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Partial pressure of oxygen and carbon dioxide in alveolar air and arterial blood, lung diffusion capacity and its components, ventilation parameters, ventilation-perfusion ratio were determined in healthy people aged 60-89 (45 subjects) and aged 20-31 (19 subjects, controls). In elderly and old people PO2 in arterial blood was found to decrease with increasing alveolar-arterial PO2 gradient. In other words, arterial hypoxemia was determined by the disturbance in gas exchange between alveolar air and blood of lung capillaries. The diffusion capacity of lung decreased at the expense of membrane factor. Its age-related dynamics was mainly due to a decrease in the pulmonary diffusion surface occurring because of improper coordination of ventilation and perfusion in the lungs. The discrepancy of pulmonary ventilation and perfusion proved to be the leading factor of arterial hypoxemia in late ontogenesis.

On the basis of literature analysis and clinical experience, a classification of external respiratory failure (ERF) is suggested. The types of ERF can be as follows: 1) pulmonary ventilation failure; 2) gas diffusion failure; 3) pulmonary blood flow failure; 4) respiration control failure; and 5) ambient air gas composition change. The forms of ERI can be classified as acute, subacute and chronic. The stages of ERF include the following: I (compensatory) with pulmonary ventilation function drop of degree I-III (of an obstructive, restrictive and mixed type) and without hypoxemia, normo- or hypocapnia; II (subcompensatory) with the same pulmonary ventilation failures, moderate or serious hypoxemia, normo- or hypocapnia; III (decompensatory) with hypoxemia and hypercapnia or extremely severe hypoxemia in combination with normo- or hypocapnia.