LAUSR

Pulmonary vascular tone is known to be sensitive to both local alveolar PO2 and PCO2. Although the effects of hypoxia are well studied, the hypercapnic response is relatively less understood. We assessed changes in regional pulmonary blood flow in humans in response to hypercapnia using previously developed MRI techniques. Dynamic measures of blood flow were made in a single slice of the right lung of seven healthy volunteers following a block-stimulus paradigm (baseline, challenge, recovery), with CO2 added to inspired gas during the challenge block to effect a 7 Torr increase in end-tidal CO2. Effects of hypercapnia on blood flow were evaluated based on changes in spatio-temporal variability (fluctuation dispersion, FD) and in regional perfusion patterns in comparison to hypoxic effects previously studied. Hypercapnia increased FD 2.5% from baseline (relative to control), which was not statistically significant (p = 0.07). Regional perfusion patterns were not significantly changed as a result of increased FiCO2 (p = 0.90). Re-analysis of previously collected data using a similar protocol but with the physiologic challenge replaced by decreased FIO2 (FIO2= 0.125) showed marked flow redistribution (p = 0.01) with the suggestion of a gravitational pattern, demonstrating hypoxia has the ability to affect regional change with a global stimulus. Taken together, these data indicate that hypercapnia of this magnitude does not lead to appreciable changes in the distribution of pulmonary perfusion, and that this may represent an interesting distinction between the hypoxic and hypercapnic regulatory response.