![]() In the presence of a modest reduction in TLC and a well-preserved RV, the reduction in FRC is manifested by an increase in inspiratory capacity and a very marked decrease in the expiratory reserve volume (ERV) ( 28). The RV is usually well preserved ( 5, 11, 48, 63, 67), and the RV-to-TLC ratio remains normal or slightly increased ( 5, 28). Many studies report an association between increasing body weight and decreasing TLC ( 11, 28, 42) however, the changes are small, and TLC is usually maintained above the lower limit of normal, even in severe obesity ( 11, 28, 63). However, the effects of obesity on the extremes of lung volumes, at total lung capacity (TLC) and RV, are modest. In obesity, the reduction in FRC may become so marked that the FRC approaches residual volume (RV). There is an exponential relationship between BMI and FRC ( 28, 40), with a reduction in FRC detectable even in overweight individuals ( 28). This effect reflects a shift in the balance of inflationary and deflationary pressures on the lung due to the mass load of adipose tissue around the rib cage and abdomen and in the visceral cavity ( 52). The most consistently reported effect of obesity on lung function is a reduction in the functional residual capacity (FRC) ( 28, 40). Thus obesity has effects on lung function that can reduce respiratory well-being, even in the absence of specific respiratory disease, and may also exaggerate the effects of existing airway disease. Bronchoconstriction has the potential to enhance the effects of obesity on airway closure and thus on ventilation distribution. Greater airway closure during tidal breathing is associated with lower arterial oxygen saturation in some subjects, even though lung CO-diffusing capacity is normal or increased in the obese. Marked reductions in expiratory reserve volume may lead to abnormalities in ventilation distribution, with closure of airways in the dependent zones of the lung and ventilation perfusion inequalities. A low FRC increases the risk of both expiratory flow limitation and airway closure. Nevertheless, the reduction in FRC has consequences for other aspects of lung function. Spirometric variables decrease in proportion to lung volumes, but are rarely below the normal range, even in the extremely obese, while reductions in expiratory flows and increases in airway resistance are largely normalized by adjusting for lung volumes. However, obesity has little direct effect on airway caliber. The reduction in FRC and in expiratory reserve volume is detectable, even at a modest increase in weight. In obese people, the presence of adipose tissue around the rib cage and abdomen and in the visceral cavity loads the chest wall and reduces functional residual capacity (FRC). ![]()
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