Stimulus—response curves for the pulmonary vascular bed to hypoxia and hypercapnia

Abstract

1. In anaesthetized open‐chest cats and dogs, blood flow and gas tensions were measured in a circuit inserted into a pulmonary vein while ventilating the lobe which it drained with low O 2 and high CO 2 mixtures. 2. Both hypoxic and hypercapnic mixtures caused a reduction in blood flow from the lobe. 3. Stimulus—response curves relating blood flow to pulmonary venous P O 2 and P CO 2 were obtained. Those for hypoxia were usually asymptotic in shape; the curves became steep below 100 torr and flow sometimes fell to zero. The mean reduction in blood flow for every 20 torr fall in P O 2 was 15·7% in cats and 11·8% in dogs. Those for hypercapnia were steep at first but levelled out at high P CO 2 values; the maximum reduction in flow was 40–60% as vasoconstriction was only observed over a limited P CO 2 range. 4. Hypoventilation of the lobe led to a reduction in blood flow. This was mainly attributable to hypoxia though other factors such as hypercapnia may sometimes have contributed. 5. Total occlusion of the bronchus of an O 2 ‐filled lobe caused blood flow to fall in two phases. The first phase could be attributed to a rise in P CO 2 and the second to a fall in P O 2 . 6. The results confirm the hypothesis that hypoxia is an important factor regulating local blood flow in relation to local ventilation.