INTERACTION OF METABOLIC AND BEHAVIORAL RESPIRATORY CONTROL DURING HYPERCAPNIA AND SPEECH

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Abstract
The interaction of the metabolic and behavioral respiratory control systems was examined by analyzing ventilatory responses to hypercapnia during normal speech. Studies were performed in 7 healthy subjects who breathed through a tight-fitting face mask. Hyperoxic progressive hypercapnia to an alveolar PCO2 [CO2 partial pressure] of 70 mm Hg was induced by rebreathing, and ventilatory responses were analyzed on a breath-by-breath basis. In control rebreathing runs, the subjects remained silent; in talking runs, they spoke or read aloud after the mixed venous PCO2 plateau had been established. All subjects reported that they were considerably less dyspneic during talking runs than during control runs. The response of instantaneous minute volume of ventilation (.ovrhdot.VI) to CO2 was markedly decreased during speech (from a mean of 3.7-1.0 l/min per mm Hg); the responses of respiratory frequency (f) and tidal volume (VT) were correspondingly decreased; and there was marked breath-to-breath variability in .ovrhdot.VI, f and VT despite progressive hypercapnia. During control runs, mean inspiratory and expiratory flow rates increased progressively from breath to breath, which resulted in a negative relationship between inspired VT and inspiratory time (TI). In contrast, during talking runs mean inspiratory and expiratory flow rates were held constant (.simeq. 2.0 and 0.5 l/s, respectively) regardless of the degree of hypercapnia, and there was a positive relationship between VT and TI. During normal activities the behavioral respiratory control system can override the metabolic control system within wide limits. In addition, the regulation of inspiratory flow rate during phonation, despite wide flucutations in VT and TI, increases the possibility that during control runs inspiratory flow was also the regulated variable. The orderly changes in VT and TI were secondary to the progressive increase in inspiratory flow.