Effects Of Chronic Intermittent Hypobaric Hypoxia On Heart Rate Variability In Conscious Rats

Objective: In recent years, the cardioprotective effect of chronic intermittent hypobaric hypoxia (CIHH) has been recognized and causes the public attention more and more. The study on the mechanism of CIHH cardioprotection is a hot topic of research in clinical medicine, space medicine and high altitude medicine. Numerous studies have shown that exposure of rats to CIHH induces cardioprotective effects, promoting the tolerance of myocardium to ischemia/reperfusion (I/R) injury, enhancing the recovery of cardiac function from I/R and preventing I/R induced-arrhythmia. The mechanisms of CIHH cardiac protection involves oxygen transportation, energy metabolism, neurohumoral regulation, antioxidase, stress protein, adenosine, ATP sensitive potassium channel, mitochondrion, calcium control, nitric oxide and protein kinase. The control of cardiac autonomic nervous system, including sympathetic and parasympathetic nerves, plays an important role in normal cardiac function, and also involves in a lot of pathological process of cardiac diseases. The cardiac health condition and the ability of cardiac autonomic regulation can be evaluated by heart rate variability (HRV). However, little is known about the effect of CIHH on cardiac autonomic nerves system or whether the modulation of cardiac autonomic nervous system participates in the cardioprotection during adaption to CIHH. The aim of this study was to observe the effect of CIHH on HRV in the conscious rats during the whole process of CIHH treatment using telemetering technique, and to explore the effect of CIHH on the modulation of cardiac autonomic nervous system or the role of cardiac autonomic nervous system in CIHH cardiac protection.Methods: Male adult Sprague-Dawley rats were randomly divided into two groups: control group (CON) and CIHH treatment group (CIHH). The rats in CIHH group were exposed to hypoxia simulated 5000 m high altitude (oxygen 11.1%) in a hypobaric chamber for 42 days, 6 hrs/day. Control rats were living in the same environment as CIHH rats with free access to food and water, except no CIHH treatment. The body weight and general activity of rats was recorded in every week.Standard leadⅡECG was recorded continiously in conscious rats through Dataquest A.R.T. System (DSI, USA) before, during, and after the treatment of CIHH everyday. HRV was evaluated with spectrum analysis method and parameters include total power (TP), extremely low-frequency component (VLF), low-frequency component (LF), the high-frequency component (HF), LF/HF and heart rate (HR).All data were expressed as means±SD. T-test was used for comparison between two groups. P<0.05 was taken as a significant difference.Results: (1) Body weight of rats was increased along with the rats growing. There was no significant difference of body weight between CIHH and CON rats (P<0.05).(2) TP and VLF in CIHH rats were not different compared with CON rats under normoxia condition, but were significantly lower than those in CON rats during CIHH in hypobaric chamber since the 4th week of CIHH (P<0.05). Under acute hypoxia environment, TP and VLF in CON rats were decreased significantly (P<0.05), while the change of TP and VLF in CIHH rats was not significant (P>0.05).(3) LF and LF/HF in CIHH rats were not different compared with CON rats under normoxia condition. During CIHH in hypobaric chamber in the 1st week of CIHH, LF and LF/HF in CIHH rats were increased markedly (P<0.05), and gradually recovered back to normal level in the 2nd week of CIHH. Under acute hypoxia condition, LF was similar compared with the normoxia, but HF significantly lower(P<0.05)which caused LF/HF augmented(P<0.05); however, no significant changes were found on HF, LF and LF/HF in CIHH during exposure to acute hypoxia. (4) HR in CON rats under normoxia condition decreased gradually, obviously lower than the baseline in four weeks(P<0.05), and lower than that in CIHH rats in the 6th week(P<0.05). HR in CIHH rats was not changed significantly no matter in normoxia or CIHH conditions (P>0.05). During acute hypoxia, HR increased, but the increasing of HR was much smaller in CIHH(10%)than that in CON rats(34%, P<0.05).Conclusion: CIHH has no significant effect on the basic HRV in rat, but effectively antagonizes the changes of HRV and HR induced by acute hypoxia, which might related with the enhancement of sympathetic modulation during simulated CIHH and prevention of depression of vagal tone during acute hypoxia.