Comparison of oscillometry devices using active mechanical test loads

Noninvasiveness, low cooperation demand and the potential for detailed physiological characterisation have promoted the use of oscillometry in the assessment of lung function. However, concerns have been raised about the comparability of measurement outcomes delivered by the different oscillometry devices. The present study compares the performances of oscillometers in the measurement of mechanical test loads with and without simulated breathing. Six devices (five were commercially available and one was custom made) were tested with mechanical test loads combining resistors (R), gas compliances (C) and a tube inertance (L), to mimic respiratory resistance (R(rs)) and reactance (X(rs)) spectra encountered in clinical practice. A ventilator was used to simulate breathing at tidal volumes of 300 and 700 mL at frequencies of 30 and 15 min(−1), respectively. Measurements were evaluated in terms of R, C, L, resonance frequency (f(res)), reactance area (A(X)) and resistance change between 5 and 20 or 19 Hz (R(5–20(19))). Increasing test loads caused progressive deviations in R(rs) and X(rs) from calculated values at various degrees in the different oscillometers. While mean values of R(rs) were recovered acceptably, some devices exhibited serious distortions in the frequency dependences of R(rs) and X(rs), leading to large errors in C, L, f(res), A(X) and R(5–20(19)). The results were largely independent of the simulated breathing. Simplistic calibration procedures and mouthpiece corrections, in addition to unknown instrumental and signal processing factors, may be responsible for the large differences in oscillometry measures. Rigorous testing and ongoing harmonisation efforts are necessary to better exploit the diagnostic and scientific potential of oscillometry.