipitous oxygen desaturation occurs when the patient is rendered apneic during airway management.10 Data from Jense suggest that, during rapid sequence induction (RSI) in the morbidly obese patient, apneic time before the development of hypoxemia permits only one intubation attempt before hypoxemia ensues.9

The morbidly obese patient also has a restrictive lung defect resulting in a decreased vital capacity, expiratory reserve volume, and inspiratory capacity.9,11 Auler et al found that morbidly obese patients under general anesthesia show a higher resistance throughout the respiratory system.12

The presence of morbid obesity is considered by many to be a predictor for difficult mask ventilation.4,9,13 Adequate bag-mask-ventilation (BMV) requires an open airway and a tight mask seal. The difficulty of creating a patent airway in the morbidly obese and maintaining a competent mask seal in the face of elevated airway pressures sufficient to overcome the restrictive defect imparted by obesity mitigate against effective BMV. Anterior translation of the mandible (jaw thrust) to affect airway opening has been shown to be more difficult and less effective in the obese.14 In the cited study, nine nonobese and nine obese subjects were anesthetized and given neuromuscular blocking agents. Once apneic, and with steady airway pressure applied via a nasal device, the oropharynx and velopharynx (nasopharynx) were visualized with an endoscope. The cross-sectional areas were measured, both in the resting state and with a jaw thrust applied. In both groups of patients, the jaw thrust improved the cross-sectional area of the oropharynx. However, while an improvement with the jaw thrust maneuver occurred in the measurements of the velopharynx in the nonobese population, no improvement was noted to occur with this maneuver in the obese patients. The authors found that obstruction persisted in the lateral plane rather than the in the A-P dimension, and postulated that this was due to the redundant soft tissue around the tonsillar pillars closing in from the sides as the tonsillar pillars were stretched antero-posteriorly. This may explain why CPAP or PEEP augments ventilation in the obese patient, as both laterally splint the airway.14,15 It may also explain why the LMA has been found to be an effective rescue device in the obese population (see later).

18.2.3 What Are the Special Considerations in Patients with Obstructive Sleep Apnea?

About 5% of morbidly obese patients have obstructive sleep apnea (OSA).4 In studying over 6000 subjects, Nieto16 reported that the majority of patients with OSA are not obese. Consequently, questioning patients regarding OSA should not be reserved for only the obese. The presence of snoring may be the only indicator of OSA in the general population. Snoring and obesity are important predictors for difficult BMV.13

Although difficult to quantify, a direct correlation may exist between difficult tracheal intubation and OSA.17 Some controversy exists, however, and opposing data are present in the literature. In their study, Neligan et al showed that OSA is not a risk factor for difficult intubation. They did show, however, that male gender as well as high Mallampati scores (≥III) did predict difficulty in establishing an airway.18 Chung et al followed up with sleep studies on a number of patients who were found to be difficult or failed intubations at the time of surgery. Sixty-six percent of these were subsequently diagnosed with OSA.19

In patients with OSA, airway patency is disturbed by relaxation of pharyngeal dilator muscles during sleep. The upper airway is soft, pliable, and narrow in these patients, which makes it collapsible during sleep. Turbulent air flow through these structures produces vibrations (snoring) and collapse (apnea).20 This obstruction continues until the level of sleep is interrupted and the individual regains pharyngeal muscle tone. This snoring-obstruction-apnea cycle can be exacerbated by drugs or alcohol. Consequently, sedatives, particularly the long-acting agents given in the perioperative period, can have a pronounced deleterious effect on the ability of this patient population to maintain airway patency when asleep.4 There is little question that these factors lead to increased perioperative risk for morbidity and mortality. Numerous papers address the management issues surrounding the perioperative care of this patient population.21 Unfortunately, most of it is expert opinion, rather than evidence based. A useful reference is the published guidelines by the American Society of Anesthesiologists. Again, these are positions based primarily on expert consensus.

The obesity hypoventilation syndrome (OHS), also known as Pickwickian syndrome, is characterized by chronic respiratory insufficiency, with both obstructive and restrictive features on pulmonary function testing. Chronic hypoxemia and hypercarbia, polycythemia, somnolence, pulmonary hypertension, and right ventricular dysfunction (cor pulmonale) characterize this condition. These patients all exhibit a marked reduction in hypoxic and hypercarbic drives, measuring one-sixth and one-third the response to that of controls.22 Although some similarities exist between OSA and OHS, they are not the same disease. As pointed out earlier, not all patients with OSA are obese, and patients with OHS do not necessarily have OSA. Due to the significant underlying pulmonary and cardiac dysfunction with the OHS population, they are at significant perioperative risk.

