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Respiratory Duty Cycles in Individuals With and Without Airway Hyperresponsiveness. BACKGROUND: The respiratory duty cycle (Ti/Ttot) can influence bronchoprovocation test results and nebulized drug delivery. The Ti/Ttot has not yet been examined in individuals with airway hyperresponsiveness (AHR) in typical bronchoprovocation test conditions. This study investigated the mean Ti/Ttot in participants with and without AHR and whether the Ti/Ttot changes with increasing bronchoconstriction. METHODS: Fifteen participants with AHR and fifteen participants without AHR completed this randomized crossover study. An ultrasonic spirometer was used for continuous measurement of the Ti/Ttot as participants inhaled room air or aerosolized solution. Each participant completed two methacholine challenges, one using a continuous-output vibrating mesh nebulizer/ultrasonic spirometer and one with the nebulizer only. Prior to each methacholine challenge, participants inhaled room air and aerosolized saline through the nebulizer/spirometer setup to record baseline Ti/Ttot data. RESULTS: The mean Ti/Ttot findings [95% CIs] during room air inhalation were 0.392 [0.378-0.406] and 0.447 [0.426-0.468] in participants with and without AHR, respectively (P < .001). The mean Ti/Ttot during saline inhalation were 0.389 [0.373-0.405] and 0.424 [0.398-0.450] in participants with and without AHR (P = .040). The Ti/Ttot showed a nonsignificant downward trend with progressive methacholine-induced bronchoconstriction. CONCLUSIONS: The mean Ti/Ttot in participants with AHR closely resembles the assumed Ti/Ttot of 0.40 recommended for standard use when calculating methacholine challenge results. Since the Ti/Ttot did not change significantly over the course of a methacholine challenge, the same Ti/Ttot can be used to calculate the dose of methacholine inhaled, regardless of the level of bronchoconstriction. TRIAL REGISTRY: ClinicalTrials.gov; No.: NCT03505489; URL: www.clinicaltrials.gov.