Effects of Obesity on Respiratory Function

TLDR

Obesity is expected to alter respiratory function because it changes the relationship between the lungs, chest wall, and diaphragm. The authors examined 43 massively obese, otherwise healthy young adults using spirometry, nitrogen washout, and DLco, finding that respiratory changes were either proportional to obesity (ERV, DLco) or limited to extreme obesity (VC, TLC, MVV). Compared with predicted values, most subjects’ pulmonary function fell within 95% confidence limits, except those with extreme obesity, and weight loss in 29 participants improved VC, ERV, and MVV while reducing DLco, indicating that obesity generally does not preclude use of standard predictors and abnormal tests should be attributed to intrinsic disease unless extreme obesity is present.

Abstract

Obesity, because it alters the relationship between the lungs, chest wall, and diaphragm, has been expected to alter respiratory function. We studied 43 massively obese but otherwise normal, nonsmoking, young adults with spirometry, lung volume measurement by nitrogen washout, and single-breath diffusing capacity for carbon monoxide (DLco). Changes in respiratory function were of two types, those that changed in proportion to degree of obesity — expiratory reserve volume (ERV) and DLco—and those that changed only with extreme obesity— vital capacity, total lung capacity, and maximal voluntary ventilation. When compared with commonly used predicting equations, we found that mean values of subjects grouped by degree of obesity were very close to predicted values, except in those with extreme obesity in whom weight (kg)/height (cm) exceeded 1.0. In 29 subjects who lost a mean of 56 kg, significant increases in vital capacity, ERV, and maximal voluntary ventilation were found, along with a significant decrease in DLco. Because most subjects fell within the generally accepted 95% confidence limits for the predicted values, we concluded that obesity does not usually preclude use of usual predictors. An abnormal pulmonary function test value should be considered as caused by intrinsic lung disease and not by obesity, except in those with extreme obesity.