18.2.4 Is Tracheal Intubation More Difficult in Morbidly Obese Patients?

There is some disagreement as to whether morbid obesity predicts difficult intubation. The incidence of difficult intubation in the morbidly obese population has been reported to be approximately 13% to 20 %.23-25

However, in a study of 100 morbidly obese patients with BMIs of greater than 40 kg·m−2, Brodsky et al26 concluded that obesity, per se, was not a predictive factor in determining difficulty of intubation. Of the many parameters measured in the study population, the only two that correlated with difficult laryngoscopy following rapid-sequence induction with cricoid pressure were large neck circumference and high Mallampati scores. A neck circumference (measured at the level of the thyroid cartilage) of 40 cm was associated with a 5% incidence of difficult intubation. In the same study, difficult intubations were encountered in 35% of patients with a neck circumference of 60 cm.26 Of interest, larger neck circumference has also been associated with increasing severity of obstructive sleep apnea.27

In a separate study, Ezri also found that obesity, by itself, was not a predictor of difficult intubation.28

18.3.1 Are Obese Patients at Increased Risk of Aspiration?

Obesity has been frequently listed as a risk factor for aspiration. However, recently this belief has been challenged by a number of studies that have found no increase in gastric volumes or acidity in obese subjects.29,30 Maltby et al31 demonstrated that gastric emptying was no different in obese than in nonobese patients and suggested that the same guidelines for fasting can be applied to both patient populations. Aspiration risks and prophylaxis should be applied in the obese patients using the same criteria as in the nonobese. For a detailed discussion of aspiration and risks, see Chapter 5.

18.3.2 How Should the Morbidly Obese Patient Be Positioned for Airway Management?

Because of the decreased oxygen reserve in this patient population, it is crucial to position these patients carefully for tracheal intubation prior to induction of anesthesia. Appropriate positioning prior to the induction of anesthesia can significantly reduce the apnea time required for intubation as well as increase the oxygen reserve. While recent literature has questioned the advantage of the sniffing position over simple head extension,32 these studies still advocate the sniffing position for obese patients. Proper sniffing position has been defined as head extension and a 35 degree flexion of the neck onto the chest.33

Achieving adequate sniffing position in a patient of ideal body weight may mean only placement of a pillow under the neck and extending the head. However, in a morbidly obese patient, optimal position often requires building a ramp of sheets or towels under the shoulders, neck, and head (Figures 18-1 and 18-2). This is referred to as "extreme sniffing" or "ramped" position. As an alternative to blankets, towels, and pillows, the Troop® Elevation Pillow can be used (see Figure 49-2). A comparison study of 60 morbidly obese patients by Collins et al,34 demonstrated a significant improvement of laryngoscopic grade in patients in the "ramped" position. Determinants of proper "ramped" positioning have been described as follows: at least a 90 degree angle between the mandible and chest; the face higher than the chest; external auditory meatus at the same horizontal level as the sternal angle.34

A second and important positioning principle involves the use of the reverse Trendelenberg position. Bed placement of 30 degree head-up tilt increases functional residual capacity (FRC) and compliance (both lung and chest wall), thereby permitting greater degrees of pulmonary oxygen reserve.35 Adding continuous positive airway pressure (CPAP) of 10 cm of H2O pressure has been shown to be an effective maneuver in reducing the degree of atelectasis associated with induction. Patients receiving positive end-expiratory pressure (PEEP)/CPAP prior to induction can be expected to have higher PaO2 values than control groups that have not undergone the maneuver.36,37

18.3.3 How Should Medications Be Dosed in the Morbidly Obese Patient?

When determining the dosage of a drug, the ideal body weight (IBW) or lean body weight (LBW) are generally used. The LBW is a measured value, but can be estimated by the formula:

LBW = (a) (weight) - b (weight2/[100 × height])2

where a and b = 1.1 and 128, respectively, for men, and 1.07 and 148, respectively, for women, weight in kg, and height in m.38

The ideal body weight (as presented earlier), can be approximated by a simple formula.

IBW (kg) = height (cm) - x

where x is 100 for males and 105 for females.1

The use of total body weight (TBW) to calculate the dose is particularly problematic for lipophylic drugs. For some lipophylic agents, the use of TBW leads to an over calculation of the required dosage, and hence toxicity.

