tion-Perfusion Ratio of the Underventilated Elements (the Slow Compartment) of the Lung*

The degree of unevenness in distribution of inspired gas is of interest because it affects gas turnover in the alveoli and hence alveolar gas composition (1, 2). Since the various body or-gans are not in direct contact with the alveolar gas, this unevenness affects them only indirectly. From the standpoint of the peripheral tissues, the most important factor by which lung perform-ance may be judged is the gas content of arterial blood. Poorly ventilated alveoli are rather in-nocuous if they are also poorly perfused. On the other hand, if considerable blood flow is present, these alveoli can be a source of serious embarrass-ment to the body since they have a marked in-fluence upon arterial gas content. Several methods for measuring distribution of pulmonary blood flow have been devised. Some are based on injection or inhalation of radioactive materials (3-6), but are too complicated or too costly for general use. Finley (7) has reported a method of estimating perfusion of poorly venti-lated alveoli that requires simultaneous record-ing of alveolar CO2 and N2 (from which alveolar 02 is calculated) and arterial 02 tension. Briscoe and others (8) have described a method of evalu-ating pulmonary ventilation and perfusion that gives more detailed information but requires measurement of seven different parameters. The desirability of a less complicated method of measuring the perfusion of the poorly venti-lated alveoli and of estimating their ventilation-perfusion ratio led to the present study. Inert gas washout is followed in the expired gas and i