Effect of Hypocapnia on Cerebral Oxygen Metabolism and Blood Flow in Ischemic Cerebrovascular Disorders

Effects of hypocapnia on cerebral oxygen consumption (CMRO2) and blood flow (CBF) in cerebral ischemia were studied in 19 patients. The CMRO₂ did not change significantly during hypocapnia within the whole group of patients, because 10 out of 19 cases showed a decrease (p < 0.001) and other 9 showed an increase (p < 0.01) of CMRO₂ during hypocapnia. The first 10 showed higher resting CMRO₂ (p < 0.001) and arteriovenous differences of oxygen content (AVDO₂; p < 0.02) than the other 9 However, the resting CBF and CO₂ reactivity to hypocapnia were not different between them, and clinical situations were also similar. A dissociation between flow and metabolism was suggested in the first 10 with rather preserved CMRO₂, while reduced metabolic demands were suggested in the other 9. Different responses of CMRO₂ to hypocapnia are expected in cerebral ischemia, i.e. in cases with rather preserved CMRO₂ it decreases despite an AVDO₂ increase, suggesting a capability of CMRO2 to respond to CBF reduction, while it increases in cases with more decreased CMRO₂, as the AVDO₂ increase exceeds the CBF reduction to maintain the decreased CMRO₂ for a further CBF reduction. The vascular CO₂ reactivity, therefore, might be maintained to be constant between these patients.