The human ventilatory response to stimulation by transient hypoxia.

1. The detailed pattern of transient changes in breathing pattern has been studied following a brief hypoxic stimulus (three breaths of nitrogen) in nine healthy subjects. All showed an increase in ventilation of which the magnitude and relative contributions of volume and frequency varied between subjects. 2. Ventilation, tidal volume, inspiratory, expiratory and total breath time were recorded or derived breath-by-breath; for each of these variables, several test sequences were time-averaged at half-second intervals for each individual; similarly, time-averages were obtained for percentage changes from base line over all nine subjects. 3. There ws an increase in inspiratory time accompanying the increasing tidal volume, in all but two subjects. This was statistically significant over all subjects, and in five individuals. Frequency changes were the resultant of alterations in the two phases; when total breath duration decreased it was always linked to a decrease in expiratory time. 4. Further analysis of the initial part of the response suggests that an increase of the duration of an inspiration may be the first change allowing an increase in tidal volume, before the 'drive' increases; this may be a dynamic feature of the control system whatever the nature and site of action of the stimulus.