Perhaps the best way to calculate the appropriate dose of these agents is to employ the adjusted body weight (ABW):

ABW39 = IBW + 0.4 (TBW - IBW)

There are a number of reasons for not using the TBW in calculating drug dosing. In the obese person, not all of the excess weight is fat. Lean mass is increased in the obese person, and represents 20% to 40% of the increased weight and in addition, blood supply to adipose tissues is quite low, accounting for only 5% of cardiac output. For all practical purposes, protein binding (a major influence on volume of distribution) in obese patients is similar to that in those who are nonobese.40

18.4.1 Discuss the Appropriate Methods for Denitrogenation of the Morbidly Obese Patient

While the term preinduction oxygenation or "preoxygenation" has been used by most to reflect denitrogenation of the lungs, it is more appropriate to use the term "denitrogenation," as it more accurately reflects the desired end result of nitrogen being washed out of the lungs. Two denitrogenation methods have been described and are equally effective for the obese patient.9 They include: (1) four vital capacity breaths of 100% oxygen and (2) breathe 100% oxygen normally (tidal-volume breathing) through a snug-fitting facemask for 5 minutes. Denitrogenation employing CPAP with 100% oxygen showed no significant difference when compared with standard denitrogenation techniques in one study,10 but proved beneficial in another.36

18.4.2 How Effective Is the Laryngeal Mask Airway in the Obese Patient?

A number of studies and case reports have been published recently, illustrating the effectiveness of the various laryngeal mask devices in providing ventilation and oxygenation in obese patients. Doyle and colleagues reported the management of the airway in a patient weighing 445 kg (980 lb and BMI of 163 kg·m−2). After failing awake tracheal intubation using a flexible bronchoscope, they successfully placed a size 5 LMA ProSeal™ awake and anesthesia was induced using sevoflurane.41 A surgical airway was then performed.

In a study involving 60 obese patients (BMI >30), Natalini et al have shown that both LMA Classic™ and LMA Proseal™ are effective in providing mechanical ventilation.42 In another efficacy study of the LMA ProSeal™ (LMAP) in the morbidly obese, Keller43 induced anesthesia in 60 morbidly obese (BMI >35 kg·m−2) patients and inserted an LMAP. Insertion of the LMAP was successful in all patients, 90% on the first attempt, and the remaining 10% on the second attempt. Following adequate oxygenation, the LMAP was removed, and laryngoscopic intubation was successful in 54 patients (90%) on the first attempt and four patients (7%) on the second attempt. Failure to intubate the trachea occurred in the remaining two patients (3%). These patients had the LMAP reinserted and the surgery proceeded using the extraglottic airway. No significant hypoxemia was reported in any patient during the study.

Frappier et al44 studied 118 morbidly obese surgical patients with BMIs of greater than 45 kg·m−2. Following induction, all initially underwent laryngoscopy to determine the Cormack/Lehane grade. Subsequently, they all underwent attempted tracheal intubation using the intubating laryngeal mask airway (ILMA). Tracheal intubation was successful in 114 patients (96.3%). Laryngoscopic intubation was successful for the remaining four patients with failed ILMA attempts. No correlation between laryngoscopic grade and failure with the ILMA was evident.

18.4.3 What Are the Plans to Secure the Airway of This Patient?

Pulseless ventricular tachycardia renders this patient a crash airway (Chapter 2). Airway management of this patient should take into account the principles discussed earlier. Management of the cardiac emergency should be guided by the most recent ACLS guidelines provided by the American Heart Association. This would include initiation of cardiac compressions and defibrillation as indicated.

As with any emergency intubation, every effort is made to ensure that the first attempt is the best attempt:

• Best person
• Best position
• Best paralysis
• Best BURP
• Best length and type of laryngoscope blade

If possible, placing the patient in a ramped position (Figure 18-2) with reverse Trendelenberg immediately has the potential to improve pulmonary mechanics, improve the success of BMV, and optimize the ability to provide preintubation oxygenation saturations. Help should be summoned and adjuncts and alternative airway devices, such as the rigid fiberoptic laryngoscopes or video laryngoscopes (Plan B and Plan C), should be immediately available. Direct laryngoscopy may not be difficult, unless there is a history of difficult intubation (OSA, previously failed intubation, etc) or predictors of difficulty, such as a thick neck or a high Mallampati score elicited before collapse.

