Chapter 298: The Patient With Morbid Obesity

Since 1980, worldwide obesity has more than doubled. In 2008, more than 1.4 billion adults, 20 and older, were overweight. Of these, over 200 million men and nearly 300 million women were obese. Sixty-five percent of the world population resides in countries where overweight and obesity kill more people than underweight. In 2010, more than 40 million children under the age of 5 were overweight.¹

In children, an age- and sex-specific percentile for body mass index (BMI) determines weight status rather than the BMI categories used for adults, because children's body composition varies as they age and varies between boys and girls.

The Centers for Disease Control and Prevention defines overweight as a BMI at or above the 85th percentile and lower than the 95th percentile for children of the same age and sex.² Obesity is a BMI at or above the 95th percentile for children of the same age and sex.² The World Health Organization definition is as follows: a BMI ≥25 is overweight, whereas a BMI ≥30 is obesity.¹

Obesity is an independent risk factor for acute coronary syndrome, especially in those <40 years old.^3,4 Atypical symptoms may pose a problem with acute coronary syndrome diagnosis.^5,6 About 11% of cases of congestive heart failure are attribuTable to obesity alone.⁷ The physical deconditioning of obesity manifest by orthopnea, dyspnea, and lower extremity swelling is very similar to symptoms of acute congestive heart failure, making diagnosis problematic. Plain chest x-ray findings of congestive heart failure may be obscured by redundant overlying soft tissue and hypoventilation. Brain natriuretic peptide levels are lower in the obese patient than in the nonobese.^8,9 Cardiomyopathy may affect up to 10% of patients with a BMI >40 kg/m² and those with a long duration of significant obesity.¹⁰ Obesity is a risk factor for venous thromboembolism¹¹ and its recurrence once anticoagulation therapy is withdrawn.¹²

The increase in the prevalence of type 2 diabetes is closely linked to the upsurge in obesity. About 90% of type 2 diabetes is attribuTable to excess weight.¹³ Obesity has been strongly associated with insulin resistance in normoglycemic persons and in individuals with type 2 diabetes.¹⁴

The accumulation of fat impairs the function of ventilation in obese children and adults.^15,16,17 Reductions in forced expiratory volume in 1 second, forced vital capacity,^15,16 total lung capacity, functional residual capacity, and expiratory reserve volume are associated with increasing BMI.¹⁸

Obesity is a well-recognized risk factor for obstructive sleep apnea. Forty percent of people who are obese have obstructive sleep apnea, and approximately 70% of people with obstructive sleep apnea are obese.¹⁹

Increased fat deposition in the pharyngeal area along with reduced operating lung volumes associated with obesity reduce upper airway caliber, modifying airway configuration, which in turn increases upper airway collapsibility. Thus, airways are predisposed to repetitive closure during sleep.²⁰ Daytime sleepiness increases and may be associated with accidental trauma.¹⁹

Cor pulmonale and hypercapnic respiratory failure are common. Obesity hypoventilation syndrome (Table 298-1) was first described over 50 years ago.^21,22 The most common symptoms are (1) respiratory failure, (2) severe hypoxemia, (3) hypercapnia, and (4) pulmonary hypertension.^22,23,24

The Broselow tape inaccurately predicts actual weight in one third of children.²⁵ The significance of this inaccuracy has not been studied in depth. A weight-estimation formula based on mid-arm circumference is reliable for use in school-age children and may be an alternative to the Broselow tape.²⁶ The formula is as follows: weight (kg) = (mid-arm circumference [cm] – 10) × 3. When compared to the Argal, Advanced Pediatric Life Support, and Best Guess formulas, Krieser et al²⁷ found that parental estimation of weight was more accurate.²⁷

The concern for equipment weight capacity in the adult patient with obesity is an important determination for imaging. Scales in most EDs have a maximum weight capacity of 150 kg. Mechanized beds that weigh patients are not common in the ED but might be a consideration. Patients with obesity tend to significantly underestimate their own weight. A variety of formulas are available to estimate weight in adults who are obese using height and waist, hip, and arm circumference. The formula developed by Crandall et al²⁸ seems to require the least amount of time and patient manipulation. Two distinct formulas for nonpregnant females and males have been developed as follows:

Improper blood pressure cuff width and circumference will artificially elevate pressure readings. The standard adult blood pressure cuff is too short for patients with an arm circumference of 32 cm or larger. Patients who are overweight or obese will require cuffs larger in size.

