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Effect of recurrent episodic hypocapnic, eucapnic, and hypercapnic hypoxia on systemic blood pressure.

We have described a rat model that responds to chronic (8 h/day, 35 days) repetitive nonapneic episodic (cycled every 30 s) hypocapnic hypoxia with sustained increase in systemic blood pressure. Because the usual blood gas change of apnea is mildly increased CO2, we hypothesized that episodic hypoxia ranging from eucapnea to hypercapnia might cause a greater chronic increase in blood pressure than hypocapnic hypoxia in this model. Five groups of male Sprague-Dawley rats were studied: unhandled group received no treatment, sham group received compressed air in their chambers, hypocapnic hypoxic group received episodic hypoxia for 35 days, eucapnic hypoxic group received the same level of hypoxia but with 7-10% inspired fraction of CO2, and hybercarbic hypoxic group received hypoxia with 11-14% inspired fraction of CO2. Mean arterial blood pressure was measured in unrestrained conscious animals at baseline and after 35 days under their respective study conditions. Neither episodic eucapnic nor hypercarbic hypoxia had any additional effect on the changes in chronic diurnal blood pressure compared with hypocapnic hypoxia. These results suggest that the sympathetic nervous system or other neurohumoral systems contributing to chronic diurnal blood pressure elevation may be maximally stimulated by hypoxia or there may be some protective mechanism limiting the blood pressure response to asphyxia in this rat model.