Effects of hypercapnia, hypoxia, and rebreathing on heart rate response during apnea

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Abstract
Five series of breathholding (BH) experiments were conducted on 8 male subjects to determine the effects of apnea per se, hypercapnia and hypoxia on the development and maintenance of BH bradycardia. All BH were 90 s in duration and were achieved by face immersion at room temperature. Of the 5 series of BH, one was a continuous BH and the remaining included rebreathing at 15-s intervals while the face remained immersed. Comparison of heart rate (HR) responses between continuous BH with air and that interrupted every 15 s by rebreathing without improving the alveolar gas composition yields the attenuating effect of respiratory activity. The hypercapnic effect was calculated from the difference in HR responses between 2 series of BH with O2, in which hypoxia was not present and rebreathing was common to both; in one, rebreathing was through a CO2 scrubber, thus creating different alveolar CO2 levels in the 2 series. The effect of hypoxia was deduced by finding the difference between the total bradycardial response (continuous BH) and the summed effects of hypercapnia and apnea per se. Apnea and hypoxia reduced the HR by 19 and 18%, respectively, from the pre-BH value, and hypercapnia increased HR by 6% from the pre-BH level, thus accounting for the total 31% reduction in HR in a continuous BH.