Autonomic regulation of heart rate and peripheral circulation: Comparison of high altitude and sea level residents

1. To evaluate the activity of the autonomic nervous s stem on the heart and perihperal circulation in native high-altitude residents, during a Rimalayan expition we studied 12 men (age: 48±4, mean±SEM). life-long resident in a villa e at 4800 m (Sumdo village. Zanskar, India) and 7 healtiy sea-level residents (age: 37±4) after 7 days of acclimatization (acclimatized lowlanders) at the same altitude. Furthermore 25 sea level residents (age: 46±2) underwent the same protocol at sea level. 2. R-R interval RR), respiratory signal, non-invasive blood pressure, an6 skin arteriolar blood flow were evaluated in three different conditions: during free breathin in supine position and during controlled breathing (at 0.15 Hz) in Supine and upright position and analysed by autoregressive spectral analysis [low- (around 0.1 Hz ) and by Hz).and high-frequency (respiratory) fluctuations, LF and HF, markers of sympathetic and vagal activity, respectively]. 3. High altitude residents showed in supine position a higher RR than acclimatized lowlanders, similar to sea-level residents. RR variabilit was reduced in acclimatized lowlanders compared to both high-altitude residents and sea level residents. Systolic blood pressure (SBP) did not show significant differences between the three groups. High altitude residents showed in supine lower LF in RR signal compared to sea-level residents, and, Compared to acclimatized lowlanders, hi her HF and lower LF/HF ratio; high-altitude residents stowed a reduction in skin microcirculation variabilit compared to sea-level residents, but this was ei ht fold greater than in acclimatized lowlander. thus ingicatin a much greater vasoconstriction in acclimatized lowlanders thsn in high-altitude residents. 4. In upright position, high-altiude residents showed the. same behaviour as sea-level residents, with increase in LF- RR. and decrease in HF and LF-SBP. Acclimatized lowlanders showed similar directional trends though not significant changes for RR-LF. 5. After one week of acclimatization, lowlanders still manifested sympathetic activation and skin vasoconstriction; high-altitude residents did not show reduced vagal tone compared to sea-level residents, but a mild vasoconstriction appeared to be present. In conclusion, normal or enhanced vagal tone and preserved vasomotion are probably evidence of adaptation at hi h altitude hypoxia.