Manual of Emergency Airway Management, 3rd Edition

35.The Morbidly Obese Patient

Sarah H. Wiser

Richard D. Zane

The Clinical Challenge

The World Health Organization defines obesity by using body bass index (BMI), with “normal” defined as a BMI of 18.5 to 24.9 kg/m². Obesity is present when BMI is 30 or higher, morbid or severe obesity when BMI is 40 or higher, and superobesity is a BMI in excess of 50. The 1999–2002 National Health and Nutrition Examination Survey estimates that one-third of U.S. adults are obese, representing a dramatic increase from the prior survey in 1994 and underscoring the epidemic of obesity within the United States.

Approach to the Airway

As for all patients, the approach to managing the airway of obese patients requires a structured, methodical assessment to identify the specific predictors of difficult bag-mask ventilation (BMV), cricothyrotomy, extraglottic device use, and tracheal intubation. Patient attributes differ, and some obese patients may have multiple anatomical risk factors for difficulty in addition to obesity, whereas others may not. Nevertheless, morbidly obese patients develop both physiological and anatomical changes that can make airway management all the more challenging.

The degree of physiological and anatomical changes correlates with the extent of obesity, and dictates the level of airway difficulty. The physiological and anatomical changes associated with morbid obesity are listed in Box 35-1. The main effects of obesity on airway management are (a) rapid arterial desaturation, secondary to a decreased functional residual capacity (FRC) and increased oxygen consumption; (b) difficult BMV, due to increased risk of obstruction from excess pharyngeal adipose tissue and increased resistance due to the weight of the chest wall and the mass of abdominal contents limiting diaphragmatic excursion; and (c) difficult laryngoscopy, intubation, and cricothyrotomy.

Obesity affects almost every aspect of normal physiological function, most notably the respiratory and cardiovascular systems. Obese patients often have baseline hypoxemia with a widened alveolar–arterial oxygen gradient, which is primarily due to ventilation-perfusion (V/Q) mismatching. Lung volumes develop a restrictive pattern with multiple disturbances, the most important of which is decreased FRC. Notably, these indices change exponentially with the degree of obesity. The fall in FRC has been ascribed to “mass loading” of the abdomen and splinting of the diaphragm. FRC may be reduced to the extent that it falls within the range of closing capacity, thus leading to small airway closure and V/Q mismatch. The FRC declines further when the individual assumes the supine position, resulting in worsening of the V/Q mismatch, right-to-left shunt, and arterial hypoxemia. Although the vital capacity, total lung capacity, and FRC may be maintained in mild obesity, they can be reduced by up to 30% in morbidly obese patients and up to 50% in severe or superobese patients. The decreased FRC causes rapid oxyhemoglobin desaturation during the apneic phase of rapid sequence intubation (RSI), even in the setting of adequate preoxygenation (see Chapter 3).

The work of breathing is increased 30% to 400% in morbidly obese patients because of decreased chest wall compliance, increased airway resistance, and an abnormal diaphragmatic position. These changes limit the maximum ventilatory capacity. The obese patient has elevated oxygen consumption and carbon dioxide production due to the metabolic activity of the excess body mass.

Cardiovascular changes in obesity include increased extracellular volume, cardiac output, left ventricular end diastolic pressure, and left ventricular hypertrophy. The absolute total blood volume is increased, but it is relatively less on a volume/weight basis when compared to lean patients (50 mL/kg vs. 75 mL/kg). Cardiac morbidity, including hypertension, ischemic heart disease, and cardiomyopathy, correlates with progressive obesity.

BOX 35-1 Physiological and Anatomical Changes Associated with Obesity

Other changes include an increase in renal blood flow and glomerular filtration rate (GFR), fatty infiltration of the liver, and a propensity for diabetes mellitus and obstructive sleep apnea (Box 35-1).

