Quantitative Relationships Between Hypoxemia and Disorder of Pulmonary Function **

Hypoxemia is a common finding in many diseases of the lung. The degree of hypoxemia depends upon which aspect of pulmonary function is deranged as well as upon the severity of the abnormality. The degree of hypoxemia, the presence or absence of carbon dioxide retention, and the effects of exercise and oxygen breathing on the hypoxemia provide information about the nature and extent of the underlying abnormality of pulmonary function. This information may be of value in the differential diagnosis and treatment of patients with pulmonary disease. In addition, knowledge of the degree of hypoxemia which may result from impairment of each function of the lung provides insight into the relative severity of the hypoxemia that may result from a given disease. For example, a 50 per cent right-to-left shunt in the lung results in serious hypoxemia, whereas 50 per cent reduction of the diffusing capacity of the lung has little effect on the arterial oxygen saturation. The accompanying classification of hypoxemia (Table 1) provides a useful framework for a discussion of this subject and of its treatment. Hypotonic hypoxemia refers to situations in which the arterial oxygen tension, and thus the arterial oxygen content, is subnormal; isotonic hypoxemia refers to situations in which the arterial oxygen content is reduced but the arterial oxygen tension is normal. Tissue anoxia also results from the actions of various poisons on cellular enzyme systems, so-called histotoxic anoxia, and in diseases in which circulatory impairment exists (occlusive arterial disease and cardiac failure), but these abnormalities are beyond the scope of the present discussion. Hypotonic hypoxemia, which can be produced in normal subjects by reducing the inspired oxygen tension, occurs when the alveolar ventilation is diminished relative to metabolic needs, when the distribution of inspired air and of blood flowing through the lungs is not homogeneous, and when