Extent to which pulmonary vascular responses to Pco(2) and Po(2) play a functional role within the healthy human lung

Regional blood flow in the lung is known to be influenced by the alveolar Pco(2) and alveolar Po(2). For the healthy lung, the extent to which this influence is of functional importance in limiting heterogeneity in alveolar gas composition by matching regional perfusion (q̇) to regional ventilation (v̇) remains unclear. To address this issue, the efficiency of regulation (E) was defined as the percent correction to an initial perturbation in regional alveolar gas composition generated by the pulmonary vascular response to the disturbance. This study develops the theory to calculate E from global measurements of vascular reactivity to CO(2) and O(2) in human volunteers. For O(2), these data were available from the literature. For CO(2), an experimental component of the present study used Doppler echocardiography to evaluate the magnitude of the global vascular response to hypercapnia and hypocapnia in 12 volunteers over a timescale of ∼0.5 h. The results suggest a value for E of ∼60% over a wide range of values for v̇-to-q̇ ratio (∼0.1–10) encompassing those found in normal lung. At low v̇/q̇ (<0.65), the vascular response to O(2) forms the dominant mechanism; however, at higher v̇/q̇ (>0.65), the response to CO(2) dominates. The values for E suggest that the pulmonary vascular responses to both CO(2) and O(2) play a significant role in ventilation-perfusion matching in the healthy human lung.