Respiratory care for obese patients necessitates special approaches to ventilation and airway management

The worldwide increase in obesity has seen parallel increases in cardiovascular morbidity, type 2 diabetes mellitus, and asthma. Increasingly, obesity has been recognized as a disease entity in itself, not solely a cosmetic issue. Obesity is associated with increases in both mortality and morbidity. Obesity has reached epidemic proportions, with at least 300 million people around the world classified as obese.¹ Health care professionals will increasingly deal with overweight patients in the hospital.

The World Health Association (WHO) has defined obesity as a condition of excessive fat accumulation in the body to the extent that health and well-being are adversely affected. The WHO classification of obesity is based on the body-mass index (BMI), calculated as weight (kg) divided by height (m²). While a BMI of greater than 25 is defined as overweight, a BMI of more than 30 is defined as obese. As a widely used index to evaluate degrees of obesity, BMI shows a strong correlation with adiposity. Besides overall adiposity, however, distribution of adipose tissue appears to have important physiological consequences, with waist-to-hip ratio being a stronger predictor of cardiovascular events than BMI.

Airway Management
Obesity is frequently associated with difficult intubation. Short necks, larger neck circumferences, increased fatty tissue at the pharynx, and the large amounts of weak paratracheal tissue commonly seen in obesity can potentially reduce upper-airway caliber, leading to more challenging airway management. Juvin et al² reported difficult intubation in 2.2% of lean patients (BMI of less than 30) and 15.5% of obese patients (BMI of more than 35). Other studies,^3,4 however, have not shown a clear correlation between difficulties encountered during direct laryngoscopy and the magnitude of obesity. Nevertheless, risk factors for a difficult intubation in obese patients are a high Mallampati score, large neck circumference, limited neck mobility and mouth opening, and obstructive sleep apnea (OSA). In anticipation of potential difficulties in airway management, it might be beneficial to have airway adjuncts such as laryngeal mask airways and fiber-optic bronchoscopes readily available. Intubation while the patient is awake may be a safer option in obese patients, since sedation can cause marked derangements in pulmonary function and a tendency toward hypoxemia.⁵ Optimal positioning during face-mask ventilation and intubation is vital. A study conducted by Collins et al⁶ demonstrated that placing morbidly obese patients (BMI of more than 40) in a ramped position gives a better laryngeal view than the standard sniff position. The ramped position is achieved by elevating the upper body and head with the sternum and ear aligned in a horizontal line. Similarly, elevating the head to 25° was shown to improve oxygenation by 25%, as compared with supine positioning, in obese patients (BMI of more than 40).⁷ The inability to secure a patent airway during respiratory distress is fatal, so all measures should be taken to ensure best airway management.

Pulmonary Physiology
A high BMI and body-fat deposition impair respiratory function.^8,9 An increase in BMI typically leads to a drop in forced expiratory volume in 1 second (FEV₁), forced vital capacity (FVC), total lung capacity (TLC), functional residual capacity (FRC), and expiratory reserve volume. Besides adiposity, a central pattern of fat distribution (or abdominal obesity) was shown to be associated with lower FVC and FEV₁, as well.^9,10 FVC, TLC, and FRC may decrease by up to 30% in morbid obesity.¹¹ This general restrictive pattern in lung function assumes clinical significance only in morbid obesity. The mechanism for impairment of respiratory function in obesity remains unclear.

The restrictive pattern in pulmonary function present in obesity may be due, in part, to the deposition of fat around the chest wall, diaphragm, respiratory muscles, and ribs, as well as in the abdomen. Consequently, diaphragmatic expansion is impeded. The descent of the diaphragm into the abdominal cavity and the movement of the ribs may be limited due to fat accumulation in the abdomen. These postulations are supported by a negative correlation between BMI and measures of central obesity (such as waist-to-hip ratio) and lung volumes or spirometry results. As a result of these abnormalities, chest-wall compliance is reduced.

A less-compliant lung is evident in obesity, especially when patients are sedated/paralyzed.⁵ This can be explained by four mechanisms: collapse of the alveoli, changes in the elastic characteristics of lung tissues plus surface-lining film alterations, pulmonary vascular engorgement, or a decrease in the number or size of lung units. A lower-than-normal lung volume predisposes obese patients to expiratory flow limitation and, thus, to higher intrinsic positive end-expiratory pressure (PEEP). Hence, the resistance of the respiratory systems of obese patients tends to be higher than normal.

In the context of the pulmonary derangements present in obesity, the work of breathing is higher than normal. The high respiratory system impedance and resistance, as well as intrinsic PEEP, contribute to the heightened respiratory load. In order to compensate for this physiological disadvantage, a higher-than-normal respiratory drive, greater diaphragmatic pressure output, and (in severe obesity) the use of accessory muscles may be required to meet respiratory need.¹² Respiratory muscles that are chronically overworked can potentially result in inefficiency due to dysfunction. Obesity is commonly associated with a rapid, shallow breathing pattern and dyspnea.

