Morbidly obese individuals have increased risk for multiple disorders, among them impaired respiratory function; RTs need to be aware of specific problems among this population
Approximately 65% of American adults are overweight, according to the Centers for Disease Control and Prevention (CDC).1 Thirty percent of these individuals are obese, and 4.7% are classified as morbidly obese (class III obesity).2 In the absence of a national registry, the prevalence of critically ill obese patients in the United States is unknown. On the basis of a retrospective study spanning more than 7 years of intensive care data (1995-2002), it is estimated that the incidence of morbidly obese patients requiring intensive care treatment approaches 14 cases per 1,000 ICU admissions per year.3 Clinical interventions must be adjusted to this patient population and potential complications from morbid obesity guarded against.
As more of this patient population is admitted to critical care units, the respiratory therapist needs to understand the scope of care and interventions that are required to optimize clinical outcomes.
Classifications and Prevalence of Obesity
Obesity is epidemic among the general population in the United States. The National Health and Nutrition Examination Survey (NHANES) conducted in 1999-2002 estimated that 66% of the US population is overweight.4 To arrive at this statistic, NHANES used the body mass index (BMI), in which >25 kg/m2 is considered overweight, a BMI >30 kg/m2 is obese, and a BMI >40 kg/m2 is morbidly obese. Although the prevalence of all levels of obesity is escalating, extreme obesity has sustained the greatest proliferation.5 The World Health Organization revealed that obesity is more prevalent among certain ethnic and racial groups and that gender and age both play a role in the likelihood of obesity.6 Among men, obesity is equally prevalent for Mexican Americans, blacks, and whites; but among women, blacks and Mexican American are more likely to be obese. The likelihood of having a BMI >25 tends to increase around age 35 and decline around age 75 in both genders. Women are more likely than men to be obese in all age groups; the percentage of woman with a BMI of 30 kg/m2 is greater than for men.7
Physical Consequences of Obesity
Obesity increases the risk of diabetes, heart disease, hyperlipidemia, arthritis, sleep apnea, gallstone formation, and certain cancers, including cancers of the breast, colon, pancreas, and kidney.8 These health risks not only exist secondary to the excess weight, but also burden various organs and joints and impair the immune system. Having too much adipose tissue can cause a release of endogenous hormones and proinflammatory cytokines, which have been linked to causing a systemic inflammatory response.9
Obesity and Its Impact on Respiratory Function
Obesity imposes a restrictive load on the thoracic cage, by both directly adding weight to the thoracic cage and indirectly because of the large abdominal panniculus, which can impede the motion of the diaphragm when the person is supine. In addition, obese patients, particularly men, may experience increased respiratory resistance and resultant airflow limitations that may be related to breathing at lower lung volumes, and increases in pulmonary blood volume, or both. Obesity characteristically causes a decrease in functional residual capacity that becomes significant only in the presence of coexisting conditions such as obstructive lung disease, in which airway closure occurs at lower lung volume, leading to hypoxemia.
Obesity in otherwise healthy patients causes little interference with lung function at rest. Generally, vital capacity and total lung capacity remain normal except in the most severe instances of morbid obesity.
Obese patients can experience significant dyspnea during exercise because of the increased work required to move the heavy chest and abdomen and because of overall poor conditioning. The tachypneic shallow breathing pattern during exercise in morbidly obese patients reflects the combined effects of this mass loading and diminished compliance of the respiratory system.
In patients with impaired ventilatory drive, the mechanical workload imposed by obesity may not be countered by increased respiratory effort. Under such circumstances, chronic daytime hypercapnia may develop, most commonly in the setting of obstructive sleep apnea, but also in the absence of sleep-disordered breathing. The pathogenetic role of obesity in obstructive sleep apnea may relate in part to fatty encroachment on the upper airways.
Endotracheal intubation can be a daunting task in critically ill obese patients. Limited neck mobility and mouth opening accounted for most cases of difficult intubation in obese subjects.10 Short neck distance, receding mandible, and prominent teeth have been advanced as potential causes for difficult intubations. When performing tracheal intubation with the morbidly obese patient placed in the supine position, the functional residual capacity decreases. This is the consequence of a reduction in the expiratory reserve volume. Arterial desaturation occurs rapidly, and attempts to maintain adequate gas exchange by using mask and bag ventilation may be difficult to accomplish. Because of these challenges, several clinicians have recommended conscious intubation when laryngeal structures cannot be visualized or when the actual body exceeds twice the ideal body weight. End-tidal CO2 indicators may prove unreliable in monitoring intubation in the presence of a widened arterial-to-alveolar Paco2.11
For obese patients requiring tracheotomy, standard tracheostomy tubes are typically too short and too curved for proper positioning given the distance between the skin and the trachea. Consequently, they are more likely to be dislodged or occluded. It has been advocated to insert a custom-fitted tracheostomy tube in morbidly obese patients. Percutaneous dilational tracheostomy is considered a poor choice for morbidly obese patients with large, thick necks, but recently in a case series of 13 obese patients, the procedure was associated with a high success rate and a low complication rate.12
The mechanical properties of the total respiratory system, the lung, and the chest wall of morbidly obese patients are characterized by marked derangements compared with subjects of normal weight. As lung volumes are reduced and airway resistance increased, a tidal volume based on the patient’s actual weight is likely to result in high airway pressures, alveolar overdistention, and barotrauma. The current consensus favors initial tidal volume be calculated according to ideal body weight and then minute ventilation adjustments made secondary to arterial blood gases results. Due to reduced compliance of the total respiratory system, airway inflation pressure should not exceed 35 cm/H2O. The addition of positive-end-expiratory pressure (PEEP) should be used to facilitate alveolar recruitment and prevent atelectasis. The addition of 10 cm/H2O in sedated, paralyzed morbidly obese patients reduced elastance of the respiratory system and improved oxygenation when compared to nonobese subjects.13
Recently, the use of airway pressure release ventilation (APRV) has gained popularity as a ventilatory strategy in the obese patient population.14 Utilizing a high mean airway pressure and a long inspiratory time, APRV helps overcome the thoracic and abdominal impedance association in this patient population. Ventilator liberation via a level of CPAP greater than 10 cm H2O may help in reducing the work of breathing by unloading respiratory muscle work by providing airway splitting and compensating for imposed impedance. Current lung protective ventilatory strategies need to be utilized in this patient population but with customization. The PEEP levels may need to be increased to compensate for the large thoracic and abdominal mass.
In conclusion, the respiratory care practitioner needs to understand the pathophysiology and increased respiratory work associated with obesity. The ability to custom tailor clinical interventions and therapeutic modalities will be paramount in optimizing care to this patient population. If future trends are correct, this will become a major focus of all health care personnel. Ventilatory strategies and weaning techniques need to be geared toward a clinical end point that is patient specific for this growing patient population.
Kenneth Miller, MEd, RRT-NPS, is clinical educator, respiratory care, Lehigh Valley Hospital, Allentown, Pa.
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