• The heat transfer surface is usually made
of stainless steel or aluminium and rarely plastic
• Both aluminium & stainless steel have good thermal
conductivity and are readily coated with polymers to minimise blood
interactions
• However with long term extra corporeal life support, inorganic
emboli related to an aluminium heat exchanger (no longer sold) have been found
in the brain. Problems with corrosion in aluminium heat exchangers resulting in
blood leaks and circuit contamination have resulted in the preferential use of
stainless steel
• Intravascular aggregation has been observed at rapid cooling
rates
• Blood damage in the form of protein denaturation occurs at
temperatures greater than 43°C
Protein molecules are large and complex. Their activity depends on
their shape and chemical composition. The denaturing or inactivating, of
proteins involves the changing of their shape &/or breaking off attached
chemical groups. Denatured enzymes may loose their catalytic properties, the
anti-body-antigen mechanism may be altered, hormones may loose their function
etc.. Surface electrical charges on these molecules are altered, they become
sticky and form aggregates. Soluble proteins may become insoluble and
precipitate. Denatured globulins may cryoprecipitate (ppt when exposed to
hypothermia). Lipid components freed from lipoproteins coalesce to form fat
emboli. Some proteins become more active resulting in complement and platelet
activation.