Numerical studies on the effect of lowering temperature on the oxygen transport during brain hypothermia resuscitation

There have been arguments about the advantage and shortcoming of hypothermia on the brain resuscitation during circulation arrest. People usually accepted that hypothermia may decrease the cerebral oxygen demands, which is beneficial for the patient to sustain longer time when subjected to a hypoxia. However, there are also quite a few disputes claiming that the blood viscosity would increase with the reduction of temperature, which may lead to an increase of cerebral vascular resistance and thus worsen the hypoxia state. To resolve this critical issue, a heat transfer model was established to characterize the thermal response of brain tissue during hypothermia resuscitation. Combined with this model, a compartmental model taking account of the temperature effect was further developed to analyze the transient oxygen partial pressure (PO₂) distribution over the successive branches of the vascular network during circulation arrest. Using the morphological and physiological data of a sheep brain, effects of lowering temperature on the oxygen consumption dynamics were studied. Calculations indicated that the lower the temperature, the slower the decreasing rate for the PO₂. Although immediately lowering the brain temperature may induce an evident increase in blood viscosity and subsequently a decrease in blood flow rate, which is responsible for oxygen delivery, it seems to always result in a monotonic increase of PO₂. The results show a good qualitative accord with the experimental data. They also present better understanding on the transient oxygen transport in brain hypothermia during circulation arrest.