Increased chest wall weight, increased facial girth, and redundant pharyngeal tissue all contribute to defining obesity as an independent risk factor for difficult BMV (see Chapter 7). Obese patients tend to have a smaller pharyngeal space because of deposition of adipose tissue into the tongue, tonsillar pillars, and aryepiglottic folds. Patients with obesity have an increased risk of having obstructive sleep apnea and facial hair (in men), which are also independent risk factors for difficult BMV. Difficult BMV should be anticipated in the obese patient, often requiring a two-person technique with both oral and nasopharyngeal airways in place. In severe or superobese patients, BMV may simply be impossible as the mask seal pressure required to overcome the increased weight and resistance may be far in excess of that possible with a bag and mask. In addition, challenging BMV is associated with difficult intubation in 30% of the cases. Intubation difficulty is also associated with increased neck circumference and high Mallampati scores. Cricothyrotomy is more difficult because of the increase in neck circumference, the thickness of the subcutaneous tissues, anatomical distortions, and adipose tissue obscuring landmarks, often requiring deeper and longer incisions. Extraglottic devices may not be able to overcome the high resistance of the weighted chest wall and restricted diaphragms.

Technique

Morbidly obese patients vary with respect to airway difficulty, and a methodical LEMON assessment is essential to anticipate and plan appropriately for intubation (see Chapter 7). When the airway appears particularly difficult, the difficult airway algorithm advocates careful preparation and often an awake laryngoscopy or a fiberoptic approach with topical anesthesia and systemic sedation.

Proper positioning is essential in obese patients in order to ensure the best attempt at direct laryngoscopy and tracheal intubation. Ideally, the patient should to be propped up on linens, or on a commercially available pillow, from the midpoint of the back to the shoulders and head for proper positioning, as shown in Figure 35-1A and B. To confirm proper positioning, the patient should be viewed from the side, and an imaginary horizontal line should be able to be drawn from the external auditory meatus to the angle of Louis. This position facilitates intubation and both spontaneous and BMV, thus improving preoxygenation and prolonging the duration of time before arterial desaturation with apnea.

To determine the best technique, the risks and benefits of managing the airway with the patient awake versus unconscious are weighed. No matter which route chosen, the proper airway equipment must be available and checked for proper functioning, and help needs to be readily available in the event intubation proves to be difficult. When performing direct laryngoscopy, a short-handled laryngoscope can be easier to insert because the chest prevents the longer handle from gaining blade access to the mouth. The laryngeal mask airway (LMA) has been shown to be an effective, temporary ventilatory device in morbidly obese patients, but in the superobese patient, the pressure required to overcome the weight of the chest will likely exceed the seal pressure of the LMA, making ventilation difficult or impossible. The intubating laryngeal mask airway (ILMA) has been shown to be effective in providing both ventilation and serving as a conduit to tracheal intubation. Rigid fiberoptic intubating devices and the lighted stylet have been shown to be successful in managing the obese airway as well. The insertion technique for the lighted stylet is the same as in the nonobese patient, although one may encounter a greater diffusion of light depending on the degree of obesity, and room lighting may need to be dimmed. During direct laryngoscopy, the bougie may be helpful when only the posterior arytenoids or the tip of the epiglottis are visible.

BMV often requires two providers using two-handed bilateral jaw thrust and mask seal, with oropharyngeal and nasopharyngeal airways in place and the airway pressure relief valve and mask seal set so that continuous positive airway pressure (5–15 cm H₂O) is delivered to the pharynx. Relaxation of the upper airway muscles during RSI will often cause collapse of the adipose-laden, soft-walled pharynx between the uvula and epiglottis, making BMV and tracheal intubation more difficult, and greatly reinforcing the need to use oral and nasal airways.

Cricothyrotomy may be extremely challenging in the severely obese patient because the chin may be directly contiguous with the chest wall, making identification of and access to anatomical landmarks difficult. In the moderately obese patient, care must be taken to ensure that landmarks are found. This step may require one or two assistants whose sole role is to hold or retract neck, facial, and chest fat folds. As in all patients, cricothyrotomy is a tactile procedure. In the obese patient, direct visualization will likely be impossible.

