| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Guest Access | Sign In via User Name/Password
|
|
|
|||||||
Guest Access | Sign In via User Name/Password |
|||||||||
* From the Pulmonary Division, University of Alberta, Edmonton, AB, Canada.
Correspondence to: Richard L. Jones, PhD, FCCP, 2E4.42 WMC, University of Alberta, Edmonton, AB, Canada T6G 2B7; e-mail: rjones{at}cha.ab.ca
Abstract
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.
Key Words: body mass index • lung volumes • obesity • pulmonary function testing
Obesity is a major health issue for North Americans. There has been a steady trend of increasing obesity over the past several decades,1 with the prediction of 20% of the US population being morbidly obese by the year 2010.2 A similar trend exists in Canada.3 Obesity impacts on many areas of clinical medicine, including pulmonary medicine, where it is debated if obesity is linked to asthma,45 or whether the obesity, due to its effect of decreasing lung volumes and increasing airway resistance,678910 causes symptoms that simply mimic asthma.1112 The purpose of this study was not to enter the obesity/asthma debate, but many obese people do have symptoms suggestive of lung disease,1112 so many of them are sent by their doctors for pulmonary function tests (PFTs). Therefore, it is important to understand the relationship between body mass index (BMI) and lung function to properly interpret PFTs. Our study is unique in that we show for the first time the correlation between BMI and lung volumes.
Several previous studies71013141516 have reported that increased body weight decreases lung volumes, but many of the previous studies have been small; they included subjects with coexisting morbidities such as cardiovascular disease,17 or they were conducted with the subjects in the supine position.7 Also, some studies1216171819 reported only spirometry results or they included only two groups of subjects: those with lower BMI and those with high BMI.13142021 Despite the different study populations, there is general agreement that obesity, even morbid obesity, has relatively little effect on vital capacity (VC) or total lung capacity (TLC). However, functional residual capacity (FRC) and expiratory reserve volume (ERV) can be severely decreased1415172022 as a result of the altered chest wall mechanics in obesity.2123242526 One study showed that mild obesity decreases FRC and ERV in patients with cardiovascular disease,17 but others131527 reported that much more severe obesity is required to decrease FRC and ERV. Others28 studied obese subjects before and after surgery-induced decreases in body weight, and showed that decreasing body weight has the expected positive impact on lung mechanics.
We have had much experience interpreting PFTs, and we noticed that FRC is often decreased in patients who have only a modest increase in BMI. We have also observed a high frequency of normal airway function in many obese patients with asthma receiving bronchodilators. This is not unexpected, since a previous report11 from our institution showed that obesity is associated with a higher ratio of FEV1 to FVC, while at the same time it is, parenthetically, associated with a higher incidence of bronchodilator use. We hypothesized that in patients with normal FEV1/FVC ratio, there would be a significant decrease in lung volumes as BMI increased, and that some of the lung volumes would be decreased at modest values for BMI.
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 selected for this study had significant airway obstruction, but others had evidence for interstitial lung disease or cardiovascular disease and some gave nonreproducible effort. Specifically, patients comprising this study had the following characteristics: (1) BMI > 20 kg/m2; (2) white race; (3) no diagnosis of cardiopulmonary or chest wall disease, but a working diagnosis of asthma was permitted; (4) normal FEV1/FVC ratio ( 90% of predicted)29; (5) normal forced expired flow at 75% of the FVC as derived from the data of Knudson et al.30 The lower limit of normal (LLN) was taken as 25% of the FVC per second (the LLN for forced expired flow at 75% of FVC for a person with an FVC of 2 L would be 0.5 L/s); (6) residual volume (RV) below the upper limit of normal (ULN)29; and (7) single-breath diffusing capacity of the lung for carbon monoxide (DLCO) above the LLN29 after adjusting for the patient’s alveolar volume according to the method of Johnson.31 Values for DLCO above the ULN were included.
The two laboratories from which the PFTs were reviewed and selected for this study are accredited by the Alberta College of Physicians and Surgeons. Prior to the study, both laboratories were inspected by a committee of the College composed of experts in lung function testing. The inspections included assessment of testing procedures, technician knowledge, infection control, safety, record keeping, and accuracy of data interpretation. Both laboratories use SensorMedics (Northridge, CA) 
max 22 systems including a 6200 Autobox for measuring lung volumes. The plethysmograph software was Vmax version 06-1B (Viasys; Yorba Linda, CA), which adjusts the volume calibration for the size of the patient being tested. All patients tested were referred to these laboratories primarily by family physicians and internists for lung function testing. All patients arrived for testing with a PFT requisition stating the diagnosis or working diagnosis, symptoms, and medications. During the 5 months of selection for this study, approximately 1,700 tests were reviewed; and it was from this sample that the 373 patients were selected. We selected patients according to the criteria listed above and subdivided the patient results into BMI categories of 20 to 25 kg/m2 (normal weight), >25 to 30 kg/m2 (overweight), >30 to 35 kg/m2 (mild obesity), >35 to 40 kg/m2 (moderate obesity), and > 40 kg/m2 (morbid obesity).32
The lung volumes and FEV1/FVC ratio are represented as percentage of the predicted value with the predicted values coming from a study by Gutierrez et al.29 Predicted ERV was obtained by subtracting the predicted RV from the predicted FRC.29 TLC was determined by FRC plus inspiratory capacity, and RV was determined by TLC minus VC.
The results were analyzed using linear or nonlinear exponential regression to assess the effects of BMI on lung volumes. Differences between BMI groups were analyzed using analysis of variance (ANOVA) with a Tukey post hoc analysis. Significance was taken as p < 0.05 for all tests.
