Abstract
BACKGROUND: The purpose of this study was to investigate physiological phenotypes of asthma in obesity. RESEARCH QUESTION: Do physiological responses during bronchoconstriction distinguish different groups of asthma in people with obesity, and also differentiate from responses simply related to obesity? STUDY DESIGN AND METHODS: Cross-sectional study of people with obesity (31 with asthma and 22 without lung disease). Participants underwent methacholine challenge testing with measurement of spirometry and respiratory system impedance by oscillometry. RESULTS: Participants had class III obesity (BMI, 46.7 ± 6.6 kg/m(2) in control subjects and 47.2 ± 8.2 kg/m(2) in people with asthma). Most participants had significant changes in peripheral airway impedance in response to methacholine: in control subjects, resistance at 5 Hz measured by oscillometry increased by 45% ± 27% and area under the reactance curve (AX) by 268% ± 236% in response to 16 mg/mL methacholine; in people with asthma, resistance at 5 Hz measured by oscillometry increased by 52% ± 38% and AX by 361% ± 295% in response to provocation concentration producing a 20% fall in FEV(1) dose of methacholine. These responses suggest that obesity predisposes to peripheral airway reactivity. Two distinct groups of asthma emerged based on respiratory system impedance: one with lower reactance (baseline AX, 11.8; interquartile range, 9.9-23.4 cm H(2)O/L) and more concordant bronchoconstriction in central and peripheral airways; the other with high reactance (baseline AX, 46.7; interquartile range, 23.2-53.7 cm H(2)O/L) and discordant bronchoconstriction responses in central and peripheral airways. The high reactance asthma group included only women, and reported significantly more gastroesophageal reflux disease, worse chest tightness, more wheeze, and more asthma exacerbations than the low reactance group. INTERPRETATION: Peripheral airway reactivity detected by oscillometry is common in obese control subjects and obese people with asthma. There is a subgroup of obese asthma characterized by significant peripheral airway dysfunction by oscillometry out of proportion to spirometric airway dysfunction. This peripheral dysfunction represents clinically significant respiratory disease not readily assessed by spirometry.