| Summary: | It has been proposed that the naturally-occurring respiratory oscillation in P<sub>a,CO<sub>2</sub></sub> carries sufficient information on the rate of metabolism to be implicated in the generation of the hyperpnoea of hypoxic exercise. As a P<sub>CO<sub>2</sub></sub> signal of this frequency can be transduced by the carotid chemoreceptors but not by those of the medulla, it would be of interest to examine the manner in which hypoxia and a CO<sub>2</sub> transient interact at the carotid body. In the present study the responses of healthy subjects to an induced P<sub>A,CO<sub>2</sub></sub> oscillation (of two breaths' period) were examined against a background of graded hypoxia at rest and during moderate rhythmic exercise. The responses were measured on a breath-by-breath basis in a variety of inspiratory and expiratory variables including times, volumes, ventilations and flows. In brief it was found that:- (i) oscillating responses were largely confined to P<sub>A,O<sub>2</sub></sub> less than 72 torr; (ii) their incidence in hypoxia was potentiated by exercise;(iii) FRC oscillated, on average, more consistently than any other output variable; (iv) at times, an inspiratory stimulation occurred on the same breath as an expiratory depression; (v) a linear multiplication between hypoxia and the P<sub>A,CO<sub>2</sub></sub> oscillation was occasionally observed, the amplitude of the response increasing as hypoxia became more intense. Linear muliplication was more frequent in exercise. The patterns of oscillating responses were interpreted in terms of a carotid body involvement, with hypoxia and the P<sub>CO<sub>2</sub></sub> oscillation multiplying at receptor level, and a subsequent degree of complexity being introduced by the central timing of the carotid body discharge at the brain-stem. The potentiating effects of exercise may have resulted from a reduced attenuation of the P<sub>CO<sub>2</sub></sub> oscillation in transit from the lung to the carotid body, and an improved central timing of the carotid body discharge.
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