Smooth Reference Equations for Slow Vital Capacity and Flow–Volume Curve Indexes

Abstract

We derived reference values for slow vital capacity (VC) and flow-volume curve indexes (FVC, FEV(1), and flows) from the 1,185 tracings provided by 1,039 "normal" subjects who participated in one or both cross-sectional surveys of the Po River Delta study in 1980-1982 and in 1988-1991. Definition of "normal" was based on negative answers to questions on respiratory symptoms/diseases or recent infections, current/past tobacco smoking, and work exposure to noxious agents. Reference equations were derived separately by sex as linear regressions of body mass index (BMI = weight/height(2)), BMI-squared, height, height-squared, and age. Age entered all the models by natural cubic splines using two break points, except for the ratios FEV(1)/VC and FEV(1)/FVC. Random effects models were applied to adjust for the potential intrasubject correlation. BMI, along with height and age, appeared to be an important predictor, which was significantly associated with VC, FEV(1), FVC, FEV(1)/FVC, and PEF in both sexes, and with FEV(1)/VC and FEF(25-75) in females. Natural cubic splines provided smooth reference equation curves (no "jumps" or "angled points") over the entire age span, differently from the conventional reference equations. Thus, we recommend the use of smooth continuous equations for predicting lung function indexes, along with the inclusion of BMI in the equations.

The Difference Between Slow and Forced Vital Capacity Increases With Increasing Body Mass Index: A Paradoxical Difference in Low and Normal Body Mass Indices

“…1 Several authorities have reported the effect of weight on VC. 5,6 The VC decreases with increasing BMI. Obesity reduces lung compliance, 15 and, as a consequence, it decreases VC.…”

“…4 Obesity can decrease a patient's vital capacity, as reflected in both FVC and SVC measurements. [5][6][7] To our knowledge, however, it is not known whether body mass index (BMI) affects FVC more than SVC. If obesity reduces FVC more than SVC, the FEV 1 /FVC ratio may be artificially low in obese individuals, leading to underdiagnosis of OAD.…”

Age- and height-based prediction bias in spirometry reference equations

“…Until recently no statistical techniques allowed accurate and continuous description of pulmonary function from childhood to old age. However, with the advent of new techniques, notably the GAMLSS technique [10,[20][21][22][23] it is now possible to summarise pulmonary function across the whole age range. In addition, it allows the lower limit of normal (usually the fifth percentile), which varies appreciably with age [10,15,16,22], to be modelled properly.…”

Weight history from birth through childhood and youth in relation to adult lung function, in Danish juvenile obese and non-obese men

“…6,7 The initial increase is thought to be a 'muscularity effect', while the decrease with high BMI values may be due to an 'obesity effect' on the mobility of the thorax. 7 Pistelli et al 6 reported that in a male non-smoking healthy Italian population, the value at which the direction of the association between lung function and BMI changed was 25.7 kg/m 2 . Our results are in agreement with these findings, showing a strong detrimental effect of adult BMI among those who are overweight or obese, and no effect among those normal and underweight.…”

“…A few studies have reported that pulmonary function increased with increasing BMI within lower BMI values and then decreased with further increases. 6,7 Moreover, weight gain has been associated with the longitudinal decline of both FEV 1 and forced vital capacity (FVC) in general population samples. 8,9 Some studies have explored the relationship between BMI and lung function in childhood; 10 however, to our knowledge, the association between BMI in childhood and adult lung function has not been investigated.…”