It is not hard to imagine that the oxygen consumption and carbon dioxide production in obesity are higher than normal. The high metabolic demand may be attributed to excess adipose tissue and increased work of breathing from altered respiratory mechanics. Patients with morbid obesity are prone to hypoxemia and a widened alveolar-arterial oxygen gradient. This is due to ventilation-perfusion mismatching from alveolar collapse and airway closure at the lung bases in the presence of high levels of oxygen consumption. This abnormality is magnified when patients are sedated, paralyzed, or supine.

Pulmonary Management
The impact of obesity on respiratory function has important implications for the respiratory care of this specific group of patients in critical care. Most patients are sedated or paralyzed in the intensive care unit (ICU). The complications of high resistance and low compliance of their respiratory systems, as well as the poor pulmonary oxygenation frequently encountered in obesity, are exaggerated when morbidly obese patients are sedated/paralyzed. Therefore, mechanically ventilating them is potentially challenging. When mechanical ventilation is initiated, it is always convenient to set a tidal volume according to the patient's actual body weight. Since chest-wall and lung compliance are lower, FRC is lower and airway resistance is higher in obese patients; they are at risk of high peak airway pressures, and alveolar distention and barotrauma may occur, if tidal volumes are set based on their actual body weights. Instead, the initial tidal volume should be calculated according to the patient’s ideal body weight. The end-tidal carbon dioxide monitors used in ICUs can become unreliable due to the widened alveolar-arterial gradient evident in obesity. Since a ventilation-perfusion mismatch from atelectasis is the frequent cause of poor oxygenation in obese patients, it may be useful to apply therapeutic PEEP during mechanical ventilation. In fact, Pelosi et al¹³ reported significant improvements in lung compliance, chest-wall compliance, pressure-volume curve, and oxygenation with PEEP of 10 cm H₂O in morbidly obese patients. PEEP will be effective only when set sufficiently high to prevent collapse of the alveoli and airways, especially in the dependent regions. Knowing the patient’s PO₂ is helpful in the titration of PEEP to a therapeutic level. Early application of PEEP, prior to sedation or development of atelectasis, has been shown to improve oxygenation and prevent alveolar collapse.¹³ Recruitment maneuvers may be beneficial in opening collapsed regions of the lungs. The theoretical mechanism of recruitment is re-expansion of collapsed units of the lung by sustained inflation at the opening pressure.

Placing obese patients in a supine position further lowers FRC, which leads to worsening ventilation-perfusion mismatching. Hence, supine positioning should be avoided in obese patients, as it can lower pulmonary oxygenation. Rapid derecruitment of the alveoli and transient hypoxemia may occur when morbidly obese patients are supine. Reports^14,15 have shown that the reverse Trendelenburg position results in better oxygenation and respiratory-system compliance, as well as in larger tidal volumes. In instances where bedside procedures need to be performed and the patient needs to be in a supine position, a reverse Trendelenburg position may therefore be preferred.

Weaning a morbidly obese patient from mechanical ventilation can be a daunting experience, especially with the presence of pulmonary complications aggravating the existing respiratory derangements. Intubated, morbidly obese patients with pulmonary complications, besides spending more time on the ventilator, require a longer weaning time from mechanical ventilation and a higher oxygen concentration.¹⁶ Since reverse Trendelenburg position has been shown to improve respiratory mechanics, it can be postulated that caring for obese patients in that position may aid in weaning. Placing patients supine or at 90° should be avoided in conducting weaning trials. Patients with morbid obesity who have undergone gastric-bypass surgery tend to experience extended periods of mechanical ventilation and weaning.¹⁷

In a report¹⁹ on the effects of obesity on fast-track extubation for postoperative patients, the incidence of extubation failure was significantly higher in patients with BMIs of more than 30. Most patients in the study, obese or not, were successfully extubated. The decision to extubate should be approached gingerly. Once patients meet the criteria for discontinuation of mechanical ventilation, extubation should be performed in the semi-Fowler position to ensure optimal respiratory mechanics. Approximately 70% of people with OSA are obese; and some 40% of obese people have OSA.⁸ This prevalence indicates that a majority of the obese patients admitted to the ICU may have OSA and may even require concurrent nocturnal noninvasive ventilation. Its initiation after extubation should be considered for obese patients.

Conclusion
Obesity is associated with mechanical changes to the respiratory system and a heightened inflammatory state. With obesity growing worldwide, it is urgent for respiratory care providers to be cognizant of these changes.

Peggy Ler, RRT, is the respiratory therapist, Constance Lo, MBBS, MRCP, is senior consultant, and Philip Eng, MBBS, MMed, is the head and senior consultant, Department of Respiratory and Critical Care Medicine, Singapore General Hospital.

References
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