Effects of reduced temperature on oxygen transport from capillaries to brain tissue.

The normal function of the brain depends on adequate oxygen supply. Oxygen deprivation (hypoxia) can result in irreversible damage to neurons within minutes. Cooling (hypothermia) of brain tissue can reduce the rate of damage, and is used in surgeries where blood flow to the brain is interrupted, such as aortic arch reconstruction. Hypothermia affects several factors that influence tissue oxygen levels, including oxygen consumption rate, diffusivity and solubility. The goal of the present work is to predict the effects of hypothermia on the partial pressure of oxygen in brain tissue. The dependence on temperature of parameters governing oxygen transport is estimated from literature data. A theoretical model based on the Krogh cylinder configuration is used to predict the effects of hypothermia on the distribution of oxygen partial pressure in the cylindrical tissue region surrounding a capillary. For a given blood flow rate and inflowing oxygen level, tissue oxygen levels are shown to increase with decreasing temperature. Although oxygen diffusivity in tissue declines with hypothermia, the reduction in oxygen consumption leads to a net increase in predicted oxygen levels. Tissue hypoxia resulting from reductions in blood flow rate can be ameliorated by reductions in temperature. For example, if blood flow is reduced to 36% of normal, temperature reduction by 2.3°C can increase tissue oxygen levels above the hypoxic range. The results support the use of hypothermia to reduce brain damage under conditions of reduced blood flow.