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Sympathetic Function in Sherpa and Lowlanders at High Altitude

  • Author / Creator
    Busch, Stephen
  • Introduction: Low altitude dwellers (Lowlanders) that ascend to altitude are met with the challenge of a chronically low oxygen environment (hypoxia). Muscle sympathetic nerve activity (MSNA) becomes augmented under hypoxia in an attempt to redistribute blood towards oxygen critical tissues. Previous work has shown that both basal MSNA and blood pressure progressively increase in Lowlanders over the span of several days at altitude. This sympathetic response may differ in native Sherpa, who exhibit an improved cardiovascular response at altitude relative to Lowlanders. However, no studies have measured MSNA in native Sherpa. Furthermore, the sympathetic response to additional stress (sympathetic reactivity) at altitude has not been previously been measured. Hypothesis: We believe that Sherpa will exhibit lower basal sympathetic activity than acclimatized Lowlanders at high altitude. This will also correspond with lower sympathetic reactivity in Sherpa at altitude. Methods: Microneurography was used to measure MSNA in Lowlanders (n= 14) at 344m and 5050m, while Sherpa (n = 8) were measured at 5050m. MSNA burst frequency (bursts/ min), burst incidence (bursts/100 hb), and burst amplitude (% of max burst) was collected during 10 minutes of supine rest. Sympathetic reactivity was measured via end-expiratory breath hold, which produces a strong sympathetic response. Sympathetic reactivity was quantified as burst area (au) and total normalized SNA (au/min) during the last 10 cardiac cycles of each breath hold in Lowlanders and Sherpa at both altitudes. In addition, total normalized SNA was calculated during the last 15 cardiac cycles of baseline for comparison with reactivity conditions. Results: Ascent from low to high altitude saw an increase in Lowlander burst frequency (11±5 bursts/ min to 30±6 bursts/ min; Mean±SD; p<0.001) and burst incidence (25±13 bursts/ 100hb to 50±15 bursts/ 100hb; p<0.001), while Sherpas saw lower burst frequency (23±11 bursts/ min; p<0.05) and incidence (44±20 bursts/ 100hb; p<0.05) at 5050m. Burst amplitude was similar between both groups at altitude, though Lowlanders exhibited larger bursts at 5050m compared to 344m (P<0.05). Both groups showed an increase in MSNA during the last 5 cardiac cycles of breath holding prior to volitional breakpoint (P<0.05). However, burst area was lower in Sherpa compared to Lowlanders at high altitude (P<0.05) . In addition, the total normalized SNA and blood pressure responses were similar in Lowlanders between 344m and 5050m; while Sherpa exhibited a lower SNA and blood pressure response compared to Lowlanders at altitude. Discussion: In the current study we were able to collect, for the first time, direct recordings of post-ganglionic muscle sympathetic activity in native Sherpa. These results demonstrate that Sherpas exhibit lower SNA at altitude compared to Lowlanders, despite no difference in blood pressure and resting arterial oxygen saturation. We also showed that sympathetic reactivity was lower in Sherpa at 5050m through the use of voluntary breath holds, despite a similar blood pressure during breath holding as Lowlanders. These findings suggest an increased vascular responsiveness within Sherpa that is able to maintain blood pressure similar to that of acclimatized Lowlanders under reduced sympathetic activation at altitude. In addition, the findings of a similar SNA response to breath holding in acclimatized, despite higher basal MSNA at altitude, suggest a sympathetic reserve becomes reduced at altitude.

  • Subjects / Keywords
  • Graduation date
    Spring 2018
  • Type of Item
    Thesis
  • Degree
    Master of Science
  • DOI
    https://doi.org/10.7939/R3B854008
  • License
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