Prevention of thermal and condensation errors in pneumotachographic recordings of the maximal forced expiratory manoeuvre

Abstract

When recording respiratory manoeuvres using a pneumotachograph (PT), it is customary to heat the pneumotachograph head to avoid condensation errors. It is technically demanding to heat the PT correctly, and this increases the complexity of the equipment. We have studied whether a simpler method can eradicate condensation errors and preserve thermal stability of the PT. We tested the accuracy of a PT by recording the discharge of 1,000 ml of air through the PT before and after a subject had blown through it, thus causing both condensation within the PT and a rise in its temperature. We then determined whether the thermal and condensation errors could be avoided by placing the PT, between blows, on a fan that was blowing air at a constant 1.65 l · s-1. With ambient temperature at 20°C, a single blow through the PT was found to cause a subsequent 4% over-reading in volume, due to condensation and temperature changes in the PT. After five consecutive blows the error was 7%. Placing the PT on the fan for 15 s after a single blow abolished these errors, whereas a wait of 15 s did not. At an ambient temperature of 14°C there was a 5% over-reading of volume after a single blow, and an over-reading of 9% after five blows. At an ambient temperature of 26°C these errors were 3 and 5%, respectively. All of these errors were abolished by placing the PT on the fan for 15 s and 30 s, respectively. These errors were significantly greater for a Fleisch as compared to a mesh screen type of pneumotachograph. Placing a pneumotachograph, in between blows, in the flow stream of a fan blowing ambient air for 15 s minimizes errors due to condensation effects and temperature change in the pneumotachograph head.