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Many tests are available for measuring unevenness of pulmonary ventilation. In recent years tests of unevenness of pulmonary blood flow have been described, which utilize radioactive krypton (1) in patients with emphysema, radioactive oxygen in patients with mitral stenosis (2), and radioactive carbon dioxide in normal subjects (3). These studies provide quantitative data on the degree of unevenness of pulmonary blood flow but require expensive and complicated apparatus and are not easily adapted to clinical pulmonary function testing. Simpler methods of estimating unevenness of blood flow in relation to ventilation have been suggested. These are based on the rate of rise of arterial oxygen saturation (ear oximeter) during the inhalation of oxygen (4-6), analysis of gas samples obtained from various lobes of the lungs (7), continuous analysis of a single expiration (8), and the difference between the nitrogen tension of alveolar gas and urine (9, 10). These simpler methods do not provide quantitative data on the degree of unevenness of pulmonary blood flow. An ingenious graphical method of determining unevenness of perfusion of the lung in patients with chronic pulmonary emphysema has been worked out by Briscoe and co-workers (11). This method utilizes a nitrogen washout and wash-in technique and the arterial saturation (luring air-breathing. Its disadvantage is that other causes of arterial desaturation i.e., shunts and diffusion barriers cause an overestimation of the degree of unevenness of perfusion. In this laboratory we have devised a test which yields quantitative data for the determination of the fractional alveolar ventilation and pulmonary capillary blood flow that go to the well and poorly ventilated regions of the

THE DETERMINATION OF UNEVEN PULMONARY BLOOD FLOW FROM THE ARTERIAL OXYGEN TENSION DURING NITROGEN WASHOUT