Airway resistance at maximum inhalation as a marker of asthma and airway hyperresponsiveness

Asthmatics exhibit reduced airway dilation at maximal inspiration, likely due to structural differences in airway walls and/or functional differences in airway smooth muscle, factors that may also increase airway responsiveness to bronchoconstricting stimuli. The goal of this study was to test the hypothesis that the minimal airway resistance achievable during a maximal inspiration (R(min)) is abnormally elevated in subjects with airway hyperresponsiveness.

The R(min) was measured in 34 nonasthmatic and 35 asthmatic subjects using forced oscillations at 8 Hz. R(min) and spirometric indices were measured before and after bronchodilation (albuterol) and bronchoconstriction (methacholine). A preliminary study of 84 healthy subjects first established height dependence of baseline R(min) values.

Asthmatics had a higher baseline R(min) % predicted than nonasthmatic subjects (134 ± 33 vs. 109 ± 19 % predicted, p = 0.0004). Sensitivity-specificity analysis using receiver operating characteristic curves indicated that baseline R(min) was able to identify subjects with airway hyperresponsiveness (PC20 < 16 mg/mL) better than most spirometric indices (Area under curve = 0.85, 0.78, and 0.87 for R(min) % predicted, FEV1 % predicted, and FEF25-75 % predicted, respectively). Also, 80% of the subjects with baseline R(min) < 100% predicted did not have airway hyperresponsiveness while 100% of subjects with R(min) > 145% predicted had hyperresponsive airways, regardless of clinical classification as asthmatic or nonasthmatic.

These findings suggest that baseline R(min), a measurement that is easier to perform than spirometry, performs as well as or better than standard spirometric indices in distinguishing subjects with airway hyperresponsiveness from those without hyperresponsive airways. The relationship of baseline R(min) to asthma and airway hyperresponsiveness likely reflects a causal relation between conditions that stiffen airway walls and hyperresponsiveness. In conjunction with symptom history, R(min) could provide a clinically useful tool for assessing asthma and monitoring response to treatment.