Directional Sensitivity Of Dynamic Cerebral Autoregulation In Healthy Lowlanders At High Altitude And High Altitude Natives

INTRODUCTION: Dynamic cerebral autoregulation (dCA), quantified via transfer function analysis with repeated squat-stands (RSS), is unaltered throughout high-altitude (HA, 5,050 m) partial acclimatization (2 weeks) in lowlanders (LL) and HA natives. Directional sensitivity of the dCA response (i.e., the attenuated change in cerebral blood velocity when mean arterial pressure [MAP] transiently increases vs. decreases) is reported at sea level. Whether the dCA response directional sensitivity is altered in LL at HA or differs from HA natives is unknown. OBJECTIVE: To examine the dCA directional sensitivity responses in healthy LL at HA and HA natives. METHODS: MAP and middle cerebral artery mean blood velocity (MCAv) were recorded continuously during 5-min RSS at 0.05 Hz and 0.10 Hz at sea level (N = 10, 29 ± 7 yrs), initial exposure to 5,050 m (N = 8, 28 ± 5 yrs), after 2 weeks of acclimatization to HA (N = 9, 29 ± 7 yrs), and in HA natives at HA (5,050 m; N = 16, 32 ± 14 yrs). For each transition, we calculated absolute MCAv and MAP changes with respect to the transition time intervals of both variables indexing time adjusted ratios when MAP increases (ΔMCAvT/ΔMAPTINC ) and decreases (ΔMCAvT/ΔMAPTDEC ). Values were then averaged over the 5-min RSS. RESULTS: In LL, ΔMCAvT/ΔMAPTINC was lower than ΔMCAvT/ΔMAPTDEC at sea level, initial arrival to HA, and after 2 weeks of HA acclimatization at both 0.05 Hz (MAP direction: p = 0.028, condition: p = 0.598, interaction: p = 0.181) and 0.10 Hz RSS (MAP direction: p = 0.002, condition: p = 0.547, interaction: p = 0.204). When acclimatized LL were compared with HA natives, ΔMCAvT/MAPTINC was lower than ΔMCAvT/ΔMAPTDEC at 0.05 Hz (MAP direction: p = 0.001, condition: p = 0.213, interaction: p = 0.601), while ΔMCAvT/ΔMAPTINC and ΔMCAvT/ΔMAPTDEC were not different at 0.10 Hz RSS (MAP direction: p = 0.547, condition: p = 0.117, interaction: p = 0.823). CONCLUSION: Directional sensitivity of dCA is present at both 0.05 Hz and 0.10 Hz in LL at sea level. Acute HA exposure and 2 weeks of partial HA acclimatization does not appear to have an influence on the dCA directional sensitivity. In addition, dCA directional sensitivity in HA natives is not different than partially acclimatized LL. Further research is needed to understand whether atmospheric pressure and carbon dioxide could influence these findings.