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.
|
|
|
|
|
Discussion
Our results confirm the findings of many others1415172022 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 studies7101314151620 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 84% of predicted at a BMI of 30 kg/m2. The ERV decreased from 118% of predicted to 55% of predicted when BMI increased from 20 to 30 kg/m2. Therefore, the FRC of a person with a BMI of 30 kg/m2, who is on the borderline between overweight and mild obesity,32 is only 75%; and ERV is only 47% of the values for a person with a BMI of 20 kg/m2. Put another way, compared to a person with a BMI of 20 kg/m2, a person with a BMI of 30 kg/m2 has already lost 66% of the FRC and 70% of the ERV of a person with a BMI of 40 kg/m2. A person with a BMI of 35 kg/m2 has lost 86% of the FRC and 88% of the ERV of a person with a BMI of 40 kg/m2. Therefore, there is a rapid loss of FRC and ERV with modest weight gain. The exponential decrease in FRC with increasing BMI is similar to the finding of Pelosi et al,7 who also showed that increasing BMI decreases FRC exponentially. However, Pelosi et al7 performed their studies on supine and anesthetized patients, and the absolute effect of BMI on FRC in their study was greater than in our patients who were studied in the seated posture.
The lack of a significant effect of increasing BMI on RV/TLC ratio indicates that RV and TLC decrease proportionately with increasing body weight. However, the FRC/TLC ratio decreased from the 20 to 25 kg/m2 BMI group to the 25 to 30 kg/m2 and 30 to 35 kg/m2 BMI groups, indicating that FRC is more affected by BMI than is TLC until BMI exceeds 35 kg/m2, after which FRC and TLC decrease proportionately.
There is a relationship between FRC and airway resistance in obesity,78910 and airway conductance is linearly related to FRC in obesity.6 Based on the changes in FRC observed in our study and compared to a person with a BMI of 20 kg/m2, airway resistance would increase by approximately 33%, 49%, and 62% for people with BMI values of 30, 35, and 40 kg/m2, respectively; and this might contribute to previous reports1112 of obesity-related breathing symptoms.
Our finding of an increasing DLCO with BMI has been reported previously.33 However, the increases in DLCO were minimal, and our highest BMI group had an average DLCO of only 108% of predicted, which is well within the normal range.
Our regression analyses fit nicely with the normal reference values used.29 For all the lung volumes (except ERV, which was predicted by subtracting the predicted RV from the predicted FRC and for which the normal range is not available) our subjects had near 100% of predicted values at a BMI of 25 kg/m2, which was near that of the population comprising the reference set.29
In summary, we showed the effects of increasing BMI on lung volumes, and our findings will assist clinicians when interpreting PFT results from patients with normal airway function. Using a rule of thumb, physicians can expect an approximate 0.5% decrease in VC, TLC, and RV with each unit increase in BMI. DLCO increases approximately 0.3% for each unit increase in BMI. For FRC and ERV, the changes are more dramatic. Although the linear regressions between BMI and FRC and ERV were significant (p < 0.0001), the exponential relationship was more accurate in showing the relatively rapid decrease in FRC and ERV in the overweight and mild obesity states. However, using the linear regressions, FRC and ERV decrease approximately 3% and 5%, respectively, for each unit increase in BMI from 20 to 30 kg/m2. Above a BMI of 30 kg/m2, both FRC and ERV decrease approximately 1% for each unit increase in BMI. With body weight increasing in North America,123 and the reported relationship between FRC and airway resistance,6 physicians can expect an increasing frequency of complaints of shortness of breath.
The potential impact on pulmonary function laboratories should be large since currently 35% of the US population is overweight (BMI 25 to 30 kg/m2), 26% have mild-to-moderate obesity (BMI 30 to 40 kg/m2), and 5% are morbidly obese with BMI > 40 kg/m2 (derived from the data of Hedley et al34). Therefore, two thirds of the US population currently has a decreased FRC.
Limitations of the Study
Our selection criteria were based primarily on patients having a normal FEV1/FVC ratio, a DLCO above the LLN, and no documented history of any disease. It is possible that some patients in our sample had chest wall disorders, other than obesity, or that some had pleural or lung parenchymal disease that was undiagnosed and which resulted in a normal DLCO. However, each patient selected for the study had seen their primary care physician prior to being tested, and there were no indications that these kinds of disorders existed. Although all of the patients included in our sample had normal forced expired flow rates when they were tested, it is possible that some of them had real asthma and that airway function happened to be normal at the time of testing. However, none of the patients selected had increased RV, which is another indicator of peripheral airway disease.35 Another limitation is that our entire patient population was white, and the findings may not translate to other ethnic groups.
Footnotes
Abbreviations: ANOVA = analysis of variance; BMI = body mass index; DLCO = diffusion capacity of the lung for carbon monoxide; ERV = expiratory reserve volume; FRC = functional residual capacity; LLN = lower limit of normal; PFT = pulmonary function test; RV = residual volume; TLC = total lung capacity; ULN = upper limit of normal; VC = vital capacity
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.
Received for publication August 4, 2005. Accepted for publication February 22, 2006.
References
This article has been cited by other articles:
|
|
 |
|
  T. G. Babb, B. L. Wyrick, D. S. DeLorey, P. J. Chase, and M. Y. Feng Fat Distribution and End-Expiratory Lung Volume in Lean and Obese Men and Women Chest, October 1, 2008; 134(4): 704 - 711. [Abstract] [Full Text] [PDF]
|
|
Read all eLetters
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
