Human responses to propionic acid. II. Quantification of breathing responses and their relationship to perception

In 20 normal and four anosmic participants, instantaneous inhalation and ex halation flow rates were recorded in response to 15 s stimulations with cle an air or propionic acid concentrations (0.16, 1.14, 8.22 and 59.15 p.p.m., v/v) that ranged from peri-threshold for normals to clearly supra-threshol d for anosmics. Each odorant/irritant delivery to the face-mask began with an exhalation. This allowed concentration to reach full value before stimul us onset, defined as the point where the participant began to bring the sti mulus into the nose by inhalation. Two seconds after this stimulus onset, n ormals exhibited cumulative inhaled volume (CIV) declines of 39 and 14%, an d latencies of 500 and 710 ms, with presentations of 59.15 and 8.22 p.p.m., respectively. With anosmics, 59.15 p.p.m. caused a 19% decline in CIV that began at 730 ms. Examination of the first inhalation after stimulus onset shows that the CIV declines in normals were achieved by a progressive decli ne in volume (InVol), beginning with a slight drop at 1.14 p.p.m., and a ma rked decline in duration (InDur) with only the highest concentration. Anosm ics exhibited declines in InDur and InVol with only the 59.15 p.p.m. stimul us, and these declines were much more modest than the changes seen in norma ls. Comparison of these breathing results with perceptual responses from th is same experiment demonstrates that: (i) in normals, odor perception rises slightly, but breathing does not change, with the lowest concentration; (i i) the higher breathing sensitivity (declines in InVol) of normals is paral leled by both the higher nasal irritation of these individuals and the pres ence of odor sensation; (iii) InDur declines in normals only with a stimulu s concentration sufficient to cause marked nasal irritation in anosmics; an d iv) in anosmics, modest but reliable declines in both InDur and InVol mir ror the marked elevation in nasal irritation magnitude seen with only the h ighest concentration. In view of the failure of prior work to provide evide nce that olfactory activation alone can cause any of the breathing changes we observed, we conclude that some breathing parameters are quite useful as rapid and sensitive measures of nasal irritation that arises from activati on of nasal trigeminal afferents alone or in combination with the olfactory nerve.