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