Drug Dosage and Administration

Obesity, along with any associated comorbidities, affect all aspects of the pharmacodynamic and pharmacokinetic properties of medications, including absorption, onset, volume of distribution (Vd), protein binding, metabolism, and clearance. In the obese patient, there is not only an increase in the adipose tissue, but also an increase in lean body mass of about 30% of the total excess weight. The ratio of fat to lean mass increases, however, causing a relative decrease in the percentage of lean mass and water in obese patients compared to lean patients. In addition, there is an increase in blood volume and cardiac output. The Vd for a particular agent is affected by the combination of these obesity-associated factors, along with the specific lipophilicity of the drug. Protein binding is affected by an increased concentration of triglycerides, lipoproteins, cholesterol, and free fatty acids. These lipids limit the binding of some drugs, thus increasing the free plasma concentration. In contrast, increased alpha-one glycoprotein may increase protein binding of other drugs, thus decreasing the free plasma concentration. For most agents that undergo hepatic metabolism, there is minimal change despite the high incidence of fatty infiltration of the liver. Agents handled by the kidney, however, have accelerated clearance due to increased GFR. Furthermore, obese patients may be more sensitive to the effects of sedative drugs, opioids, and anesthetic agents. These pharmacokinetic and pharmacodynamic changes can make the net effect of these agents unpredictable. Thus, monitoring of clinical end points is important in addition to empirical drug dosing based on published data.

In general, the lipophilicity of the agent can indicate the dosage requirement. Most anesthetic agents are lipophilic, thus an increase in Vd and dose of the drug is expected, but this is not consistently demonstrated in pharmacological studies because of factors such as end-organ clearance or protein binding. Less lipophilic agents have little or no change in Vd and, therefore, should be dosed according to ideal body weight (IBW) or lean body weight (LBW). Unfortunately, few studies have investigated the effects of obesity on the disposition of anesthetic agents, so for many drugs it is unclear if weight-related dosage adjustments should be made and whether these should be based on actual weight, ideal weight, or a percentage of the actual body weight. See Table 35-1 for specific recommendations.

Postintubation Management

The changes in the anatomy and physiology of obese patients have important implications for ventilator management. The initial tidal volume should be calculated based on IBW and then adjusted according to airway pressures, with the success of oxygenation and ventilation indicated by pulse oximetry and capnography, or arterial blood gas monitoring. Generally, the use of positive end-expiratory pressure is recommended to prevent end-expiratory airway closure and atelectasis, particularly in the posterior lung regions. In severe or superobesity, it may be necessary to ventilate the patient in the semierect position to move the weight of the breasts, abdominal fat, or pannus off the chest wall.

Portable bedside radiographs are usually of poor quality in the obese patient, limiting their clinical value, although one can usually determine if the endotracheal tube (ETT) is in a mainstem bronchus.

When considering extubation of the obese patient, a conservative approach should be taken. Review documentation regarding the difficulty of BMV and tracheal intubation, and consider the possibility of the patient requiring emergent reintubation.

Tips and Pearls

· The predicted difficulty in intubation combined with the decreased physiological reserve in obese patients makes timely airway management important, and the decision to intubate cannot be delayed.

· Most tracheostomy tubes will not be long enough for the morbidly obese patient; a 6-mm inner diameter ETT may be advanced through the cricothyrotomy incision.

Evidence

1. Is obesity an independent risk factor for difficult intubation? Classically, obesity has been considered an independent risk factor for difficult intubation; however, with increasing clinical airway experience with these patients, this has become debatable. Brodsky et al. (1) looked at 100 obese patients and characterized whether they were difficult to intubate. They found that the obese population encounters a frequency of difficult intubations comparable to the general surgical population (1). Conversely, a study by Juvin et al. (2) in obese versus lean patients showed that obesity was associated to an increased risk of difficult intubation. Further analysis of these studies demonstrated that the level of BMI above that defined as obesity was not an independent risk factor for intubation difficulties, whereas Mallampati scores of 3 and 4 were risk factors. Brodsky et al. (1) additionally found that increasing neck circumference was the most important independent risk factor for difficult intubation in this patient population, with a 5% risk at 40 cm and a 35% risk at 60 cm. Despite finding similar risk factors, these two groups disagree as to whether obesity should be considered an independent risk factor for difficult intubation. Notably, the different outcome between the two reports may be accounted for by different definition of “difficult airway.” In fact, Brodsky et al. had a 12% incidence of “problematic” intubations, although this was not further defined.

