EFFECT OF ALTERATIONS IN MENTAL ACTIVITY ON THE BREATHING PATTERN IN HEALTHY-SUBJECTS

The overall output from the respiratory centers is regulated by an automatic metabolic control system in the brainstem and by higher neural centers under direct voluntary control. An understanding of the constancy with which respiration is controlled can be obtained by measuring the breath-to-breath variability in breathing pattern. We hypothesized that different forms of mental activity would alter the variability of breathing pattern. To test this hypothesis, we measured breathing pattern on a breath-by-breath basis during resting wakefulness and during four conditions of altered mental activity. Measurements were obtained with a calibrated respiratory inductive plethysmograph, and variability was assessed by calculations of the coefficients of variation. We also examined the effect of the altered states of mental activity on the mean values of the breathing pattern components. We found that noxious stimulation increased the variability of all the breathing pattern indices, audiovisual stimulation tended to increase the variability of tidal volume (VT), and mental arithmetic had no effect. In addition, the variability of breathing pattern was Increased during rapid eye movement sleep and decreased during Stage IV sleep. The variability of VT and expiratory time were greater than that of inspiratory time (TI) across the different states of mental activity. Significant correlations were observed between VT and TI and between VT and frequency (f) during Stage IV sleep. With regards to the mean values, mental arithmetic, audiovisual stimulation, and noxious stimulation all increased minute ventilation and mean inspiratory flow. The increase in minute ventilation was achieved solely by an increase in f during mental arithmetic and audiovisual stimulation and by an increase in both f and VT during noxious stimulation. These results demonstrate that higher centers affect the breath-to-breath variability of the breathing pattern and that they exerted a greater influence on the variability of respiratory volume then on that of respiratory timing.