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BMI and lung volumes
Lung function in obese patients with asbestos diseases; crystal ball or linear regression?
Equations for BMI vs Lung Volumes
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Rafael E. de la Hoz, physician The Mount Sinai Medical Center
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rafael.delahoz{at}mssm.edu Rafael E. de la Hoz
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I read with interest Jones and Nzekewu's article on the effects of body mass index on lung volume measurements. Would it be possible to publish or obtain the linear regression equations that he derived for the lung volume measurements? Thanks. Rafael E. de la Hoz, MD |
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Roger K.A. Allen, Thoracic and Sleep Physician Wesley Hospital, Auchenflower, Brisbane, Australia
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rogerallen{at}internode.on.net Roger K.A. Allen
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Obesity is the commonest cause of a restrictive ventilatory defect in my practice and I never report lung function until I first see the patient's BMI although I notice that BMI is not always a reliable indicator of obesity in very muscular men such as soldiers. I also see large numbers of patients with a variety of asbestos-related diseases i.e. pleural plaques, pleural thickening, effusions and mesothelioma, and often with co-existing COPD and obesity. The commonest cause of an improvement in lung function in this group is weight loss for what ever reason. The "obesity factor" is important as the severity of the restrictive defect plays an important role in the estimation of severity of the asbestos diseases (and other diseases such as silicosis), prognosis and therefore costs awarded to the successful plaintiff. I have also observed that the drop in lung function due to obesity seems to be at a BMI of around 30 but this is by no means as linear and predictable as one might think. Co- existing causes of restrictive defects with decreased lung compliance eg from pleural thickening comlicates matters even more. To cut to the chase, I would be interested to learn from the authors of this paper if they have any data on this subject and in particular looking at obese patients with asbestos diseases who lose or gain weight during a period of observation. I am also interested in any attempt to look at this correlation using HR CT scans of the chest and measurements of lung and chest wall compliance, again in obese subjects. My suspicion is that the linear regressions, formulae etc derived for obesity in healthy obese individuals may be significantly different with regards the variable of obesity in this subset of patients. The application of these derived formulae to obese patients with restricive diseases may be an erroneous extrapolation of data. As one who soberly weighs these matters up on a daily basis for court reports, I look forward to some enlightenment. |
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Richard L. Jones, Pulmonary Physiologist University of Alberta
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rjones{at}cha.ab.ca Richard L. Jones
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The request from Dr. de la Hoz for the equations representing the effects of BMI on lung volumes are provided below. Please note that the linear regression equations for FRC and ERV were broken into BMI = 20-30 and BMI >30 because the best fit relationships for these lung volumes were exponential. Linear Regression Equations: DLCO = 91.9 + 0.31(BMI), VC = 109.1 - 0.48(BMI), TLC = 110.1 - 0.50(BMI), RV = 113.4 - 0.53(BMI) FRC(BMI 20-30) = 167.8 - 2.79(BMI), FRC(BMI >30) = 109.8 - 0.96(BMI), ERV(BMI 20-30) = 195.6 - 4.42(BMI), ERV(BMI >30) = 74.5 - 1.11(BMI) Expnential Equations: FRC = 231.9*e(-0.067(BMI)) + 55.17 (The (-0.067(BMI)) is an exponent) ERV = 596.6*e(-0.083(BMI)) + 6.66 (The (-0.083(BMI)) is an exponent) Richard L. Jones, PhD |
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