Comparison of oscillometry devices using active mechanical test loads

Abstract

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.

FIGURE 1

Schematic arrangement of the mechanical test loads.

FIGURE 2

Representative tracing of pressure, flow and volume (blue) signals with “intra-breath” resistance (black) and reactance (red) computed at an 11-Hz oscillation frequency in test load M2.

FIGURE 3

Impedance measurements in the mechanical test loads a) M1 and b) M6.

FIGURE 4

Values of a) resistance (R), b) compliance (C), c) inertance (L) and d) the fitting error (F) obtained from model fitting to impedance data measured in the mechanical test loads M1–M6, with the different devices and modes. Horizontal lines indicate values from fitting to calculated impedances. ^#: C>0.06 L·hPa⁻¹.

FIGURE 5

Values of a) resonance frequency (f_res), b) reactance area (A_X) and c) the frequency dependence of resistance (R_5–20(19)) calculated from impedance data measured in the mechanical test loads M1–M6 with the different devices and modes. Horizontal lines indicate values obtained from the calculated impedance data. Zero crossing of reactance occurred below (^#) or above the measured frequency range (^¶).