Microcirculatory and Rheological Adaptive Mechanisms at High Altitude in European Lowlander Hikers and Nepalese Highlanders

Background: Physical activity at high-altitudes is increasingly widespread, both for tourist trekking and for the growing tendency to carry out sports and training activities at high-altitudes. Acute exposure to this hypobaric–hypoxic condition induces several complex adaptive mechanisms involving the cardiovascular, respiratory and endocrine systems. A lack of these adaptive mechanisms in microcirculation may cause the onset of symptoms of acute mountain sickness, a frequent disturbance after acute exposure at high altitudes. The aim of our study was to evaluate the microcirculatory adaptive mechanisms at different altitudes, from 1350 to 5050 m a.s.l., during a scientific expedition in the Himalayas. Methods: The main haematological parameters, blood viscosity and erythrocyte deformability were assessed at different altitudes on eight European lowlanders and on a group of eleven Nepalese highlanders. The microcirculation network was evaluated in vivo by conjunctival and periungual biomicroscopy. Results: Europeans showed a progressive and significant reduction of blood filterability and an increase of whole blood viscosity which correlate with the increase of altitude (p < 0.02). In the Nepalese highlanders, haemorheological changes were already present at their residence altitude, 3400 m a.s.l. (p < 0.001 vs. Europeans). With the increase in altitude, a massive interstitial oedema appeared in all participants, associated with erythrocyte aggregation phenomena and slowing of the flow rate in the microcirculation. Conclusions: High altitude causes important and significant microcirculatory adaptations. These changes in microcirculation induced by hypobaric–hypoxic conditions should be considered when planning training and physical activity at altitude.