Localized hypothermia aggravates bleeding in the collagenase model of intracerebral hemorrhage.

Animal studies testing whether therapeutic hypothermia is neuroprotective after intracerebral hemorrhage (ICH) have been inconclusive. In rodents, ICH is often produced in the striatum by infusing collagenase, which causes prolonged hemorrhaging from multiple vessels. Our previous data shows that this bleeding (hematoma) is worsened by systemic hypothermia given soon after collagenase infusion. In this study we hypothesized that localized brain hypothermia would also aggravate bleeding in this model (0.2 U of collagenase in 1.2 μL of saline). We also evaluated cooling after intrastriatal thrombin infusion (1 U in 30 μL of saline)-a simplified model of ICH thought to cause bleeding. Focal hypothermia was achieved by flushing cold water through an implanted cooling device attached to the skull underneath the temporalis muscle of adult rats. Previous work and data at this time shows this method cools the striatum to ∼33°C, whereas the body remains normothermic. In comparison to normothermic groups, cooling significantly worsened bleeding when instituted at 6 hours (∼94 vs. 42 μL, p=0.018) and 12 hours (79 vs. 61 μL, p=0.042) post-ICH (24-hour survival), but not after a 24-hour delay (36-hour survival). Rats were cooled until euthanasia when hematoma size was determined by a hemoglobin-based spectrophotometry assay. Cooling did not influence cerebral blood volume after just saline or thrombin infusion. The latter is explained by the fact that thrombin did not cause bleeding beyond that caused by saline infusion. In summary, local hypothermia significantly aggravates bleeding many hours after collagenase infusion suggesting that bleeding may have confounded earlier studies with hypothermia. Furthermore, these findings serve as a cautionary note on using cooling even many hours after cerebral bleeding.