Effect of obesity on breathlessness and airway responsiveness to methacholine in non-asthmatic subjects

Background: Obesity is associated with increased prevalence and incidence of asthma, but the mechanism is unknown. Obesity reduces lung volumes, which can increase airway responsiveness, and increases resistive and elastic work of breathing, which can increase dyspnea. Objective: To determine if the intensity of dyspnea due to airway narrowing or if airway responsiveness is increased in obese, non-asthmatic subjects. Subjects: Twenty-three obese (BMI (body mass index) >= 30 kg m(-2)) and 26 non-obese (BMI <30 kg m(-2)) non-asthmatic subjects, aged between 18 and 70 years. Methods: High-dose methacholine challenge was used to determine the sensitivity and the maximal response to methacholine. Respiratory system resistance (Rrs) and reactance were measured, using the forced oscillation technique, as indicators of resistive and elastic loads during challenge. Perception of dyspnea was measured by the Borg score during challenge. Static lung volumes were measured by body plethysmography. Results: Static lung volumes were reduced in the obese subjects. There were no significant differences in the sensitivity or maximal response to methacholine between obese and non-obese subjects. The magnitude of change in Rrs was similar in both groups, but obese subjects had more negative reactance after challenge (P = 0.002) indicating a greater elastic load. The intensity of dyspnea was greater in obese subjects (P = 0.03). Conclusions: Obesity reduces lung volumes, but does not alter the sensitivity or maximal response to methacholine. However, obese subjects have enhanced perception of dyspnea, associated with greater apparent stiffness of the respiratory system, and may therefore be at greater risk of symptoms.