BTPS CORRECTION FOR CERAMIC FLOW SENSOR

Several commercially available spirometers use unheated ceramic elemen ts as flow sensors to determine flow and calculate volume of air. The usual method of correcting the resulting flow and volume values to bod y temperature pressure saturated (BTPS) is to apply a constant factor approximately equal to 30 percent of the full BTPS correction factor. To evaluate the usual BTPS correction factor technique, we tested seve ral sensors with a mechanical pump using both room air and air heated to 37 degrees C and saturated with water vapor. The volume signals use d to test the sensors were volume ramps (constant flow) and the first four American Thoracic Society (ATS) standard waveforms. The percent d ifference in FEV(1) obtained using room vs heated-humidified air (prop ortional to the magnitude of the BTPS correction factor needed) ranged from 0.3 percent to 6.2 percent and varied with the number of maneuve rs previously performed, the time interval between maneuvers, the volu me of the current and previous maneuvers, and the starting temperature of the sensor. The temperature of the air leaving the sensor (exit te mperature) showed a steady rise with each successive maneuver using he ated air. When six subjects performed repeated tests over several days (each test consisting of at least three maneuvers), a maneuver order effect was observed similar to the results using the mechanical, rathe r than static, BTPS correction factor is needed for accurate estimatio ns of forced expiratory volumes and to reduce erroneous variability be tween successive maneuvers. Use of exit air temperature provides a mea ns of estimating a dynamic BTPS correction factor, and this technique may be sufficient to provide an FEV(1) accuracy of less than +/-3 perc ent for exit air temperatures from 5 degrees to 28 degrees C.