Objective
This study tested whether functional somatic symptoms are associated with exaggerated increases in self-reported anxiety and somatic complaints in response to stress and CO2-enriched air breathing, and whether this association exists in parallel to or in the absence of exaggerated physiological responses.
Methods
Out of 499 young somatically healthy undergraduate women, 18 participants high in functional somatic symptoms (HSS group) and 18 participants low in symptoms (LSS) were selected. They were submitted to mental stress, mild physical exercise and relaxation during conditions of normal breathing, breathing compressed normal air, and breathing compressed 5% CO2-enriched air. In all conditions, self-reported anxiety and somatic symptoms and respiratory and autonomic responses were assessed.
Results
HSS participants reported, as compared to LSS, more tenseness, anxiety, and somatic symptoms at baseline and increased responses to mental stress and during 5% CO2 breathing, but not in response to exercise. However, no evidence was found for a corresponding exaggerated respiratory or autonomic response.
Conclusion
A young, female, and nonclinical population with numerous functional somatic symptoms and high levels of anxiety is characterized by an exaggerated perception of a normal physiological response.

This study tested various sources of changes in respiratory
sinus arrhythmia (RSA). Twenty-two healthy participants
participated in three experimental conditions (mental stress,
relaxation, and mild physical exercise) that each consisted
of three breathing parts (normal breathing, breathing compressed
room air, and breathing compressed 5% CO2-enriched
air). Independent contributions to changes in RSA were found
for changes in tonic vagal modulation of heart rate, central
respiratory drive (i.e., PaCO2), respiratory depth,
and respiratory frequency. The relative contributions to changes
in RSA differed for mental stress and physical exercise. It
is concluded that uncorrected RSA will suffice to index
within-subject changes in tonic vagal modulation of heart rate
in most situations. However, if the central respiratory drive
is expected to change, RSA should ideally be corrected for changes
in PaCO2, respiratory depth, and respiratory frequency.

The hypothesis that biased symptom perception toward excessive symptoms is common when relatively normal chronic patients enter symptom-relating situations, irrespective of emotional variables, was tested in 19 women with severe asthma, 18 with somatization-like characteristics, and 18 controls. Each underwent three experimental conditions: mental stress, resting, and physical exercise. Each condition included three breathing conditions: breathing normally, normal compressed air, and 5.5% CO2-enriched compressed air. Results yielded no group differences in physiological measures, e.g. elevated CO2 in exhaled air (end-tidal partial pressure of CO2, PetCO2), or lung function. Asthma patients experienced more breathlessness, and somatization-like participants more breathlessness, miscellaneous symptoms, and subjective stress than controls. Although these differences suggested acquired biased symptom perception, as it turned out, breathlessness in asthmatics was more influenced by PetCO2 and less by subjective stress compared to controls. Likewise, breathlessness in somatization-like participants was similarly influenced by PetCO2 and subjective stress compared to controls, and miscellaneous symptoms were even more influenced by PetCO2 and less by subjective stress compared to controls. It was concluded that acquired sensitivity to physiological activity associated with habitual symptoms may account for excessive symptoms in patients with chronic health problems.