UBC‐Nepal Expedition: Haemoconcentration underlies the reductions in cerebral blood flow observed during acclimatization to high altitude

• Published in: Experimental physiology 2019-12, Vol.104 (12), p.1963-1972
• Main Authors: Howe, Connor A., Ainslie, Philip N., Tremblay, Joshua C., Carter, Howard H., Patrician, Alex, Stembridge, Mike, Williams, Alex, Drane, Aimee L., Delorme, Eric, Rieger, Mathew G., Tymko, Michael M., Gasho, Chris, Santoro, Antoinette, MacLeod, David B., Hoiland, Ryan L.
• Format: Article
• Language: English
• Subjects: Acclimatization; Altitude; Blood flow; Blood pressure; Cerebral blood flow; Diuresis; Erythropoiesis; Gases; haematocrit; haemodilution; Hematocrit; high‐altitude; hypoxia; Ultrasound
• ISSN: 0958-0670; 1469-445X
• DOI: 10.1113/EP087663

New Findings What is the central question of this study? The aim was to evaluate the degree to which increases in haematocrit alter cerebral blood flow and cerebral oxygen delivery during acclimatization to high altitude. What is the main finding and its importance? Through haemodilution, we determined that, after 1 week of acclimatization, the primary mechanism contributing to the cerebral blood flow response during acclimatization is an increase in haemoglobin and haematocrit. The remaining contribution to the cerebral blood flow response during acclimatization is likely to be attributable to ventilatory acclimatization. At high altitude, an increase in haematocrit (Hct) is achieved through altitude‐induced diuresis and erythropoiesis, both of which result in increased arterial oxygen content. Given the impact of alterations in Hct on oxygen content, haemoconcentration has been hypothesized to mediate, in part, the attenuation of the initial elevation in cerebral blood flow (CBF) at high altitude. To test this hypothesis, healthy men (n = 13) ascended to 5050 m over 9 days without the aid of prophylactic acclimatization medications. After 1 week of acclimatization at 5050 m, participants were haemodiluted by rapid saline infusion (2.10 ± 0.28 l) to return Hct towards pre‐acclimatization values. Arterial blood gases, Hct, global CBF (duplex ultrasound) and haemodynamic variables were measured after initial arrival at 5050 m and after 1 week of acclimatization at high altitude, before and after the haemodilution protocol. After 1 week at 5050 m, the Hct increased from 42.5 ± 2.5 to 49.6 ± 2.5% (P