SPIROMETRY WITH A FLEISCH PNEUMOTACHOGRAPH - UPSTREAM HEAT-EXCHANGER REPLACES HEATING REQUIREMENT

The exact temperature of the head of an unheated Fleisch pneumotachogr aph (PT) during recording is not known, and variation in its temperatu re may lead to errors in measuring spirometric indexes. We measured PT head temperature during blows from five normal subjects, recorded by using a PT with and without an upstream heat exchanger to condition th e air to the ambient temperature that was set in a climate chamber. Gr oup mean (+/-SD) temperature of a thermocouple (TC) placed inside the PT head was 11.8 +/- 1.9 degrees C with 7 degrees C ambient, 25.4 +/- 1.3 degrees C at 23 degrees C, and was 37.2 +/- 0.3 degrees C at 37 de grees C. The between-subject range of temperature for this TC was 7.5 degrees at 7 degrees C, 5.5 degrees at 23 degrees C, and 1.1 degrees a t 37 degrees C. The mean within-subject within-blow variation of tempe rature for this TC was 10.0 degrees and 3.3 degrees C for ambient of 7 degrees and 23 degrees C, respectively. At the usual ambient temperat ure in a laboratory, these differences in temperature lead to a 3.6% b etween-subject bias in recording, and the within-subject differences l ead to 2.6% underreading of peak expiratory flow and a 0.5% overreadin g later in the blow which makes ATPS-to-BTPS correction erroneous or d ifficult to perform. With the use of an upstream heat exchanger, the g roup mean temperature was 8.7 +/- 0.4 degrees, 23.2 +/- 0.2 degrees, a nd 37.1 +/- 0.2 degrees C at the three ambient temperatures, respectiv ely, and the within-subject within-blow variation was reduced to <1 de grees C. A heat exchanger placed upstream of the PT satisfactorily con ditioned expired air to the ambient temperature and removed the error.