Effect of Cervical Sympathectomy and Hypoxia on the Heterogeneity of O2 Saturation of Small Cerebrocortical Veins

This study evaluated the hypothesis that the sympathetic nervous system was one of the factors increasing the heterogeneity of cerebrocortical venous O 2 saturation and this heterogeneity would be greater during hypoxia when cervical sympathetic activity was elevated. Thirty-two male Long–Evans rats were either sham operated ( n = 16) or received bilateral cervical sympathectomy ( n = 16). One-half of the animals ( n = 8) in each treatment were challenged by hypoxia (8% O 2 in N 2 ). Cerebral blood flow was determined in five brain regions with [ 14 C]iodoantipyrine. Oxygen saturation was measured microspectrophotometrically in small cerebrocortical arteries and veins. The degree of hypoxic hyperemia was not significantly different between sham-operated and sympathectomized rats. Cortical venous O 2 saturations, indicating the balance between O 2 supply and consumption, were significantly more heterogeneous in the sham-operated group under both normoxic and hypoxic conditions. The coefficient of variation (CV = 100 × SD/mean) for the normoxic sham-operated animals was 24.9% and the average venous O 2 saturation was 53.8%. During hypoxia, venous O 2 saturation was significantly decreased to 43.1% without a change in CV (24.5%). Sympathectomy significantly reduced this heterogeneity through a reduction in the number of low O 2 saturation veins (CV = 13.2%) under normoxic conditions and the effect was similar under hypoxic conditions (CV = 15.3%). In both sham-operated and sympathectomized groups, hypoxia elicited a significantly higher cerebrocortical O 2 consumption. Thus, bilateral cervical sympathectomy improved the O 2 supply in selective cerebrocortical regions with high O 2 extraction. However, the effect of sympathetic innervation on the heterogeneity of cerebrocortical venous O 2 saturation was not potentiated by hypoxia. This may suggest that peripheral sympathetic fibers innervate a fixed number of cerebrocortical vessels and, regardless of its intensity, this innervation exerts similar effects on the cerebrocortical venous O 2 saturation.