Regional Pulmonary Blood Flow

unteers were studied, nine sitting and seven supine. During hypoxia there was a slight but significant increase in relative perfusion of the upper portions of the lungs in both the sitting and supine subjects. During recovery from hypoxia, blood flow distribution differed significantly from the control. The erect subjects showed increased relative perfusion of the lung bases and the supine subjects showed increased relative perfusion of the upper zones. Comparison of the distribution of inhaled and intravenously injected isotope showed that in the sitting subjects the altered distribution during hypoxia tended to make alveolar oxygen tension more uniform. In the supine subjects, however, the shift in blood flow increased the perfusion of the regions with the lowest ventilation/ perfusion, tending to accentuate uneven alveolar oxygen tension. Therefore it does not seem that the altered blood flow distribution during hypoxia was due to selective vasoconstriction in the regions of lowest alveolar oxygen tension, but rather that vasoconstriction was greatest in the lower lung zones because the vessels there are more responsive to hypoxia. During mild acute hypoxia, vasoconstrictor tone does not seem to effectively match ventilation and perfusion. The altered distribution of pulmonary blood flow during recovery from hypoxia suggests the occurrence of posthypoxic vasodilation. Failure to consider this possibility may lead to erroneous interpretation of pulmonary hemodynamic measurements made after the inspired oxygen concentration has been changed. Received for publication 5 August 1968. INTRODUCTION In a wide variety of human and animal experiments inhaled oxygen concentration has been shown to influence pulmonary vascular resistance (1). von Euler and Liljestrand (2) suggested that pulmonary blood flow distribution is regulated by a local action of alveolar oxygen tension on the blood vessels which produces optimal matching of ventilation and perfusion. This hypothesis stimulated many studies of the effect of regional hypoxia on local pulmonary vascular resistance but it is still unknown whether alveolar oxygen tension significantly influences blood flow distribution under physiological conditions. Fishman, Himmelstein, *Fritts, and Cournand (3) found no consistent reduction of blood flow to the hypoxic lung when 8-12% oxygen was breathed through a bronchospirometric catheter and Fishman, after reviewing the considerable literature on this subject, concluded that under normal conditions the effect of local gas composition on the distribution of blood flow was small when compared with gravitational and mechanical influences (1). However, Arborelius (4) claimed that a consistent reduction of blood flow to the hypoxic lung occurs during unilateral inhalation of 15% oxygen, a relatively mild hypoxic stimulus. He suggested that the variable results of other workers were due to the inaccuracy of oxygen uptake as a measure of flow to the hypoxic lung and that his observations based on the recovery of intravenously injected 'Kr from each lung supported the hypothesis of von Euler and Liljestrand. Since the introduction of radioactive gas methods for the study of regional ventilation and perfusion a great deal more has been learned about the factors influencing the distribution of blood and gas in the lungs. West, Dollery, and Naimark (5) showed that in isolated perfused lungs blood flow was highest in the dependent part of The Journal of Clinical Investigation Volume. 48 1969 301 Downloaded from http://www.jci.org on October 28, 2017. https://doi.org/10.1172/JCI105986