In obese people, the presence of adipose tissue around the rib cage and abdomen and in the visceral cavity loads the chest wall and reduces functional residual capacity (FRC). The reduction in FRC and in expiratory reserve volume is detectable, even at a modest increase in weight. However, obesity has little direct effect on airway caliber. Spirometric variables decrease in proportion to lung volumes, but are rarely below the normal range, even in the extremely obese, while reductions in expiratory flows and increases in airway resistance are largely normalized by adjusting for lung volumes. Nevertheless, the reduction in FRC has consequences for other aspects of lung function. A low FRC increases the risk of both expiratory flow limitation and airway closure. Marked reductions in expiratory reserve volume may lead to abnormalities in ventilation distribution, with closure of airways in the dependent zones of the lung and ventilation perfusion inequalities. Greater airway closure during tidal breathing is associated with lower arterial oxygen saturation in some subjects, even though lung CO-diffusing capacity is normal or increased in the obese. Bronchoconstriction has the potential to enhance the effects of obesity on airway closure and thus on ventilation distribution. Thus obesity has effects on lung function that can reduce respiratory well-being, even in the absence of specific respiratory disease, and may also exaggerate the effects of existing airway disease.

ABSTRACr A rapid, simple method for measuring bronchial responsiveness to inhaled histamine is described. The method was used to obtain dose response curves in 50 atopic subjects with varying respiratory and nasal symptoms. The cumulative dose of histamine which caused a 20% fall in the one second forced expiratory volume (PD2>FEV.) varied between 0*046 and greater than 39 ,umol and correlated with the severity of symptoms. The reproducibility of the PD2,FEv,, determined from duplicate measurements in 15 subjects with varying degrees of bronchial responsiveness was found to be satisfactory. When the PD2OFEV, from this rapid method was compared with that obtained from the dosimeter method no significant difference was found. The dose delivered by this method was shown to be cumulative. May 1983 drive the nebuliser, so the equipment is cumbersome.

There is an urgent need for research to better understand the mechanisms of asthma in the obese, and to develop new therapies specifically targeted to this unique patient population.