Cerebral pressure-flow relationship directional sensitivity in healthy lowlanders and natives at high altitude

Whether cerebral pressure-flow relationship directional sensitivity, which represents the attenuated changes in cerebral blood velocity in response to transient increases, compared with decreases, in mean arterial pressure (MAP), is altered in lowlanders at high altitude or differs between lowlanders and Sherpa (a well-adapted highlander population of the Nepalese Khumbu region) is unknown. Both MAP and middle cerebral artery mean blood velocity (MCAv) were recorded continuously during 5-min repeated squat-stands (RSS) at 0.05 Hz and 0.10 Hz at sea level (n = 10), initial exposure to high-altitude (n = 8), after 2 wk of partial acclimatization to high-altitude (n = 9), and in Sherpa (n = 16). For each transition, we calculated absolute and relative MCAv and MAP changes with respect to the transition time intervals of both variables indexing time adjusted ratios when MAP increases (ΔMCAv_T/ΔMAP_T^INCREASE and %MCAv_T/%MAP_T^INCREASE) and decreases (ΔMCAv_T/ΔMAP_T^DECREASE and %MCAv_T/%MAP_T^DECREASE). Regardless of altitude conditions, %MCAv_T/%MAP_T^INCREASE was lower than %MCAv_T/%MAP_T^DECREASE [0.05 Hz RSS: (P = 0.007); 0.10 Hz RSS (P = 0.003)] in lowlanders. Partially acclimatized lowlanders and Sherpa had lower %MCAv_T/%MAP_T^INCREASE than %MCAv_T/%MAP_T^DECREASE at 0.05 Hz (P = 0.007), but comparable metrics at 0.10 Hz RSS (P = 0.971). These findings indicate acute exposure and partial acclimatization to high altitude do not alter the cerebral pressure-flow relationship directional sensitivity compared with sea level measures in lowlanders. In addition, the hysteresis-like pattern in Sherpa is not different when compared with partially acclimatized lowlanders.NEW & NOTEWORTHY Cerebral blood flow changes are buffered when blood pressure increases versus decreases. The current findings suggest that acute exposure and partial acclimatization to high altitude do not influence the presence of this phenomenon in lowlanders. Preservation of this hysteresis-like pattern across all altitude conditions may protect the cerebral microvasculature from hypoxia-induced increases in blood pressure. However, cerebral pressure-flow directional sensitivity is absent in Sherpa, a well-adapted highlander population, when blood pressure oscillates at higher frequency.