Objectives: The mechanisms by which hypothermia improves cardiac arrest (CA)-induced brain damage are unclear. The authors hypothesized that mild hypothermia induced before CA attenuates brain edema formation by preventing neutrophil-mediated dysfunction of the endothelial cell junction proteins.
Methods: Eighteen rats were randomized to normal control surgery (group 1, n = 6), normothermic (37.5 degrees C) CA (group 2, n = 6), or hypothermic (34 degrees C) CA (group 3, n = 6). Hypothermia was induced with external cooling before CA in group 3. Cardiac arrest was induced by 8 minutes of asphyxiation. Brain edema was determined by wet-to-dry weight ratio and cerebral spinal fluid pressure (CSFP). Brain neutrophil content was determined by myeloperoxidase (MPO) activity, and occludin degradation was assessed by western blotting.
Results: Normothermic CA significantly increased brain wet-to-dry weight ratio from 4.52 +/- 0.04 in group 1 to 4.80 +/- 0.04 in group 2 (p = 0.0003) and CSFP from 3.6 +/- 0.9 in group 1 to 8.9 +/- 0.9 mm Hg in group 2 (p = 0.004). Mild hypothermia before CA in group 3 significantly reduced brain wet-to-dry weight ratio (4.68 +/- 0.03, p = 0.008 vs. group 2) and CSFP (3.8 +/- 0.5 mm Hg, p = 0.004 vs. group 2). Cardiac arrest increased brain MPO from 0.07 +/- 0.025 in group 1 to 0.16 +/- 0.02 units/gram brain weight in group 2 (p = 0.006) that was not decreased by hypothermia before CA (0.12 +/- 0.02 in group 3 (p = 0.07 vs. group 2). There was no occludin proteolysis in any group.
Conclusions: Mild hypothermia before CA decreases CA-induced brain edema. The hypothermia-elicited reduction in brain edema does not appear to be neutrophil-dependent and the early brain edema formation may not involve the proteolysis of occludin.