Forced oscillation measurements in bronchial challenge with exercise and bronchodilator response in children with recurrent respiratory symptoms

Background: Measurements of respiratory impedance with the forced oscillation technique (FOT) are considered useful for testing exercise‐induced bronchoconstriction (EIB) and bronchial reversibility (BR) with beta‐agonists in children. However, few data exist on the relationship between FOT and gold‐standard spirometric parameters during these tests. Aims: To evaluate the relationship between FOT and spirometric variables during EIB and BR in patients attending our Pulmonary Unit for recurrent respiratory symptoms. Methods: In 63 outpatients aged 11.0±2.7 yr (M/F: 38/25), we assessed spirometry, FOT at 8 Hz, FENO, allergy skin‐prick tests and medical respiratory records. Spirometry was repeated 1, 5, 10, 15 and 20 minutes after exercise as well as FOT (3, 18 min). EIB was defined as a FEV1 fall >10%. Bronchial response to inhaled salbutamol (“D” prefix) for FEV1, FEF25‐75, respiratory resistance (Rrs) and reactance (Xrs) was assessed 20 min after exercise. Results: Subjects with EIB (n=12) had similar Rrs, Xrs and FENO values and slightly lower FEV1/FVC and FEF25‐75 than those without EIB. In the whole sample, baseline Rrs and Xrs, both in inspiratory (insp) and expiratory (exp) phase correlated with the post‐ exercise fall in FEF25‐75 but not with the fall in FEV1. Impedance measurements at 3 and 18 minutes following exercise correlated with the fall in both spirometric parameters, especially with the fall in FEF25‐75 (e.g. Rrs insp 3′ vs. fall in FEF25‐75: r=‐0.57; Xrs exp 18′ vs. fall in FEF25‐75: r=0.57, p<0.001 for both). After salbutamol, DRrs and DXrs correlated better with DFEV1 than with DFEF25‐75 (e.g. DRrs vs. DFEV1: r=‐0.52, p<0.001). Conclusion: FOT measurements of both respiratory resistance and reactance at an oscillation frequency of 8 Hz are particularly suited to assess peripheral airway obstruction following exercise. The same variables mainly reflect post‐bronchodilator changes at a mid‐proximal airway level as measured by variations in FEV1. Interpretation of FOT measurements at a low oscillation frequency depends on the type of airway response (exercise or beta agonist) that is being tested.