PREVENTION OF THERMAL AND CONDENSATION ERRORS IN PNEUMOTACHOGRAPHIC RECORDINGS OF THE MAXIMAL FORCED EXPIRATORY MANEUVER

When recording respiratory manoeuvres using a pneumotachograph (PT), i t is customary to heat the pneumotachograph head to avoid condensation errors. It is technically demanding to heat the PI correctly, and thi s 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 an d a rise in its temperature. We then determined whether the thermal an d condensation errors could be avoided by placing the PT, between blow s, on st fan that was blowing air at a constant 1.65 l.s(-1). With amb ient temperature at 20 degrees C, a single blow through the PT was fou nd to cause a subsequent 4% over-reading in volume, due to condensatio n 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 a bolished these errors, whereas a wait of 15 s did not. At an ambient t emperature of 14 degrees C there was a 5% over-reading of volume after a single blow, and an over-reading of 9% after five blows. At an ambi ent temperature of 26 degrees C these errors were 3 and 5%, respective ly. Ah of these errors were abolished by placing the PI 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. P lacing a pnemuotachograph, in between blows, in the flow stream of a f an blowing ambient air for 15 s minimizes errors due to condensation e ffects and temperature change in the pneumotachograph head.