Induction agents are not indicated in the crash airway. Neuromuscular-blocking agents (eg, succinylcholine) may be required in the event the first attempt at oral intubation fails and residual muscle tone is suspected to be a factor contributing to that failure (see Crash Intubation Algorithm, Chapter 2). However, it is of questionable benefit to administer succinylcholine in the presence of a pulseless VT.

The risk of aspiration depends upon a number of factors, including the fact that it is an emergency, her stomach is likely to be full either from gastric juices or gas from attempts at bag-mask-ventilation, and a positive history of gastro-esophageal reflux disease (GERD), to name a few. As failed or difficult intubation increases the risk of aspiration, cricoid pressure is employed while managing the airway. However, if intubation or airway management proves difficult in the presence of cricoid pressure, the pressure can, and should be released. It is important to recognize that cricoid pressure has never been proven to be protective, and has been demonstrated to render airway management more difficult in some situations (see Chapter 5 for detailed discussion).

If the decision to intubate her trachea was to be made prior to her arrest, the choice to use neuromuscular-blocking agents to facilitate that intubation would depend on the level of confidence one has that the airway can be secured. Inherent in this is the anticipated ease of oxygenation with BMV or EGD in the event that intubation fails. If the decision is made to paralyze, the selection and dosing of induction agents should be based on the hemodynamic stability of the patient. Should effective gas exchange not be possible, an awake look may be employed to further evaluate the potential for a successful intubation (see Chapter 2).

18.4.4 What Are the Rescue Options for the Failed Airway in the Morbidly Obese Patient?

The first rescue from failed BMV is improved BMV. BMV is likely to be difficult in this patient due to soft tissue collapse of the upper airway, difficult or impossible mask seal, and noncompliant lungs due to her obesity and pulmonary edema. Having an experienced assistant to facilitate two-handed BMV technique may be crucial (see Chapter 7 for details).

Management decisions in the face of the failed airway depend on whether it is a 'can't intubate, can ventilate' failed airway or a 'can't intubate, can't ventilate' (CICV) failed airway. In the former situation, there is time and the practitioner may select an EGD or an alternative airway device or technique of greatest familiarity. As discussed earlier, the LMA has been demonstrated to be an effective device in the morbidly obese patient due to its ability to splint the airway. If found to be effective, ventilation and oxygenation can be facilitated. However, with the high pressures that may be required, adequate ventilation may prove to be challenging. In that scenario, switching to an intubating LMA (LMA Fastrach™) may allow subsequent intubation. Another option, particularly if one already has an LMA Classic™ in place, is to employ the Cook Aintree® Intubation Catheter (AIC). The AIC is a hollow catheter that slides over a flexible bronchoscope (FB).45 This technique allows use of the FB to facilitate intubation even in the situation where the practitioner has only low or average skills with the flexible scope. The loaded scope can then be passed through the LMA Classic™ and advanced through the vocal cords and into the trachea. The LMA Classic™ and the bronchoscope are removed, leaving the AIC behind. An endotracheal tube (ETT) can then be advanced over the AIC and the catheter removed, leaving the ETT in place.

In the latter situation (CICV), there is no time and a cricothyrotomy must be performed. While preparing to perform the surgical airway, an EGD may be attempted concurrently.

In the CICV situation, it is appropriate to use alternative intubating techniques, such as the intubating LMA, the Bullard Laryngoscope, or the Glidescope® guided by the skill of the practitioner. These devices have been shown to be effective in tracheal intubation in morbidly obese patients.

Due to the short, thick neck, performing a surgical airway in this morbidly obese patient would undoubtedly prove to be difficult.

The incidence of morbid obesity is increasing in our society, posing both long- and short-term risks to the patient requiring airway management. Associated medical conditions affecting vital organ system reserve influence our decision making when airway management is required. Obesity alone may not predict difficult laryngoscopy, therefore, highlighting the importance of thorough airway examination. However, a thick neck or high Mallampati scores do predict difficulty with laryngoscopic intubation in obese patients. Difficulty with mask-ventilation is common. Preparation for failure is important. The various airway devices highlighted in this chapter have been shown to be effective in the obese patient.

It should be expected that rapid desaturation and hypoxemia may occur following induction and paralysis. However, proper positioning to maximize FRC and recruit alveoli and proper denitrogenation practices will minimize desaturation.

Drug dosing adjustments due to obesity remains poorly understood for many agents. If time permits, careful titration to effect with intravenous induction agents may prove to be effective and safer than dosing according to body weight.

Finally, the recognition of failure is crucial, as is the recognition of what type of failure has occurred. Management pathways are guided by how much time there is, and the experience of the practitioner.