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Cerebral Metabolism during Cord Occlusion and Hypoxia in the Fetal Sheep: A Novel Method of Continuous Measurement Based on Heat Production
Author(s) -
Hunter Christian J.,
Blood Arlin B.,
Power Gordon G.
Publication year - 2003
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2003.048082
Subject(s) - cerebral blood flow , fetus , blood flow , anesthesia , hypoxia (environmental) , arterial blood , medicine , chemistry , oxygen , biology , pregnancy , genetics , organic chemistry
This study was undertaken to validate a new method of measuring cerebral metabolic rate in the fetal sheep based on heat production in a local region of the brain. Heat production was compared to oxygen use in 20 near‐term fetuses during basal conditions, moderate hypoxia and cord occlusion. Thermocouples were placed to measure core and brain temperature and a composite probe placed in the parietal cortex to measure changes in cortical blood flow (CBF) using laser Doppler flowmetry and tissue P O2 using fluorescent decay. Catheters were inserted in a brachiocephalic artery and sagittal sinus for blood sampling. With moderate hypoxia, induced by administering 10−12 % oxygen to the ewes, fetal arterial P O2 declined from 23 ± 1 to 11 ± 1 Torr and brain tissue P O2 fell from 7.6± 0.7 to a nadir of 0.8 ± 0.4 Torr, while CBF increased to 139 ± 5 % of baseline. Cortical heat production, calculated as the product of CBF, the temperature gain from artery to brain tissue, and the specific heat of blood, decreased by 45 ± 11 % in parallel to similar declines in oxygen uptake. With severe asphyxia induced by complete cord occlusion for 10 min, fetal arterial P O2 declined from 23 ± 1 to 9 ± 2 Torr and brain tissue P O2 fell from 7.0 ± 0.7 to essentially 0 Torr while CBF decreased 40 ± 5 %. Cortical heat production decreased by 78 ± 6 % while oxygen use declined by 90 ± 3 %. Glucose uptake increased significantly relative to oxygen use and lactate concentration increased in sagittal sinus blood. We conclude that local measurements of heat production in the brain provide a useful index of overall metabolic rate, closely reflecting oxygen use in moderate hypoxia and indicating a significant contribution from anaerobic metabolism during severe asphyxia.