The CO₂, O₂, and N₂ gradients between alveolar air and arterial blood during steady-state exercise in healthy non-smoking young and elderly men at normoxia and at hypoxia

Abstract

The CO₂, O₂, and N₂ gradients between alveolar air and arterial blood were measured during a mild steady-state exercise test in three groups of non-smoking men, 20–29, 40–49, and 63–70 years old, respectively. Breathing a low O₂-mixture (15.8% O₂) the (a-A) D CO₂ and the (a-A) D N₂ decreased during exercise, compared to room air breathing, when the same exercise level was imposed, indicating a more homogeneous distribution of alveolar ventilation and perfusion under the influence of hypoxia. However, the (A-a) D O₂ remained constant during both conditions, i. e. room air and hypoxia breathing. It is well known that the O₂ gradient increases under both conditions with age. The failure to demonstrate an (a-A) D N₂ when a large (A-a) D O₂ is present, suggests the presence of large compartments of the lungs with a low diffusion/perfusion relation (rarification of capillaries and a relatively high number of shortly segmented capillaries). The low diffusion/perfusion regions seem to contribute much more to the large (A-a) D O₂ than do the less important low ventilation/perfusion compartments. Such a constellation of maldistibution between alveolar ventilation, diffusion, and perfusion seems to be characteristic for the aging lung of non-smokers performing mild exercise.