Background: when using oscillometry, theoretical reasons support the use of admittance (Y) instead of impedance (Z) for assessing changes in lung mechanics within the same patient (Farré, ERJ,2022), as changes in Y are not affected by upper airway shunt. The current study aims to compare changes in both Y and Z with changes in spirometry due to bronchodilation (BD) in children with persistent asthma.
Methods: we retrospectively analyzed data collected on children (6?12 y). Oscillometry (RESMON PRO FULL, Restech srl, Italy) followed by spirometry was performed pre and post 400 mcg salbutamol. A positive response to BD was defined as an increase in FVC or FEV1 >10%predicted (Stanojevic, ERJ, 2022). We computed the AUC of the ROC curve considering absolute and relative (%baseline) changes of oscillometry parameters at 5Hz from Z (module, resistance (R5) and reactance (X5)) and Y (module, conductance (G5) and susceptance (B5)).
Results: we included 56 children (Table 1), with 17 showing a positive response to BD.
| Parameter | Median (IQR) |
| Age [years] | 9.6 (8.0;11.2) |
| Height [cm] | 138(129;147) |
| BMI [kg/m2] | 17 (16;19) |
| Males | 40 |
| Ethnicity | 22 african; 32 caucasian; 2 indian |
| baseline FVC % predicted | 93 (85;101) |
| baseline FEV1 %predicted | 93 (83;100) |
Y and Z parameters resulted in similar AUC (Table 2). G5% baseline presented the highest AUC.
| Parameter | Absolute change | Change as % baseline |
| R5 | 0.71 | 0.71 |
| X5 | 0.69 | 0.73 |
| Module Z5 | 0.72 | 0.73 |
| G5 | 0.72 | 0.75 |
| B5 | 0.64 | 0.64 |
| Module Y5 | 0.71 | 0.73 |
Conclusion: in these subjects with normal baseline FEV1, the agreement with spirometry was similar using Y or Z. The impact of using Y should be evaluated in smaller or more obstructed children, where the impact of the upper airway shunt is higher.