Sustained venoconstriction in man supplemented with CO2 at high altitude

Area of Science:

• Physiology
• Altitude Medicine
• Cardiovascular Research

Background:

• Venoconstriction is a known physiological response to high altitude.
• The specific drivers of this venoconstriction, hypoxia (low oxygen) or hypocapnia (low carbon dioxide), remain debated.

Purpose of the Study:

• To differentiate the effects of hypoxia and hypocapnia on venoconstriction at high altitude.
• To investigate the impact of these factors on forearm blood flow, venous compliance, and catecholamine levels.

Main Methods:

• Five male subjects were exposed to simulated high altitude (4,000-4,400 m) with supplemental CO2.
• Control subjects (four males) were exposed to similar altitudes without CO2.
• Forearm plethysmography, blood pressure measurements, and urinary catecholamine analysis were employed.

Main Results:

• Venous compliance decreased in both groups at high altitude.
• Forearm blood flow and resistance were unchanged in the CO2-supplemented group but decreased/increased respectively in the hypocapnic group.
• Urinary catecholamines increased in the CO2 group but not in the hypocapnic group.

Conclusions:

• Hypoxia is the primary cause of reduced venous compliance at high altitude.
• Hypocapnia is responsible for increased forearm resistance and decreased blood flow.
• Differences in arterial pH likely explain observed variations in urinary catecholamines.