‘Ideal’ Alveolar Air and the Analysis of Ventilation-Perfusion Relationships in the Lungs

Abstract

The partial pressure of oxygen in the alveolar air may vary widely in different parts of the lungs in patients with pulmonary disease. The composition which the alveolar air would have if it were homogeneous throughout the lungs can be detd. precisely by solving the blood and gas R. Q. equations, assuming that there is equilibrium between the alveolar air and the blood leaving the alveolar capillaries. Alveolar air of the one and only composition which satisfies these equations is called the "ideal" alveolar air. Variations in the composition of alveolar air in different parts of the lungs occur primarily because of variations in ventilation-perfusion ratio. These relationships can be analyzed using the "ideal" alveolar air concept. Physiological dead space, when calculated using the "ideal" alveolar air, includes a contribution from alveoli with a high ventilation-perfusion ratio. A ratio of dead space to tidal air in excess of 30% indicates that a significant proportion of alveoli are well ventilated but poorly perfused. Venous admixture, when calculated using the "ideal" value for blood leaving the alveolar capillaries, includes a contribution from alveoli with a low ventilation-perfusion ratio. A ratio of venous admixture to cardiac output in excess of 7% indicates that a significant proportion of alveoli are well perfused but poorly ventilated. Analysis of ventilation-perfusion relationships must be done in the normal range of oxygenation since breathing either a low oxygen mixture or a very high oxygen mixture minimizes the effects under consideration.