The difference between slow and forced vital capacity increases with increasing body mass index: a paradoxical difference in low and normal body mass indices.

BACKGROUND Obesity reduces FVC, the most commonly used measurement of vital capacity (VC) and slow VC (SVC). It is unknown whether the difference between SVC and FVC is constant in different body mass indices (BMIs). We hypothesized that the difference between SVC and FVC increases as a function of BMI. METHODS We retrospectively reviewed pulmonary function tests (PFTs) that included spirometry and plethysmography and were performed in adults from January 2013 to August 2013. A total of 1,805 PFTs were enrolled. The non-parametric Wilcoxon signed-rank test was used to compare FVC with SVC, and to compare FEV1/FVC with FEV1/SVC ratio. Spearman correlation analysis was used to determine whether BMI has an effect on the discordance between FVC and SVC. Finally, we used the McNemar test for paired binary data to compare the prevalence rate of obstruction when using different measurements of VC. RESULTS In individuals with BMI < 25 kg/m(2) and no evidence of obstruction in the PFTs, FVC was larger than SVC (P = .03), whereas in overweight and obese individuals, SVC was significantly larger than FVC. The difference between SVC and FVC was positively correlated with BMI (P < .001). One hundred thirty-one patients had a normal FEV1/FVC but low FEV1/SVC ratio. Fifty of these 131 individuals also had a normal FVC; the majority of them (46 of 50) had the PFTs for investigation of respiratory symptoms and had BMI > 25 kg/m(2) (42 of 50). CONCLUSIONS Our results indicate that FVC is larger than SVC in patients with low and normal BMI and no evidence of obstruction in the PFTs, whereas FVC is smaller than SVC in overweight and obese individual. Our findings add to the existing literature that use of FEV1/FVC may lead to underdiagnosis of obstructive airway disease in overweight and obese individuals.