Normal exercise capacity in chronic mountain sickness: how high can the hematocrit go without consequence?

The 2018 Global Research Expedition on Altitude Related Chronic Health (Global REACH) to Cerro de Pasco, Peru: an Experimental Overview

NEW FINDINGS

What is the central question of this study? Herein, a methodological overview of our research team's (Global REACH) latest high altitude research expedition to Peru is provided. What is the main finding and its importance? The experimental objectives, expedition organization, measurements and key cohort data are discussed. The select data presented in this manuscript demonstrate the haematological differences between lowlanders and Andeans with and without excessive erythrocytosis. The data also demonstrate that exercise capacity was similar between study groups at high altitude. The forthcoming findings from our research expedition will contribute to our understanding of lowlander and indigenous highlander high altitude adaptation.

ABSTRACT

In 2016, the international research team Global Research Expedition on Altitude Related Chronic Health (Global REACH) was established and executed a high altitude research expedition to Nepal. The team consists of ∼45 students, principal investigators and physicians with the common objective of conducting experiments focused on high altitude adaptation in lowlanders and in highlanders with lifelong exposure to high altitude. In 2018, Global REACH travelled to Peru, where we performed a series of experiments in the Andean highlanders. The experimental objectives, organization and characteristics, and key cohort data from Global REACH's latest research expedition are outlined herein. Fifteen major studies are described that aimed to elucidate the physiological differences in high altitude acclimatization between lowlanders (n = 30) and Andean-born highlanders with (n = 22) and without (n = 45) excessive erythrocytosis. After baseline testing in Kelowna, BC, Canada (344 m), Global REACH travelled to Lima, Peru (∼80 m) and then ascended by automobile to Cerro de Pasco, Peru (∼4300 m), where experiments were conducted over 25 days. The core studies focused on elucidating the mechanism(s) governing cerebral and peripheral vascular function, cardiopulmonary regulation, exercise performance and autonomic control. Despite encountering serious logistical challenges, each of the proposed studies was completed at both sea level and high altitude, amounting to ∼780 study sessions and >3000 h of experimental testing. Participant demographics and data relating to acid-base balance and exercise capacity are presented. The collective findings will contribute to our understanding of how lowlanders and Andean highlanders have adapted under high altitude stress.

Global Reach 2018 Heightened α-Adrenergic Signaling Impairs Endothelial Function During Chronic Exposure to Hypobaric Hypoxia

RATIONALE

Chronic exposure to hypoxia is associated with elevated sympathetic nervous activity and reduced vascular function in lowlanders, and Andean highlanders suffering from excessive erythrocytosis (EE); however, the mechanistic link between chronically elevated sympathetic nervous activity and hypoxia-induced vascular dysfunction has not been determined.

OBJECTIVE

To determine the impact of heightened sympathetic nervous activity on resistance artery endothelial-dependent dilation (EDD), and endothelial-independent dilation, in lowlanders and Andean highlanders with and without EE.

METHODS AND RESULTS

We tested healthy lowlanders (n=9) at sea level (344 m) and following 14 to 21 days at high altitude (4300 m), and permanent Andean highlanders with (n=6) and without (n=9) EE at high altitude. Vascular function was assessed using intraarterial infusions (3 progressive doses) of acetylcholine (ACh; EDD) and sodium nitroprusside (endothelial-independent dilation) before and after local α+β adrenergic receptor blockade (phentolamine and propranolol). Intraarterial blood pressure, heart rate, and simultaneous brachial artery diameter and blood velocity were recorded at rest and during drug infusion. Changes in forearm vascular conductance were calculated. The main findings were (1) chronic hypoxia reduced EDD in lowlanders (changes in forearm vascular conductance from sea level: ACh1: -52.7±19.6%, ACh2: -25.4±38.7%, ACh3: -35.1±34.7%, all P≤0.02); and in Andeans with EE compared with non-EE (changes in forearm vascular conductance at ACh3: -36.4%, P=0.007). Adrenergic blockade fully restored EDD in lowlanders at high altitude, and normalized EDD between EE and non-EE Andeans. (2) Chronic hypoxia had no effect on endothelial-independent dilation in lowlanders, and no differences were detected between EE and non-EE Andeans; however, EID was increased in the non-EE Andeans after adrenergic blockade (P=0.012), but this effect was not observed in the EE Andeans.

CONCLUSIONS

These data indicate that chronic hypoxia reduces EDD via heightened α-adrenergic signaling in lowlanders and in Andeans with EE. These vascular mechanisms have important implications for understanding the physiological consequences of acute and chronic high altitude adaptation.

High-Altitude Pulmonary Vascular Diseases

More than 140 million people permanently reside in high-altitude regions of Asia, South America, North America, and Africa. Another 40 million people travel to these places annually for occupational and recreational reasons, and are thus exposed to the low ambient partial pressure of oxygen. This review will focus on the pulmonary circulatory responses to acute and chronic high-altitude hypoxia, and the various expressions of maladaptation and disease arising from acute pulmonary vasoconstriction and subsequent remodeling of the vasculature when the hypoxic exposure continues. These unique conditions include high-altitude pulmonary edema, high-altitude pulmonary hypertension, subacute mountain sickness, and chronic mountain sickness.

Carbonic anhydrase inhibitors and high altitude illnesses

Carbonic anhydrase (CA) inhibitors, particularly acetazolamide, have been used at high altitude for decades to prevent or reduce acute mountain sickness (AMS), a syndrome of symptomatic intolerance to altitude characterized by headache, nausea, fatigue, anorexia and poor sleep. Principally CA inhibitors act to further augment ventilation over and above that stimulated by the hypoxia of high altitude by virtue of renal and endothelial cell CA inhibition which oppose the hypocapnic alkalosis resulting from the hypoxic ventilatory response (HVR), which acts to limit the full expression of the HVR. The result is even greater arterial oxygenation than that driven by hypoxia alone and greater altitude tolerance. The severity of several additional diseases of high attitude may also be reduced by acetazolamide, including high altitude cerebral edema (HACE), high altitude pulmonary edema (HAPE) and chronic mountain sickness (CMS), both by its CA-inhibiting action as described above, but also by more recently discovered non-CA inhibiting actions, that seem almost unique to this prototypical CA inhibitor and are of most relevance to HAPE. This chapter will relate the history of CA inhibitor use at high altitude, discuss what tissues and organs containing carbonic anhydrase play a role in adaptation and maladaptation to high altitude, explore the role of the enzyme and its inhibition at those sites for the prevention and/or treatment of the four major forms of illness at high altitude.

Chronic mountain sickness score was related with health status score but not with hemoglobin levels at high altitudes

Chronic mountain sickness (CMS) or lack of adaptation to live in high altitudes is related to environmental hypoxia and excessive erythrocytosis (EE) (hemoglobin >21 and >19 g/dL for men and women, respectively). Diagnosis of CMS ("Qinghai CMS Score") is based on seven signs/symptoms (breathlessness and/or palpitations, sleep disturbance, cyanosis, dilatation of veins, paresthesia, headache, tinnitus) and the score for EE. The present study was designed to determine the association between hemoglobin, Qinghai CMS score, CMS clinical score (7 signs/symptoms) and Health Status using a health survey composed of 20 items. The rate of CMS (32.6%) was higher than the rate of EE (9.7%; P<0.002). A significant inverse relationship was observed between CMS clinical score and health status score (r=-0.56 for men, and r=-0.55 for women, P<0.01). However, CMS clinical score was not different in groups with different Hb levels. Health status score was significantly higher in subjects without CMS. In conclusion, elevated hemoglobin levels were not associated with elevated CMS clinical score.