In conclusion, it is likely that obesity, by itself, is a marker for difficult airway management, and that the obese patient, like the lean patient, may, in addition, have numerous other markers of difficult airway management.

2. What is the best position for preoxygenation and intubation of the obese patient? Positioning is essential to airway management of the obese airway. In the supine position, obesity results in a relative neck extension, leading to a more anterior placement of the larynx. In addition, in the supine position the FRC approaches closing capacity, creating a V/Q mismatch, shunting, and consequent hypoxemia (3). Moreover, it may be difficult to obtain adequate preoxygenation of the patient. Altermatt et al. (4) studied the effect of supine versus sitting position on preoxygenation of obese patients. Patients preoxygenated in the sitting position had 216 seconds of safe apnea time versus 124 seconds in the supine group. Schurmann et al. (5) also showed a significant extension of the “safe apnea period” by employing a 30-degree reverse Trendelenburg position prior to induction of anesthesia. Because the weight of the chest and abdomen is displaced off the airway in this position, BMV is facilitated with lower peak inspiratory pressures (6).

As shown in Figure 35-1B, when the patient is propped on linens or a commercially manufactured device, the airway becomes well defined and removes the weight of the chest, facilitating intubation. To obtain this position, the planking of the patient needs to start at the midpoint of the back and extend to the head (7). When in doubt about the position, take a lateral view of the patient to assess if an imaginary horizontal line can be drawn from the external auditory meatus to the angle of Louis (7). See Figure 35-1A and B.

3. What is known about the ILMA in obese patients? The ILMA was designed based on MRI studies of normal weight subjects, but efficacy has been demonstrated in morbidly obese patients (8). Combes et al. (8) compared a group of lean patients to a group of obese patients using the ILMA as the primary airway tool. All patients were successfully intubated with the ILMA, but obese patients required fewer manipulations of the ILMA than lean patients and had fewer failed blind passes of the endotracheal tube and fewer esophageal intubations. Frappier et al. (9) found similar results in their study comparing standard direct laryngoscopy versus ILMA in morbidly obese patients. The ILMA not only provided adequate ventilation, but also achieved intubation in 96.7% of their patients.

The theory behind the efficacy of the ILMA is based on MRIs of obese patients. MRI has shown that decreased pharyngeal area and volume are caused by deposition of adipose tissue, predominately in the lateral pharyngeal walls (8). This information may serve to guide the ILMA into place and stabilize its position with cuff inflation. Furthermore, the ILMA provides for adequate ventilation, whereas BMV may prove to be difficult when collapse of the pharynx occurs with induction. Thus, the ILMA should be considered a valuable and complementary tool to conventional laryngoscopy.

4. What is the dose of succinylcholine to achieve the best intubating conditions? In the morbidly obese patient, increased extracellular volume is often associated with increased levels of plasma pseudocholinesterase, both factors playing a role in the duration of action of succinlycholine. Lemmen and Brodsky (10) found that obese patients receiving IBW and LBW doses of succinylcholine had significantly less blockade, with 33% and 27% having poor intubating conditions, respectively. All patients receiving total body weight (TBW) dosing displayed adequate intubating conditions, although recovery time was prolonged. Of note, even the IBW had a recovery time of 5 minutes, which is still inadequate for resuming spontaneous ventilation before hypoxemia develops in the obese patient (typically 2–3 minutes) (10). Despite the relatively long recovery time with succinylcholine, even with IBW dosing, it is wise to achieve the best intubating conditions possible, hence the recommendation for TBW dosing.

