Acquired lightheadedness in response to odors after hyperventilation

This study aimed to investigate whether lightheadedness in response to odors could be acquired through previous associations with hyperventilation-induced hypocapnia.

Diluted ammonia and acetic acid served as conditional odor cues (CSs) in a differential associative learning paradigm. Hyperventilation-induced hypocapnia (unconditional stimulus [US]) was used to induce lightheadedness. In a training phase, participants (n = 28) performed three hypocapnic and three normocapnic overbreathing trials of 60 seconds each. One odor was consistently paired with the hypocapnic overbreathing (CS+); the other (control) odor was paired with normocapnic overbreathing (CS-). In the test phase, each odor was presented once during spontaneous breathing and once during normocapnic overventilation. Lightheadedness was assessed online during each breathing trial, which was followed by an extensive hyperventilation symptom checklist. Fractional end-tidal CO2, breathing frequency, and inspiratory volume were measured throughout the experiment.

In the test phase, participants experienced lightheadedness more quickly in response to the odor that had been paired with hypocapnic overbreathing compared with the control odor. They also scored higher on the symptom "feeling unreal."

Lightheadedness in response to odors can be acquired easily. The present results may help to elucidate the paradox that both avoidance and exposure to chemicals seem to be effective in reducing symptoms in idiopathic environmental illness.