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1. The effects of changes in ventilation and/or alveolar P_CO2 on the baroreflex control of heart rate have been studied in seven experiments on six young men and women who had been trained to control tidal volume and respiratory frequency at various levels independent of alveolar P_CO2, during hyperoxia.

2. Intravenous phenylephrine provoked transient rises of directly measured arterial pressure during which individual systolic pressures (P) were linearly related to the following pulse interval (I). Baroreflex sensitivity was expressed as the slope of the regression of I on P, and reflex setting (I_ref) as I at a single reference arterial pressure (= mean P for the experiment).

3. Voluntary control of breathing had little effect on heart rate and arterial pressure (baroreflex setting), but diminished reflex sensitivity.

4. Hypercapnia regularly caused tachycardia at the reference pressure (i.e. baroreflex setting lowered). The response was completely or partly reproduced by change of P_{A, CO2} at constant ventilation in four subjects but not in two others; in them change of ventilation at constant P_{A, CO2} completely mimicked the effect of free-breathing hypercapnia.

5. Values of baroreflex sensitivity were relatively scattered. Hypercapnia caused a fall in baroreflex sensitivity in three subjects whether ventilation was fixed or free to rise. After separating the effect of voluntarily controlling ventilation, ventilation per se was without effect on reflex sensitivity.

6. It is concluded that hypercapnia and hyperpnoea have separate effects on the baroreflex, the relative magnitudes of which differ from one subject to another. Baroreflex setting and sensitivity vary independently in response to change of ventilation and of P_{A, CO2}.

This article has been cited by other articles:

				R. A. Henry, I-L. Lu, L. A. Beightol, and D. L. Eckberg Interactions between CO2 chemoreflexes and arterial baroreflexes Am J Physiol Heart Circ Physiol, June 1, 1998; 274(6): H2177 - H2187. [Abstract] [Full Text] [PDF]