The effect of low lung volume on airway function in obesity
Introduction
Obesity is characterized by reduction in lung volumes, most notably a marked reduction in functional residual capacity (FRC) and expiratory reserve volume (ERV) (Jones and Nzekwu, 2006, King et al., 2005a, Pelosi et al., 1998, Rubinstein et al., 1990). It is not clear however, whether obesity has any direct effect on airway function that is independent of its effect on lung volumes. Although FEV1 and FVC may be reduced in the obese, the FEV1/FVC ratio is usually well preserved (Lazarus et al., 1997, Schachter et al., 2001, Sin et al., 2002, Wallace et al., 1999), suggesting that there is no intrinsic airway obstruction. Similarly, some studies have shown that increased airway or respiratory system resistance in the obese can be normalized by adjustment for operating lung volume (Rubinstein et al., 1990, Zerah et al., 1993). However, other studies have shown that reduced FRC can only partially explain the associations between increased BMI and increased airway resistance (King et al., 2005a, Watson and Pride, 2005) or reduced mid expiratory flows (Rubinstein et al., 1990). Given the high prevalence of respiratory symptoms in obesity (Sahebjami, 1998) and the relationship between obesity and increased symptoms in asthma (Farah et al., 2011) an understanding of the nature of airway dysfunction in the obese may be clinically relevant.
The extent to which abnormalities in ventilation heterogeneity, airway closure and impedance in obesity are attributable to reduced FRC is unknown. The distribution of ventilation to the lung bases is reduced in severely obese individuals (Demedts, 1980, Holley et al., 1967, Wallace et al., 1999), but this probably only occurs in subjects with markedly reduced ERV (Holley et al., 1967). Non-ventilation of the lung bases would suggest the possibility of increased airway closure but the effect of obesity on airway closure is unclear, since studies that have reported increased closing capacity in obesity have been compromised by the inclusion of smokers or the lack of an appropriate control group (Farebrother et al., 1974). However, airway closure maybe present during tidal breathing, due to reduced FRC (Benedik et al., 2009a, Hedenstierna et al., 1976). Airway closure during tidal breathing may affect ventilation heterogeneity (Crawford et al., 1989), but the effects of obesity on ventilation heterogeneity have not been described. Respiratory system reactance reflects both airway closure (Lundblad et al., 2007) and ventilation heterogeneity in peripheral airways (Lutchen and Gillis, 1997), but the determinants of reduced Xrs in obesity are not known.
We hypothesize that obesity is associated with abnormalities in airway function beyond what can be explained by changes in lung volume alone. To investigate this we used measurements of airway function that are likely to be sensitive to changes in FRC. The aim was to determine the relative effect of BMI and reduced FRC on impedance measured by forced oscillation technique, ventilation heterogeneity measured by multiple breath nitrogen washout, and airway closure measured by single breath nitrogen washout, in obese and non-obese subjects.
Section snippets
Methods
Subjects were studied during a single visit to the laboratory where they underwent measurements of spirometry, lung volumes by body plethysmography, gas transfer, impedance by forced oscillation technique (FOT), single breath nitrogen washout (SBNW) and multiple breath nitrogen washout (MBNW).
Subject characteristics and standard lung function
Subject characteristics and lung function are summarized in Table 1. Seventeen subjects were non-obese and 18 were obese. The two groups were well matched for gender. There was a non-statistically significant difference in mean age of the two groups (non obese: 43.9 ± 13.6, obese: 51.9 ± 13.8 p = 0.10). The obese subjects had similar mean FEV1%predicted values compared with non-obese subjects, but had lower mean FVC% predicted, and consequently a higher mean spirometric ratio, although remaining in
Discussion
In summary we found abnormalities in airway function in obesity that were independent of the effect of low FRC. Ventilation heterogeneity in peripheral airways, as measured by Sacin, is increased in obesity, and is not related to FRC. Reduced respiratory system conductance in obesity was incompletely explained by reduced FRC. The increase in airway closure in the obese is small but combined with the reduction in FRC, airway closure is more likely to occur within the range of tidal breathing,
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