EXPIRED AIR-TEMPERATURE AT THE MOUTH DURING A MAXIMAL FORCED EXPIRATORY MANEUVER

We have studied the temperature of expired air during a maximal forced expiratory manoeuvre, because this has not previously been fully inve stigated and it will influence how flow and volume recording devices s hould be calibrated and used. Temperature was recorded with a fine the rmocouple, the response time of which was determined at various gas ve locities and for which a correction was made. Recordings during maxima l forced expiratory manoeuvres were made on 12 normal subjects and 12 subjects with chronic airflow limitation. The thermocouple was placed in the mouthpiece, so that it was at the level of the lips during a bl ow. In the normal subjects, the effect of differing inhalation protoco ls was also determined. In the normal subjects, the mean temperature w as 33.6-degrees-C at peak expiratory flow (PEF), and 34.4-degrees-C at 75% forced vital capacity (FVC), but fell to 33.4-degrees-C at FVC. I n the subjects with chronic airflow limitation, the temperature was co nstant at 35.0-degrees-C from PEF up to 50% FVC, being significantly h igher than in the normals, and fell to 33.5-degrees-C at FVC. Expired air temperature up to 50% FVC was significantly negatively correlated with absolute PEF, forced expiratory volume in one second (FEV1) and F VC. In the normals, a slow inhalation through the nose raised the expi red temperature by almost 1-degrees-C throughout the blow, whereas inh aling air at 6-degrees-C did not affect expired air temperature The ex pired air temperature can vary by up to 3-degrees-C between individual subjects, and it is influenced by the route of inhalation and the ins pired volume. For performing a maximal forced expiratory manoeuvre the least temperature variation between subjects would be obtained follow ing a slow inhalation through the mouth.