Diffusion limitation in normal humans during exercise at sea level and simulated altitude

Area of Science:

• Physiology
• Altitude Medicine
• Respiratory Physiology

Background:

• Arterial oxygen partial pressure (PaO2) is crucial for oxygen delivery.
• Understanding factors affecting PaO2 at altitude is vital for physiological adaptation.
• Ventilation-perfusion (VA/Q) inequality and diffusion limitations are potential contributors to hypoxemia.

Purpose of the Study:

• To determine the relative contributions of VA/Q inequality, diffusion resistance, shunt, and gas phase diffusion to PaO2 at simulated altitudes.
• To assess these factors at rest and during exercise in normal men.

Main Methods:

• Employed the multiple inert gas elimination technique (MIGET).
• Measured respiratory and inert gases, ventilation, and cardiac output.
• Compared actual PaO2 with values predicted by VA/Q inequality alone.

Main Results:

• At sea level and 5,000 ft, alveolar-arterial PO2 differences were primarily due to VA/Q mismatch.
• At 10,000 and 15,000 ft, exercise exacerbated the alveolar-arterial PO2 difference beyond VA/Q mismatch.
• No evidence of diffusion disequilibrium or postpulmonary shunt was found.

Conclusions:

• VA/Q inequality is the primary determinant of PaO2 at lower altitudes and rest.
• During exercise at higher altitudes, factors beyond VA/Q inequality significantly reduce PaO2.
• Diffusion limitations do not explain hypoxemia at simulated high altitudes in this study.