5. Are obese patients at greater risk for aspiration? It has been traditionally believed that obesity poses a relatively high risk of aspiration. However, this evidence has been challenged by a report from Tasch et al. (11) that showed lack of association between the degree of obesity and gastric volume or pH. Furthermore, Verdich (12) reported no difference in rates of gastric emptying or lower esophageal sphincter tone.

6. What is the evidence for drug dosing based on IBW versus LBW versus TBW? Obesity affects all aspects of pharmacokinetics and pharmacodynamics; thus, it is difficult to predict how an individual patient will respond to a particular agent. Further complicating the issue, there are few studies that have investigated the effects of obesity on the disposition of anesthetic agents. As a result, it is unclear whether weight-related dosage adjustments should be made and whether these should be based on actual weight, ideal weight, or a percentage of the actual body weight. Many drugs may necessitate a higher dosage because of the lipophilicity of the agent, but the dose is limited by cardiovascular depression, hence a LBW dosing method (13). Because others (e.g., vecuronium) have shown to have increased effect even with LBW dosing, despite the increase in Vd from the extra lean body mass, an IBW method is recommended (14). In essence, the best method for dosing an agent is by careful titration to effect (3). In some instances, titration may not be possible, thus empiric drug dosing must be performed in order to achieve a certain goal, whether it be induction or paralysis for intubation (15,16). Refer to Table 13.1 for specific recommendations.

References

1. Brodsky JB, Lemmens HJM, Brock-Utne JG, et al. Morbid obesity and tracheal intubation. Anesth Analg 2002;94:732–776.

2. Juvin P, Lavaut E, Dupont H, et al. Difficult tracheal intubation is more common in the obese than lean patients. Anesth Analg 2003;97:595–600.

3. Adams JP, Murphy PG. Obesity in anaesthesia and intensive care. Br J Anaesth 2000;85(1):91–108.

4. Altermatt FR, Munoz HR, Delfino AE, et al. Pre-oxygenation in the obese patient: effects of position on tolerance to apnea. Br J Anaesth 2005;95(5):706–709.

5. Schumann R, Jones SB, Ortiz VE, et al. Best practice recommendations for anesthetic perioperative care and pain management in weight loss surgery. Obesity Res 2005;13(2):254–266.

6. Leykin Y Pellis T, Lucca M, et al. The effects of cisatracurium on morbidly obese women. Anesth Analg 2004;99:1090–1094.

7. Brodsky JB, Lemmens HJM, Brock-Utne JG, et al. Anesthetic considerations for bariatric surgery: proper positioning is important for laryngoscopy. Anesth Analg 2003;96:1841–1842.

8. Combes X, Saurat S, Leroux B, et al. Intubating laryngeal mask airway in morbid obese and lean patients. Anesthesiology 2005;102(6):1106–1109.

9. Frappier J, Guenoun T, Journois D, et al. Airway management using the intubating laryngeal mask airway for the morbidly obese patient. Anesth Analg 2003;96:1510–1515.

10. Lemmen HJM, Brodsky JB. The dose of succinlycholine in morbidly obesity. Anesth Analg 2006;102:438–442.

11. Tasch MD, Stoelting RK. Aspiration prevention and prophylaxis: pre-op considerations. In Hagerg C, ed. Benumof's Airway Management. Philadelphia: Mosby; 2007:281–302.

12. Verdich C, Madsen JL, Toubro S, et al. Effects of obesity and major weight reduction on gastric emptying. Int J Obes Relat Metab Disord 2000;24:899–905.

13. Casati A, Putu M. Anesthesia in the obese patient: pharmacokinetic considerations. J Clin Anesth 2005;17:134–145.

14. Cheymol G. Effects of obesity on pharmacokinetics: implications for drug therapy. Clin Pharmacokinet 2000;39(3):215–231.

15. Ogunnaike BO, Jones SB, Jones DB, et al. Anesthetic considerations for bariatric surgery. Anesth Analg 2002;95:1793–1805.

16. Lemmens HJM, Brodsky JB. Anesthetic drugs and bariatric surgery. Expert Rev Neurotherapeut 2006;6(7):1107–1113.