Symptoms that occur in response to odorous substances can be learned and generalize to new substances, especially in persons with high negative affectivity. The findings further support the plausibility of a pavlovian perspective of multiple chemical sensitivity.

Objectives-Multiple chemical sensitivity is a poorly understood syndrome in which various symptoms are triggered by chemically unrelated, but often odorous substances, at doses below those known to be harmful. This study focuses on the process of pavlovian acquisition and extinction of somatic symptoms triggered by odours. Methods-Diluted ammonia and butyric acid were odorous conditioned stimuli (CS). The unconditioned stimulus (US) was 7.4 % CO 2 enriched air. One odour (CS+) was presented together with the US for 2 minutes (CS+ trial), and the other odour (CS−) was presented with air (CS-trial). Three CS+ and three CSexposures were run in a semi-randomised order; this as the acquisition (conditioning) phase. To test the eVect of the conditioning, each subject then had one CS+only-that is, CS+ without CO 2 -and one CS-test exposure. Next, half the subjects (n=32) received five additional CS+only exposures (extinction group), while the other half received five exposures to breathing air (wait group). Finally, all subjects got one CS+only test exposure to test the eVect of the extinction. Ventilatory responses were measured during and somatic symptoms after each exposure. Results-More symptoms were reported upon exposure to CS+only than to CSodours, regardless of the odour type. Altered respiratory rate was only found when ammonia was CS+. Five extinction trials were suYcient to reduce the level of acquired symptoms. Conclusion-Subjects can acquire somatic symptoms and altered respiratory behaviour in response to harmless, but odorous chemical substances, if these odours have been associated with a physiological challenge that originally had caused these symptoms. The conditioned symptoms can subsequently be reduced in an extinction procedure. The study further supports the plausibility of a pavlovian conditioning hypothesis to explain the pathogenesis of MCS. (Occup Environ Med 1999;56:295-301)

Simply breathing via a mouthpiece and pneumotachograph made end-tidal CO 2 fractional concentration (FET,CO 2 ) decrease progressively both in hyperventilators and in patients with anxiety disorders, but not in normals. At the start of the measurement the FET,CO 2 was not different between patients and healthy subjects. In patients ≤28 yrs, the decrease of FET,CO 2 resulted from a higher tidal volume, and in patients ≥29 years from an increase in frequency. After voluntary hyperventilation, the recovery of FET,CO 2 was delayed in patients, due to a slower normalization of respiratory frequency in females and in older males, and of tidal volume in younger males, and also due to less frequent end-expiratory pauses. When breathing was recorded first by means of inductive plethysmography (Respitrace), the progressive decline of FET,CO 2 seen in patients was not observed: from the onset of the recording, FET,CO 2 was reduced in patients. It did not change further when, immediately afterwards, the subject switched to mouthpiece breathing.The finding that breathing through a mouthpiece induces hyperventilation in patients and that recovery of FET,CO 2 is delayed after voluntary hyperventilation, suggests that the respiratory control system is less resistant to challenges (mouthpiece or voluntary hyperventilation) in those patients. On the other hand, the lower values of FET,CO 2 measured during recording by means of a Respitrace probably result from a challenge, prior to the recordings, induced by the fitting of the measuring device to the patient. This unsteadiness of breathing characterizes patients with hyperventilation syndrome and those with anxiety disorders, but is not sufficiently sensitive to be used for individual diagnosis.

Supported by the Research Council of the University of Leuven (grant OT/93/12).It is known that in healthy subjects breathing through a mouthpiece results in an increase of tidal volume (VT) [1][2][3], of inspiratory (t I) [2,3] and expiratory time (t E) [2], and inspiratory drive (VT/t I) [2,4]. These changes in ventilation have been attributed to: 1) the influence of the additional dead space; 2) stimulation of the nasal and oral mucosa by the noseclip and mouthpiece; 3) shift of respiratory route from unrestricted nose to mouth.For these reasons, an alternative technique, i.e. respiratory inductive plethysmography (Respitrace) is widely used in the study of the breathing pattern to avoid the influence of the mouthpiece and occlusion of the nose.As pointed out by GILBERT et al.[1], a fourth influence which might modify the natural resting ventilation is that of registration itself, by focusing the subject's attention on his breathing. The use of any recording technique, even a noninvasive one, might modify the spontaneous breathing pattern. In the present study, we tried to evaluate this influence. Materials and method SubjectsThe study was performed on two groups of healthy volunteers: 1) 42 subjects ("younger" population), 25 females and 17 males, aged 21-26 (mean 22) yrs, who were (except for two) medical students of K.U. Leuven, Belgium; 2) 32 subjects ("older" population), 15 females and 17 males, aged 35-63 (mean 47) yrs, who were recruited from outside the hospital. All 74 subjects (younger and older population) were naive to the purpose of the study. Anonymity was ensured.Before the experiment, the subjects answered four questionnaires. One dealt with medical history, two screened the level of anxiety, and one the spontaneous complaints of the subjects. To evaluate the anxiety of the subjects, the state and trait versions of the Zelfbeoordelingsvragenlijst (ZBV-DY1, ZBV-DY2) [5], which is the Dutch Seventy four subjects (40 females and 34 males), aged 21-63 yrs, were studied under three different conditions whilst their breathing was being recorded for 5 min by means of inductance plethysmography (Respitrace): 1) subjects were misled into believing that their breathing was not being recorded but that they had to wait for 5 min whilst equipment was calibrated; 2) subjects were instructed that their breathing pattern was being recorded for 5 min; 3) the subject's breathing was recorded for 5 min with mouthpiece and pneumotachograph. The first two conditions were randomized. The Respitrace was calibrated by means of multiple linear regression carried out during the 5 min period of quiet breathing through a mouthpiece.Awareness of the recording of breathing caused prolongation of inspiratory (t I) and expiratory time (t E). Breathing through the mouthpiece resulted in an increase of t I, t E and tidal volume (VT). The breathing irregularities (sighs and end-expiratory pauses) decreased when subjects were aware of the recording of breathing and nearly disappeared when subjects breathed through the mouthpiec...

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