Effects of changes in packed cell volume on the specific heat capacity of blood: implications for studies measuring heat exchange in extracorporeal circuits

Extracorporeal circuits such as cardiopulmonary bypass (CPB) and renal dial ysis machines cause active and/or passive loss of body heat. Attempts to qu antify this heat. loss are generally based on the Fick principle which requ ires knowledge of the specific heat capacity (SHC) of blood. As changes in packed cell volume are common, we investigated the effect of these changes on the SHC of blood over a range of packed cell volumes (PCV) from whole bl ood at 43.1% (3594 J kg(-1) degrees C-1) to pure Hartmann's solution (4153 J kg(-1) degrees C-1). The SHC of other fluids used during CPB was also mea sured and found to be 4139 J kg(-1) degrees C-1 and 4082 J kg(-1) degrees C -1 far normal saline and Gelofusine, respectively. The maximum variability in SHC over the range of PCV values encountered during CPB was calculated t o be small (5.5%). We conclude that: use of a constant value of SHC for cal culation of thermal energy transfer is currently justified.