The forced oscillation technique (FOT) is an emerging clinical lung function test, with commercial devices becoming increasingly available. However comparability across existing devices has not been established. We evaluated in vivo and in vitro measurements made using three commercial devices against a custom-built device (WIMR): Resmon Pro Diary (Restech srl, Italy), tremoFlo C-100 (Thorasys Medical Systems, Canada), Jaeger Masterscope CT IOS (CareFusion, Hoechberg, Germany). Respiratory system resistance Rrs and reactance Xrs at 5 Hz were examined in twelve healthy subjects (mean age 33 ± 11 years, 7 males), and in two test standards of known resistance and reactance. Subjects performed three measurements during tidal breathing on the four devices in random order. Total, inspiratory and expiratory Rrs and Xrs were calculated and compared using one-way repeated measures ANOVA and Bonferroni post-hoc tests. Rrs did not differ between devices, with <10% deviation from predicted, except for the IOS device. With Xrs, similar values were seen between the WIMR and Resmon devices and between the tremoFlo and IOS devices. No differences were observed using test standards; deviation from theoretical value was <2% for resistance and <5% for reactance. The WIMR, tremoFlo and Resmon Pro but not IOS devices measure similar Rrs, whereas there was more disparity across devices in the estimation of Xrs parameters. The discrepancy between in vivo and in vitro measurements suggest that FOT validation procedures need to take into account the breathing pattern, either using biological controls or a breathing model.

BackgroundTelemonitoring trials for early detection of acute exacerbations (AECOPD) have provided mixed results. Day-to-day variations in lung function measured by forced oscillation technique (FOT) may yield better insight. We evaluated the clinical utility of home telemonitoring of variability of FOT measures, in terms of (i) relationship with symptoms and quality of life, and (ii) the timing of variability of FOT measures and symptom changes prior to AECOPD.MethodsDaily FOT parameters at 5 Hz (resistance (R) and reactance (X); Resmon Pro Diary, Restech Srl), daily symptoms (COPD Assessment Test, CAT) and 4-weekly quality of life (St. George's Respiratory Questionnaire, SGRQ) were obtained over 8–9 months from COPD patients. Variability of resistance and reactance was calculated as the standard deviation (sd) over 7-day running windows; we also examined the effect of varying window size. The relationships between FOT versus CAT and SGRQ were assessed using linear mixed modelling, daily changes in FOT variability and CAT prior to AECOPD using one-way repeated measures ANOVA.Results15 participants with mean(sd) age 69(10) years and FEV1%predicted 39(10) had a median(IQR) adherence of 95.4(79.0–98.8)%. Variability of the inspiratory component of X (SDXinsp) related to CAT and weakly SGRQ (fixed effect estimate(95%CI) 1.57(0.65–2.49), p=0.001 and 4.41(−0.06–8.89), p=0.05, respectively). SDXinsp changed significantly on the same day as CAT (1 day before AECOPD, both p=0.02), and earlier when using shorter running windows (3 days before AECOPD, p=0.01, accuracy=0.72 for 5-day windows).ConclusionsVariability of inspiratory reactance from FOT telemonitoring reflects COPD symptoms and may be a sensitive biomarker to detect AECOPD early.

The forced oscillation technique (FOT) is gaining clinical acceptance, facilitated by more commercial devices and clinical data. However, the effects of variations in testing protocols used in FOT data acquisition are unknown. We describe the effect of duration of data acquisition on FOT results in subjects with asthma, chronic obstructive pulmonary disease (COPD) and healthy controls.FOT data were acquired from 20 healthy, 22 asthmatic and 18 COPD subjects for 60 s in triplicate. The first 16, 30 and 60 s of each measurement were analysed to obtain total, inspiratory and expiratory resistance of respiratory system (Rrs) and respiratory system reactance (Xrs) at 5 and 19 Hz.With increasing duration, there was a decrease in total and expiratory Rrs for healthy controls, total and inspiratory Rrs for asthmatic subjects and magnitude of total and inspiratory Xrs for COPD subjects at 5 Hz. These decreases were small compared to the differences between clinical groups. Measuring for 16, 30 and 60 s provided ≥3 acceptable breaths in at least 90, 95 and 100% of subjects, respectively. The coefficient of variation for total Rrs and Xrs also decreased with duration. Similar results were found for Rrs and Xrs at 19 Hz.FOT results are statistically, but likely minimally, impacted by acquisition duration in healthy, asthmatic or COPD subjects.