The American Heart Association recommends the following cuff widths when evaluating blood pressure in patients who are obese: (1) for arm circumferences ranging from 35 to 44 cm, a bladder measuring 16 cm in width is needed; (2) for circumferences from 45 to 52 cm, the bladder width should be 20 cm; and (3) in patients with short upper arm length, a 16-cm-wide cuff should be used.^29,30

Little evidence-based literature is available for appropriate dosing in obesity, and nearly none is available in the obese child. Fortunately, many drugs used in resuscitation are not lipophilic, and lean body mass is a reasonable dosing guide. Altered physiology is characterized by an increased clearance of hydrophilic drugs, a larger volume of distribution for lipophilic drugs, and a decrease in lean body mass and tissue water content, as compared to their lean counterparts.³¹ Altered mechanics can predispose the morbidly obese to systemic toxicity due to either overdosing or lack of efficacy from underdosing.^32,33

A weight-based medication schedule uses ideal body weight, total body weight, or dosing weight to avoid systemic side effects and lack of clinical efficacy by underdosing. Ideal body weight according to the Devine formula is as follows³⁴:

Dosing weight is an adjusted body weight of overweight or obese patients and is used only for drugs for which there are recommendations specifying that the actual body weight should be adjusted to use in the dose calculation.

Table 298-2 divides select drugs into ideal body weight, total body weight, and dosing weight dosing.³⁵ Fentanyl and the benzodiazepines are lipophilic and have a prolonged half-life in obese patients. With these drugs, the initial dose based on total body weight may be needed, but subsequent doses should be based on ideal body weight.³⁶ It is best to check with a pharmacist for specific dosage regimens.

Vascular access is problematic. Patients who are critically ill and morbidly obese patients require fluid administration often guided by central venous pressure and urine output.³⁷ Central venous pressure placement is extremely challenging even for the most skilled physician. In the obese patient, the distance from skin to vessel is much further than normal, anatomic landmarks are obscured (Figure 298-1, A and B), and the angle of approach may be too steep to allow cannulation even after reaching the vessel. There is no clear consensus as to the preferable site and approach to central venous catheterization. In general, there is an increased incidence of infection and deep venous thrombosis when using the femoral approach.³⁸ If this proves to be the only option, then use this site.

The internal jugular vein can be accessed with equal success to the subclavian approach in patients who are obese. The success rate might be increased with the head maintained in the neutral position, thereby reducing the risk of overlap of the internal jugular vein over the carotid artery.³⁹

A US-guided 15-cm catheter can be used to cannulate the brachial or basilic vein.⁴⁰ Another approach is the use of a pediatric central venous catheter placed into the basilic vein. The pediatric central venous catheter is 8 cm (3.15 in) in length, is a double-lumen catheter, and has 18- and 20-gauge lumens.⁴¹ Longer catheters can also be considered to guard against inadvertent dislodgement.

Attenuation severely limits the image quality of plain radiographs (Figure 298-2). Increasing exposure time can improve the image but at the expense of increased radiation. Motion artifact increases with increased exposure time. Multiple cassettes may be required if the patient is too large for a single 14 × 17–inch film. Patients may be able to stand for plain radiography if too large for the tables.⁴²

Newer CT scanners can accommodate up to 660 lbs. If the patient outweighs equipment capacity, veterinary schools and the local zoo are options.

In the patient with obesity and blunt abdominal trauma who is too large for imaging equipment, consider diagnostic peritoneal lavage.⁴³ Most standard MRIs have a maximum shoulder-to-shoulder width of 52 inches (137 cm) and weight limits of 300 to 350 lb (136 to 159 kg).

Give procedural sedation drugs and pain medications cautiously. Select doses at the lower end of the range, and titrate to effect. Local and regional anesthesia might be considered for complicated or prolonged procedures.⁴⁴

Difficulty with mask ventilation, rapid oxygen desaturation, and altered pharmacokinetics can make airway management challenging.⁴⁵ Impedance to airway management is caused by excess fatty tissue externally on the breast, neck, thoracic wall, and abdomen and internally in the mouth, pharynx, and abdomen. The respiratory compromise that is associated with morbid obesity leaves little room for error.

