Correction of thermal artifacts in plethysmographic airway resistance measurements

Specific airway resistance (sRaw) measured by body plethysmography without conditioning of the inspired air to BTPS exhibits a strong frequency dependence related to the fact that the warming and wetting of the gas in the airways is not instantaneous (R. Peslin, C. Duvivier, M. Vassiliou, and C. Gallina. J. Appl. Physiol. 79: 1958-1965, 1995). We have tested three methods in 21 healthy subjects to correct for that artifact by using a simple model, assuming a first-order thermal process characterized by a single time constant. The corrections required entering an assumed constant value for (methods 1 and 2) and/or for airway inertance (methods 1 and 3) and/or measuring the inspired gas temperature and water vapor saturation (methods 2 and 3). The frequency dependence of sRaw was measured from 0.5 to 3 Hz both with (sRawETPS) and without (sRawam) gas conditioning. With optimal values for and/or airway inertance, the mean difference between sRawam and sRawETPS was close to zero with all three methods, but the root mean square difference was significantly lower with method 2 (0.83 +/- 0.35 hPa.s compared with 1.21 +/- 0.54 and 1.20 +/- 0.49 hPa.s with methods 1 and 2, respectively). We conclude that the thermal artifact of sRaw measurements may be best corrected by using temperature measurements and an assumed time constant (0.152 s with our equipment).