Table 1

Differences between spirometry and FOT/IOS

Main principleFlow sensor/volume displacement helps measure flow rates and lung volumesForced oscillations of single frequency sound waves (FOT) or impulses of multiple frequency sound waves (IOS) are pushed into the lungs as pressure waves to measure respiratory resistance and reactance
Main parametersVolumes: FEV1, FVCZrs, Rrs, Xrs, Fres, Ax
Flows: PEFR, FEF25–75%
Patient co-operation required++++
Type of breathing manoeuvreForced exhalationTidal breathing
Variability (intra-subject)3–5%5–15%
Sensitivity to airway location
Cut off for bronchodilator response12–15% for FEV140% for R5 or X5
Cut off for bronchoconstrictor response20% for FEV150% for R5
Insight into lung mechanics++++
Standardised methodology+++++
Availability of robust reference values++++
  • FEV1: forced expiratory volume in 1 s; FVC: forced vital capacity; PEFR: peak expiratory flow rate; FEF25–75%: forced expiratory flow at 25–75% of FVC; Zrs: respiratory impedance; Rrs: respiratory resistance; Xrs: respiratory reactance; Fres: resonant frequency; Ax: reactance area; R5: respiratory resistance at 5 Hz; X5: respiratory reactance at 5 Hz.