Patients who are obese have increased intra-abdominal pressure and increased incidence of hiatal hernia and gastroesophageal reflux disease. These characteristics render patients more prone to aspiration during airway management.⁴⁶

Patients who are obese will desaturate more rapidly after preoxygenation than their lean counterparts. When no cervical spine injury is suspected, desaturation may be partially prevented by keeping the patient in a 25-degree head-up position during preoxygenation.⁴⁷

Two-person bag-valve mask (BVM) with a two-handed bilateral jaw thrust is recommended in patients who are morbidly obese. If tolerated, an oral airway may be used to prevent the tongue from occluding the airway. The early use of noninvasive positive-pressure ventilation may abate the need for endotracheal intubation. High expiratory positive pressures may be needed.

Obesity is not a contraindication for rapid sequence intubation. Advance preparation is critical, and assessment for a potential difficult airway is of utmost importance.⁴⁸

The "sniffing" position results in suboptimal positioning for laryngoscopy in patients who are obese, and this may also confound results and falsely worsen graded views.⁴⁹ The "ramping" position (Figure 298-3A) offers improved intubation conditions in patients who are morbidly obese compared to the "sniffing" position (Figure 298-3B). This position is achieved by placing multiple folded blankets under the upper body, head, and neck until the external auditory meatus and the sternal notch are horizontally aligned.⁵⁰

First give consideration to awake intubation, given that patients who are obese may be difficult to mask ventilate and rapid oxygen desaturation may occur after the ablation of spontaneous ventilation, especially in patients with a BMI greater than 40 kg/m².^51,52,53 The awake intubation may be performed either by the nasotracheal or orotracheal routes.

The relative benefits and risks of the awake intubation approach must be weighed against the merits of rapid sequence intubation, which reduces risk of aspiration, improves intubating conditions, and results in easier insertion of advanced and rescue airway devices. During rapid sequence intubation, the chance for first-pass success can be optimized by video laryngoscopy. The intubating laryngeal mask airway is effective in obesity and should be readily available because surgical access to the airway may be difficult. The bougie is a good rescue device.⁵⁴

Percutaneous and open surgical access to the airway may be difficult when landmarks are obscured by excess soft tissue and a short neck in the "cannot intubate, cannot ventilate" scenario.⁵⁵ Obesity and a short neck are associated with difficult transtracheal needle ventilation and retrograde tracheal intubation.^56,57 However, in the elective surgical airway management setting, cricothyroidotomy is technically feasible even in patients with difficult neck anatomy caused by obesity.⁵⁸ Unfortunately, the luxury of a controlled environment is often not an option for the emergency physician.

Until the proper size tracheostomy tube is located, a 6-mm-inner-diameter endotracheal tube passed through a cricothyroidotomy incision may serve as a temporizing measure.⁵⁹ There is limited literature concerning the success rates of surgical airways in patients in the emergency setting. Even in an ideal setting, cricothyroidotomy requires more than 100 seconds to achieve ventilation,⁵⁷ and the procedure is rarely performed in the ED.⁶⁰

With respect to ventilator management, adjust tidal volume as per body weight and initially keep it at 8 mL/kg of ideal body weight in patients in whom mechanical ventilation is necessary, but patients with acute respiratory distress should be ventilated with much lower tidal volume. Lower tidal volumes can be compensated for by increasing the respiratory rate to maintain normal minute ventilation and thus avoiding hypoxemia and hypercarbia.⁶¹ During prolonged ventilation, ventilatory settings are determined by serial measurements of arterial blood gasses and peak airway pressures.⁶²

In the patient who is obese, lumbar puncture is best performed in the sitting position. US may help identify vertebrae.⁶³ A formula for predicting the required lumbar puncture depth was developed by Abe et al.⁶⁴ The formula is as follows:

Success rate can increase with the awareness of the predicted depth.⁶⁵

A 22- to 24-gauge needle allows adequate flow and easier passage and may also decrease the likelihood of postpuncture headache. Fluoroscopy may be required in extreme, technically difficult, or unsuccessful cases.

Victims of motor vehicle accidents who are obese (BMI >31 kg/m²) have significantly more rib fractures, pelvic fractures, pulmonary contusions, and extremity fractures and fewer head and liver injuries.⁶⁶

Mildly overweight patients, not the morbidly obese, are less prone to intra-abdominal injury because of the protective effect of the abdominal fat, known as the "cushion effect."⁶⁷