Comparison of the effects of hypothermia at 33°C or 35°C after cardiac arrest in rats

Objectives: Hypothermia of 32 degrees C-34 degrees C induced after resuscitation from cardiac arrest improves neurologic recovery, but the optimal depth of cooling is unknown. Using a rat model, the authors tested the hypothesis that cooling to 35 degrees C between hours I and 24 after resuscitation would improve neurologic outcome as much as cooling to 33 degrees C. Methods: Halothane-anesthetized rats (n = 38) underwent 8 minutes of asphyxial cardiac arrest and resuscitation. Cranial temperature was maintained at 37 degrees C before, during, and after arrest. Between one and 24 hours after resuscitation, cranial temperature was maintained at 33 degrees C, 35 degrees C, or 37 degrees C using computer-controlled cooling fans and heating lamps. Neurologic scores were measured daily, and rats were killed at 14 days for histologic analysis. Neurons per high-powered held were counted in the CA1 region of the anterior hippocampus using neuronal nuclear antigen staining. Results: After 14 days, 12 of 12 rats (100%) cooled to 33 degrees C, 11 of 12 rats (92%) cooled to 35 degrees C, and ten of 14 rats (71%) cooled to 37 degrees C survived, with hazard of death greater in the rats cooled to 37 degrees C than in the combined hypothermia groups. Neurologic scores were worse in the rats cooled to 37 degrees C than in the hypothermia groups on days 1, 2, and 3. Numbers of surviving neurons were similar between the groups cooled to 33 degrees C and 35 degrees C and were higher than in the group cooled to 37 degrees C. Conclusions: These data illustrate that hypothermia of 35 degrees C or 33 degrees C over the first day of recovery improves neurologic scores and neuronal survival after cardiac arrest in rats. The benefit of induced hypothermia of 35 degrees C appears to be similar to the benefit of 33 degrees C.