Specific airway resistance (sRaw) measured by body plethysmography has been shown to decrease markedly with decreasing breathing frequency when the inspired air is not conditioned to body temperature, atmospheric pressure and saturation with water vapour (BTPS). The phenomenon has been attributed to noninstantaneous gas warming and wetting in the airways. The aim of this investigation was to assess whether the phenomenon was also present in a commercialized plethysmograph featuring an "electronic BTPS correction".Airway resistance (Raw) and sRaw were measured in 15 healthy subjects at six breathing frequencies ranging 0.25-3 Hz, using a constant volume plethysmograph in which a correction for non-BTPS gas conditions was applied by electronically flattening the box pressure-airway flow loop (Jaeger Masterscreen Body, version 4.0).The temperature and water vapour saturations in the box averaged 26.5±1.3°C and 59±6%, respectively. Raw and sRaw exhibited a clear positive frequency dependence in all but one subject. From 0.25 to 3 Hz Raw increased from (mean±SD) 0.62±0.55 to 1.71±+0.76 hPa·s·L -1 (p<0.001), and sRaw from 2.34±1.90 to 7.55±3.08 hPa·s (p<0.001). The data are consistent with a simple model, in which gas conditioning in the airways and external dead space occurred with a time constant of 0.39 s.We conclude that the electronic BTPS correction of the instrument was inadequate, probably because it is assumed that gas conditioning in the airways is instantaneous. We recommend that, with similar instruments, airway resistance be measured using as high a panting frequency as feasible. Eur Respir J., 1996Respir J., , 9, 1747Respir J., -1750 When a subject breathes ambient air inside a body plethysmograph, the largest part of the volume variations measured by the instrument (Vpl) is due to the warming and humidification of inspired air in the airway during the inspiratory phase and to its partial cooling during the expiratory phase. Computed according to the equation of DRORBAUGH and FENN [1] the change in Vpl occurring when a tidal volume of 0.6 L at 22°C and 50% water vapour saturation (SH 2 O) is conditioned to body temperature, atmospheric pressure and saturation with water vapour (BTPS) amounts to about 57 mL. This thermal component of Vpl is more than 10 times larger than the airway resistance (Raw) component expected in a normal subject (specific Raw (sRaw=5 hPa·s)) at a flow rate of 1 L·s -1 .

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).