EFFECTS OF SYSTEMIC HYPOXIA ON R-R INTERVAL AND BLOOD-PRESSURE VARIABILITIES IN CONSCIOUS RATS

The effects of systemic hypoxia with different levels of CO2 on R-R in terval (RRI) and systolic blood pressure (SBP) variabilities were inve stigated in conscious rats. Wistar rats chronically instrumented for t he measurement of blood pressure, electrocardiogram, and renal sympath etic nerve activity (RSNA) were exposed to hypocapnic (Hypo), isocapni c (Iso), and hypercapnic (Hyper) hypoxia. On another day, the rats wer e treated with atropine and exposed to the same type of hypoxia. Sinoa ortic denervation (SAD)-treated rats were exposed to Iso and Hyper, an d RRI and SEP variabilities before and during hypoxia were analyzed us ing the maximum-entropy method with high resolution. With regard to RR I variability, Very low frequency (VLF), low frequency (LF), and high frequency (HF) powers all decreased during Hypo, increased during Hype r, and did not change during Iso in intact rats. Changes during Hypo w ere attenuated by atropine, and those during Hyper were abolished by e ither atropine or SAD. The ratio of LF power to HF power decreased ind ependently of increases in RSNA during each type of hypoxia. On the ot her hand, there were no changes in VLF, LF, or KF power in SEP variabi lity during each type of hypoxia in intact rats. In atropine-treated r ats, LF power increased during Iso and Hyper and HF power increased du ring each type of hypoxia. There was no difference in respiratory freq uency among the three kinds of hypoxia in both intact and atropine-tre ated rats. The results suggest that arterial PCO2, level rather than r espiration frequency produces changes in powers of RRI variability thr ough changes in parasympathetic nerve activity and that with regard to SEP variability, parasympathetic nerve activity masks changes in LF p ower that reflect an increase in RSNA and those in HF power that refle ct a mechanical consequence of respiration.