Cooling strategies and transport theories for brain hypothermia resuscitation

Abstract

The brain is one of the most important organs in a biological body whose normal function depends heavily on an uninterrupted delivery of oxygen. Unlike skeletal muscles that can survive for hours without oxygen, neuron cells in the brain are easily subjected to an irreversible damage within minutes from the onset of oxygen deficiency. With the interruption of cardiopulmonary circulation in many cardiac surgical procedures or accidental events leading to cerebral circulation arrest, an imbalance between energy production and consumption will occur which causes a rapid depletion of oxygen due to the interrupted blood-flow to the brain. Meanwhile, the cooling function of the blood flow on the hot tissue will be stopped, while metabolic heat generation in the tissues still keeps running for awhile. Under such adverse situations, the potential for cerebral protection through hypothermia has been intensively investigated in clinics by lowering brain temperature to restrain the cerebral oxygen demands. The reason can be attributed to the decreased metabolic requirements of the cold brain tissues, which allows a longer duration for the brain to endure reduced oxygen delivery. It is now clear that hypothermia would serve as the principal way for neurologic protection in a wide variety of emergency medicines, especially in cerebral damage, anoxia, circulatory arrest, respiratory occlusion, etc. However, although brain cooling has been found uniquely significant in clinical practices, the serious lack of knowledge on the mechanisms involved prevents its further advancement in brain resuscitation. Compared with the expanded trials in clinics, only very limited efforts were made to probe the engineering issues involved, which turns out to be a major obstacle for the successful operation of brain hypothermia resuscitation. From the viewpoint of biothermal medical engineering, the major theories and strategies for administering brain cooling can generally be classified into three categories: heat transfer, oxygen transport and cooling strategy. Aiming to provide a complete overview of the brain hypothermia resuscitation, this article comprehensively summarizes the recent progresses made in theoretical, practical and experimental techniques in the area. Particularly, attention is paid to the mathematical models to quantify the heat and oxygen transport inside the cerebral tissues. Typical cooling strategies to effectively lower brain temperature and thus decrease oxygen consumption rate in the cerebral tissues are analyzed. Approaches to deliver oxygen directly to the target tissues are discussed. Meanwhile, some future efforts worth pursuing within the area of brain cooling are suggested.