“Reactance inversion” at low frequencies in a child undergoing treatment of a cystic fibrosis exacerbation

INTRODUCTION: Impulse oscillometry (IOS) employs high frequency sinusoidal or impulse pressure and flow waveforms to interrogate the mechanical properties of the respiratory system. It has special applications to preschool and younger children who may have difficulty performing the repetitive forced expiratory maneuvers required for spirometry. CASE PRESENTATION: We present a case illustrating improvements of respiratory system mechanics measured by IOS in a 6‐year‐old child with cystic fibrosis (CF) who demonstrated clinical and radiological improvement after a course of therapy with hospitalization and intravenous antibiotics, and initiation of a cystic fibrosis transmembrane regulator (CFTR) protein corrector/potentiator agent. We also report a new finding: observed lower than expected reactance at low compared to high frequencies (“reactance inversion”). CONCLUSION: Reactance inversion may reflect parallel pathway inhomogeneities in resistance and elastance or intrabreath airway inertance changes in young children with CF. Further study is needed in children with airway obstruction due to asthma, cystic fibrosis, and chronic lung disease of infancy to demonstrate the prevalence of this finding and whether it is specific to a measurement device.