Breathing pattern in humans: elevated CO2 or low O2 on positive airway pressure.

The purpose of this study was to determine effects on breathing pattern of pressure breathing alone and in combination with chemical stimulation. We analyzed ventilatory responses to elevated airway pressures (positive-pressure breathing, PPB) in subjects breathing air, 12% O2, or elevated CO2. Each subject sat in a body box and breathed via mouth-piece from a bag-in-box. Responses to PPB on air were increased minute ventilation (VI), tidal volume (VT), frequency (f), mean inspiratory (VT/TI) and expiratory (VT/TE) flows, decreased expiratory duration (TE) and end-tidal CO2. If end-tidal CO2 were held constant, VI, VT, and VT/TI increased less. Responses greater than predicted from summing responses to either stimulus alone were observed for VT, f, VT/TI, and VT/TE during 3 and 5% CO2 and for VT, f, and VT/TE during isocapnic hypoxia. Responses to other combined stimuli were sums of responses to the individual stimuli. Thus ventilatory responses to combined PPB and chemical stimuli cannot be predicted simply from summating responses to each independently imposed stimulus, suggesting that sensory information arises from and is integrated at multiple sites.