Profound Hypothermia Provides Neuroprotection Following Hypothermic Circulatory Arrest: Ultrastructural Observations.

Objective: To assess whether cooling to 10°C can reduce neurological injury during 75 minutes of hypothermic circulatory arrest (HCA) compared to cooling to 18°C. Methods: Twelve domestic swine were used for this prospective blind randomized study. The animals were divided into 2 groups: Group A (n=6) underwent hypothermic circulatory arrest at 18oC for 75 min, and Group B (n=6) underwent hypothermic circulatory arrest at 10oC for 75 min. At the end of the experiment, the brains were removed and immersed in paraformaldehyde. All brains were dissected in the sagital plane. Tissue blocks from the left hemisphere were cut to encompass the sensory neocortex. Results: The selected area was identified with a dissecting microscope. Samples were examined in a blind fashion using electron microscope. Two investigators were instructed to find 10 representative neurons and analyze electron micrographs of these neurons for evidence of nuclear and cytoplasmic changes. Similarly, each investigator was instructed to examine the perinuclear neuronal mitochondria for abnormalities in mitochondrial distribution. Significant differences were observed between the 2 groups in mitochondria and rough endoplasmic reticulum (RER). In 5 of the 6 animals treated with 18oC HCA, neurons had slightly dilated RER, Golgi apparatus and mitochondria. In all 6 animals treated with 10oC HCA, the structure of the cytoplasmic organelles was intact, with no apparent dilatation (p=0.015). Conclusion: This study adds further support that hypothermia at 10°C exerts better cellular protection than hypothermia at 18°C, as evidenced by these electron microscopy findings.