Chest
Volume 130, Issue 3, September 2006, Pages 827-833
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Original Research: Obesity
The Effects of Body Mass Index on Lung Volumes

https://doi.org/10.1378/chest.130.3.827Get rights and content

Background

Obesity is a major health issue in North America, and the trend is for obesity to be a more important medical issue in the future. Since obesity can cause respiratory symptoms, many obese people are referred for pulmonary function tests (PFTs). It is well known that obesity causes decreases in lung volumes, but there has never been a large study showing the correlation between body mass index (BMI) and the various lung volumes.

Design

We collected PFT results from 373 patients sent for lung function testing who had normal values for airway function but a wide range of BMIs.

Setting

The PFTs were done in two accredited outpatient laboratories.

Results

There were significant linear relationships between BMI and vital capacity and total lung capacity, but the group mean values remained within the normal ranges even for morbidly obese patients. However, functional residual capacity (FRC) and expiratory reserve volume (ERV) decreased exponentially with increasing BMI, such that morbid obesity resulted in patients breathing near their residual volume. An important finding was that the greatest rates of change in FRC and ERV occurred in the overweight condition and in mild obesity. At a BMI of 30 kg/m2, FRC and ERV were only 75% and 47%, respectively, of the values for a lean person with a BMI of 20 kg/m2.

Conclusions

We showed that BMI has significant effects on all of the lung volumes, and the greatest effects were on FRC and ERV, which occurred at BMI values < 30 kg/m2. Our results will assist clinicians when interpreting PFT results in patients with normal airway function.

Section snippets

MATERIALS AND METHODS

This was a retrospective study conducted between October 2004 and March 2005, and ethics approval was previously obtained to review the results of lung function. Data were collected from two laboratories with identical body plethysmograph systems. Three hundred seventy-three PFT results were selected for both male and female patients > 18 years old with normal forced expired flow rates and a smoking history of < 10 pack-years. The majority of the patient results that were reviewed but not

RESULTS

The numbers of female and male patients in each BMI group are shown in Table 1. There were fewer men in both the lowest and the two highest BMI groups. However, there were no significant differences in the best-fit regression lines between men and women for the effects of BMI on TLC, VC, RV, FRC, ERV, or Dlco. Therefore, we grouped the data from men and women together.

Figure 1shows the effects of BMI on TLC, VC, and RV. The 20 to 25 kg/m2 BMI group was not significantly different from the 25 to

DISCUSSION

Our results confirm the findings of many others14, 15, 17, 20, 22 who have shown that lung volumes, especially FRC and ERV, decrease as body weight increases. However, our study is unique in that it clearly shows the effects of BMI on lung volumes. This information is not available from other studies7, 10, 13, 14, 15, 16, 20 that included small numbers or limited BMI groups. An interesting finding from the regression analyses is that FRC decreased from 112% of predicted at a BMI of 20 kg/m2 to

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    Dr. Jones and Ms. Nzekwu did not receive financial support for this study, nor do they have a financial interest in the research subject area.

    Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (www.chestjournal.org/misc/reprints.shtml)

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