Interactive effects of hypoxia and hypercapnia on sympathetic nerve activity in humans

The combined effects of hypoxia and hypercapnia on the sympathetic nervous system were investigated in nine healthy subjects. Each subject was exposed to 16 gas mixtures in random order; a4X4 matrix of hypoxic (21, 16, 12, 8%) and hypercapnic (0, 2, 4, 6%) gases. Methods: Sympathetic nerve activity (SNA), O₂ saturation, arterial pressure, end tidal CO₂, and tidal volume were measured continuously before and while breathing each gas mixture for 90 sec. Changes in SNA from the preceding baseline period were determined for each gas mixture. SNA also was segmented into low lung volume phases and high lung volume phases to investigate the modulation of SNA by respiratory phase. Results: Significant main effects differences were found for hypoxia (p<0.01) and hypercapnia (p<0.001), and a trend toward an interaction was apparent. The figure illustrates the individual and interactive effects of CO₂ and O₂ on the SNA response. These effects were accentuated during low lung volume phases of respiration (p<0.05). Conclusions: These data support the hypothesis that combined hypoxia and hypercapnia produce a mutual facilitation effect on SNA which is exaggerated by withdrawal of inhibitory effects of lung inflation. Supported by NtH, HL49266.