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Obesity and pulmonary function testing

2005; Elsevier BV; Volume: 115; Issue: 5 Linguagem: Inglês

10.1016/j.jaci.2004.12.1141

1097-6825

David A. Beuther, Earl W. Sutherland,

Asthma and respiratory diseases

Obese patients frequently present with respiratory symptoms, including dyspnea, explained in part by the fact that obese individuals tend to breathe rapidly and shallowly as an adaptation to the increase in total respiratory work and resistance caused by obesity.1Kaufman B.J. Ferguson M.H. Cherniack R.M. Hypoventilation in obesity.J Clin Invest. 1959; 38: 500-507Crossref PubMed Scopus (56) Google Scholar However, the clinician is frequently asked to determine whether these findings are simply caused by obesity alone or whether they represent a respiratory illness. Accurate interpretation of spirometry performed on obese patients requires an understanding of the effect of obesity severity and distribution on lung volumes and airway size. In mild obesity, results of spirometry might be normal or might suggest a restrictive process, with a symmetric reduction in FEV1 and forced vital capacity (FVC).2Collins L. Hoberty P.D. Walker J.F. Fletcher E.C. Peiris A.N. The effect of body fat distribution on pulmonary function tests.Chest. 1995; 107: 1298-1302Crossref PubMed Scopus (223) Google Scholar Some investigators have observed a disproportionate reduction in FVC with obesity, demonstrating that body mass index (in kilograms per square meter) is significantly associated with the FEV1/FVC ratio (P < .01).3Lazarus R. Sparrow D. Weiss S.T. Effects of obesity and fat distribution on ventilatory function.Chest. 1997; 111: 891-898Crossref PubMed Scopus (253) Google Scholar In contrast, individuals with extreme obesity can demonstrate airflow limitation on spirometry. In one study of subjects with a body mass index of greater than 62 kg/m2, there was a reduction in the FEV1/FVC ratio and midexpiratory flow rate.4Biring M.S. Lewis M.I. Liu J.T. Mohsenifar Z. Pulmonary physiologic changes of morbid obesity.Am J Med Sci. 1999; 318: 293-297Crossref PubMed Google Scholar Differences in body fat distribution are also important. For example, in one study FVC, FEV1, and total lung capacity (TLC) were significantly lower in patients with a waist-to-hip ratio of 0.95 or greater compared with values in those with a waist-to-hip ratio of less than 0.95 (P < .05).2Collins L. Hoberty P.D. Walker J.F. Fletcher E.C. Peiris A.N. The effect of body fat distribution on pulmonary function tests.Chest. 1995; 107: 1298-1302Crossref PubMed Scopus (223) Google Scholar, 3Lazarus R. Sparrow D. Weiss S.T. Effects of obesity and fat distribution on ventilatory function.Chest. 1997; 111: 891-898Crossref PubMed Scopus (253) Google Scholar Full pulmonary function tests are often necessary to better characterize the spirometric abnormalities seen in the obese patient (Fig 1). The most sensitive indicator of obesity is a low expiratory reserve volume (ERV) and functional residual capacity.4Biring M.S. Lewis M.I. Liu J.T. Mohsenifar Z. Pulmonary physiologic changes of morbid obesity.Am J Med Sci. 1999; 318: 293-297Crossref PubMed Google Scholar Restriction is seen in more severe obesity, with reductions in TLC and FVC. However, residual volume is often preserved because of the relative high closing volume in relation to ERV.5Sharp J.T. Henry J.P. Sweany S.K. Meadows W.R. Pietras R.J. The effects of mass loading on the respiratory system in man.J Appl Physiol. 1964; 19 (959-6)PubMed Google Scholar These effects of obesity have been confirmed in studies of obese patients before and after bariatric surgery. In a study of 34 obese women before and 1 year after gastric banding, resulting in a mean weight loss from 113 to 82 kg, TLC, functional residual capacity, and ERV increased significantly from 93%, 77%, and 64% of predicted value to 98%, 98%, and 109% of predicted value, respectively (P < .001 for each measure).6Refsum H.E. Holter P.H. Løvig T. Haffner J.F. Stadaas J.O. Pulmonary function and energy expenditure after marked weight loss in obese women: observations before and one year after gastric banding.Int J Obes. 1990; 14: 175-183PubMed Google Scholar There are conflicting data on whether diffusing capacity is normal or increased in obesity, but this study showed that although PaO2 increased approximately 6% after surgery (P < .001), there was no significant change in diffusing capacity. The definition of asthma (reversible airflow limitation, airway hyperresponsiveness [AHR], and airway inflammation) requires no modification for obese patients with asthma, but proper interpretation of pulmonary function tests in this population might require a careful examination of lung volumes. As in nonasthmatic patients, obesity might cause a restrictive pattern on pulmonary function tests, with a symmetric reduction in airflow and a reduction in lung volume. When this physiology is combined with the airflow limitation and hyperinflation seen with asthma, a mixed pattern can result, resulting in a pseudonormalization of lung volumes. Obese asthmatic patients should still demonstrate a postbronchodilator improvement in FEV1 of at least 12% (Fig 2). Although an increased prevalence of asthma has been reported in the obese,7Camargo C.A. Weiss S.T. Zhang S. Willett W.C. Speizer F.E. Prospective study of body mass index, weight change, and risk of adult-onset asthma in women.Arch Intern Med. 1999; 159: 2582-2588Crossref PubMed Scopus (655) Google Scholar there are conflicting data on whether obesity causes AHR or airway inflammation. In general, obesity is associated with low lung volumes and therefore smaller airway caliber, and airway narrowing has been shown to worsen AHR.8Litonjua A.A. Sparrow D. Weiss S.T. The FEF25-75/FVC ratio is associated with methacholine airway responsiveness.Am J Respir Crit Care Med. 1999; 159: 1574-1579Crossref PubMed Scopus (65) Google Scholar Airway smooth muscle stretch is a potent bronchodilator, and because obese patients breathe small tidal volumes at low lung volumes, they might have reduced airway smooth muscle stretch and therefore worsening AHR.9Tantisira K.G. Weiss S.T. Complex interactions in complex traits: obesity and asthma.Thorax. 2001; 56: II64-II73PubMed Google Scholar Additionally, some investigators have observed a stronger association between obesity and asthma in women,10Chen Y. Dales R. Krewski D. Breithaupt K. Increased effects of smoking and obesity on asthma among female Canadians: the national population health survey, 1994-1995.Am J Epidemiol. 1999; 150: 255-262Crossref PubMed Scopus (216) Google Scholar which has been theorized to be the result of either an increase in estrogen levels or a smaller airway caliber in women. Obesity is a systemic inflammatory disorder associated with increased levels of leptin, TNF-α, and IL-6, but whether systemic or pulmonary inflammation has a role in asthma in patients with obesity remains to be determined.