CARDIOVASCULAR AUTONOMIC MODULATION AND ACTIVITY OF CAROTID BARORECEPTORS AT ALTITUDE

1. To assess the effects of acute exposure to high altitude on barorec eptor function in man we evaluated the effects of baroreceptor activat ion on R-R interval and blood pressure control at high altitude. We me asured the low-frequency (LF) and high-frequency (HF) components in R- R, non-invasive blood pressure and skin blood flow, and the effect of baroreceptor modulation by 0.1-Hz sinusoidal neck suction. Ten healthy sea-level natives and three high-altitude native, longterm sea-level residents were evaluated at sea level, upon arrival at 4970 m and 1 we ek later. 2. Compared with sea level, acute high altitude decreased R- R and increased blood pressure in all subjects [sea-level natives: R-R from 1002+/-45 to 775+/-57 ms, systolic blood pressure from 130+/-3 t o 150+/-8 mmHg; high-altitude natives: R-R from 809+/-116 to 749+/-47 ms, systolic blood pressure from 110+/-12 to 125+/-11 mmHg (P < 0.05 f or all)]. One week later systolic blood pressure was similar to values at sea level in all subjects, whereas R-R remained elevated in sealev el natives. The low-frequency power in R-R and systolic blood pressure increased in sea-level natives [R-R-Lf from 47 +/- 8 to 65 +/- 10% (P < 0.05), systolic blood pressure-if from 1.7 +/- 0.3 to 2.6 +/- 0.4 I n-mm Hg-2 (P < 0.05)], but not in high-altitude natives (R-R-LF from 3 2 +/- 13 to 38 +/- 19%, systolic blood pressure-if from 1.9 +/- 0.5 to 1.7 +/- 0.8 In-mmHg(2)). The R-R-HF decreased in sealevel natives but not in high-altitude natives, and no changes occurred in systolic blo od pressure-HF. These changes remained evident I week later. Skin bloo d flow variability and its spectral components decreased markedly at h igh altitude in sea-level natives but showed no changes in high-altitu de natives. Neck suction significantly increased the R-R- and systolic blood pressure-LF in all subjects at both sea level and high altitude . 3. High altitude induces sympathetic activation in sea-level natives which is partially counteracted by active baroreflex. Despite long-te rm acclimatization at sea level, high-altitude natives also maintain a ctive baroreflex at high altitude but with lower sympathetic activatio n, indicating a persisting high-altitude adaptation which may be gener ic or due to baroreflex activity not completely lost by at least 1 yea r's sea-level residence.