Lifelong high‐altitude hypoxia induces arterial baroreflex adaptations: new insights and future directions

Abstract

The arterial baroreflex control of blood pressure (BP) represents a fundamental negative feedback mechanism to maintain BP homeostasis. Unencapsulated, mechanically sensitive, free nerve endings located in the carotid sinus bifurcation and aortic arch relay afferent signals to the cardiovascular control areas in the brainstem regarding beat-to-beat oscillations in arterial BP. These afferent inputs elicit reflex changes in efferent parasympathetic and sympathetic activity to the heart and blood vessels modulating heart rate (cardiac baroreflex) and peripheral vasoconstrictor outflow (sympathetic baroreflex) in order to regulate arterial BP at rest and during cardiovascular stressors such as acute hypoxia (Fadel, 2008). Notably, high-altitude (HA) hypoxia elicits a robust increase in sympathetic outflow, which is significantly larger when compared with acute hypoxic exposure to a similar level. However, at present, the impact of long-term exposure to HA hypoxia on arterial baroreflex control of arterial BP remains poorly understood.In a recent issue of The Journal of Physiology, Simpson et al. (2019) eloquently filled this important information gap by studying Lowlanders and native Highlanders (Sherpas) at low altitude (LA) and HA. The authors sought to characterize the possible adaptations on arterial baroreflex control of BP elicited by lifelong HA exposure in Highlanders and the impact of acclimatization at HA in Lowlanders. To this end, 14 Lowlanders (two women) and nine male Highlanders (Sherpas, living at > 3440 m) were studied on two separate occasions: pre-expedition, LA test (344 m for Lowlanders and 1400 m for Highlanders) and following gradual ascent to 5050 m. Importantly, LA measurements of Sherpas was conducted > 4 days after descent from their resident altitude and the HA measures were conducted 10-14 and 10-20 days above 2860 m for Sherpas and Lowlanders, respectively. Muscle sympathetic nerve activity (MSNA) was directly recorded, from the peroneal nerve, via microneurography. Cardiac and sympathetic baroreflex control of BP were assessed by bolus injection of the vasoactive drugs sodium nitroprusside and phenylephrine to lower and raise BP, respectively (i.e. modified Oxford test). In addition, to assess the possible contribution of peripheral chemoreflex, the modified Oxford test was also performed during acute hypoxia (breathing 11% oxygen) at LA and during acute hyperoxia (breathing 100% oxygen) at LA and HA. The main findings were as follows: (1) despite similar resting BP, Sherpas have a higher MSNA burst frequency at HA than Lowlanders at LA, but it was lower when compared to Lowlanders following ascent to HA; (2) the sensitivity (i.e. gain) of sympathetic baroreflex control of BP is preserved in Lowlanders at HA and is similar to that of Sherpa counterparts, although the operating point was reset upward; (3) Lowlanders at HA presented a decrease in cardiac baroreflex sensitivity compared to at LA, but it was similar to Highlanders at HA; and (4) manipulation of peripheral chemoreceptor d...

Supraglottic jet oxygenation and ventilation decreased hypoxemia during gastrointestinal endoscopy under deep sedation at high altitudes: a randomized clinical trial

“…Although the ramifications of high-altitude hypoxia might have been well appreciated [3,[19][20][21][22][23], there have been few reports on the influence of high-altitude hypoxia on anesthesia management. Compared with previous studies, [8,[24][25][26][27] the occurrence of moderate hypoxia was more common in the nasal cannula group (47.2%).…”