X5 values measured on Impulse Oscillometry (IOS) help differentiate healthy, asthmatic, COPD and ILD subjects from one another.

Introduction and Aims: Respiratory reactance (Xrs) of the lung measures inertance (I) and the elastic properties or compliance of lung periphery expressed as capacitance (C) on Impulse Oscillometry. C and I are in opposite in phase with each other and at low frequencies (5Hz) the capacitance properties of the small peripheral airways dominate, making X5 a negative value in healthy human subjects. We aimed to compare the X5 values between healthy subjects and patients with asthma, COPD and interstitial lung disease (ILD). Methods: 275 male and female subjects (85 healthy, 106 asthmatic, 48 COPD and 36 ILD) underwent pre and post-bronchodilator IOS measurements with 400 mcg inhaled salbutamol. X5 values were compared with each other. Results: The median X5 values (%predicted) were -333.3 for healthy subjects, 511.1 for ILDs, 1500 for asthmatics and 2533 for COPD subjects (Fig 1). Conclusion: X5 (% pred) values help differentiate healthy from asthmatic, COPD and ILD subjects and also between each other. Larger studies may help identify specific cut off values to help differentiate between asthma, COPD and ILD.