Extracranial hypothermia during cardiac arrest and cardiopulmonary resuscitation is neuroprotective in vivo

Abstract

There is increasing evidence that ischemic brain injury is modulated by peripheral signaling. Peripheral organ ischemia can induce brain inflammation and injury. We therefore hypothesized that brain injury sustained after cardiac arrest (CA) is influenced by peripheral organ ischemia and that peripheral organ protection can reduce brain injury after CA and cardiopulmonary resuscitation (CPR). Male C57Bl/6 mice were subjected to CA/CPR. Brain temperature was maintained at 37.5°C ± 0.0°C in all animals. Body temperature was maintained at 35.1°C ± 0.1°C (normothermia) or 28.8°C ± 1.5°C (extracranial hypothermia [ExHy]) during CA. Body temperature after resuscitation was maintained at 35°C in all animals. Behavioral testing was performed at 1, 3, 5, and 7 days after CA/CPR. Either 3 or 7 days after CA/CPR, blood was analyzed for serum urea nitrogen, creatinine, alanine aminotransferase, aspartate aminotransferase, and interleukin-1β; mice were euthanized; and brains were sectioned. CA/CPR caused peripheral organ and brain injury. ExHy animals experienced transient reduction in brain temperature after resuscitation (2.1°C ± 0.5°C for 4 minutes). Surprisingly, ExHy did not change peripheral organ damage. In contrast, hippocampal injury was reduced at 3 days after CA/CPR in ExHy animals (22.4% ± 6.2% vs. 45.7% ± 9.1%, p=0.04, n=15/group). This study has two main findings. Hypothermia limited to CA does not reduce peripheral organ injury. This unexpected finding suggests that after brief ischemia, such as during CA/CPR, signaling or events after reperfusion may be more injurious than those during the ischemic period. Second, peripheral organ hypothermia during CA reduces hippocampal injury independent of peripheral organ protection. While it is possible that this protection is due to subtle differences in brain temperature during early reperfusion, we speculate that additional mechanisms may be involved. Our findings add to the growing understanding of brain-body cross-talk by suggesting that peripheral interventions can protect the brain even if peripheral organ injury is not altered.

FIG. 1.

Auricular canal temperature is not different from brain parenchymal temperature. Temperature measured is plotted by minute within the cardiac arrest (CA)/cardiopulmonary resuscitation (CPR) sequence (baseline=BL, arrest=CA, return of spontaneous circulation=ROSC; time point in minutes follows the phase abbreviation, e.g., ROSC10=10 minutes after return of spontaneous circulation). Temperature was monitored within the auricular canal (“Ear”), within the brain parenchyma (“Brain”), and within the temporalis (“Temporalis”) muscle during CA/CPR and recovery. There was no significant difference between auricular and brain temperature at any time point. Temporalis temperature overestimated brain temperature, statistically significant at the ROSC20 time point (mean±SEM, n=9).

FIG. 2.

Extracranial hypothermia induces transient brain parenchymal cooling. Temperature was measured within the brain parenchyma with a temperature probe placed 2 mm deep within the left parietal region and is plotted by minute within the CA/CPR sequence (baseline=BL, arrest=CA, return of spontaneous circulation=ROSC, time point in minutes follows the phase abbreviation, e.g., ROSC10=10 minutes after return of spontaneous circulation). Mice treated with extracranial hypothermia experienced a transient drop in brain parenchymal temperature despite external warming of the cranium (maximal 2.1°C±0.33°C at 6 minutes post-ROSC) (*p<0.05, mean±SEM, n=9).

FIG. 3.

CA/CPR induces multiorgan injury. Blood urea nitrogen (BUN), serum creatinine, aspartate aminotransferase (AST), alanine aminotransferase (ALT), and hippocampal CA1 cell death were measured in animals subjected to sham procedure or CA/CPR 3 days following the procedure. Neurobehavioral scoring was performed at 24 hours after procedures (*p<0.05, mean±SEM [BUN, sCr, AST, ALT, % CA1 Cell death] or median±interquartile range [neuroscore] presented n=4 sham and 15 CA/CPR).

FIG. 4.

Renal and hepatic function at 3 days post-CA/CPR is not altered by extracranial hypothermia. BUN and serum creatinine, AST, and ALT were measured at 3 days after CA/CPR and were not different between treatment groups (mean±SEM, n=15/g).

FIG. 5.

Neurohistological outcome of CA/CPR at 3 and 7 days post-arrest. Animals subjected to CA/CPR with extracranial hypothermia suffered significantly less neuronal death than those subjected to whole-body normothermic arrest as measured at 3 days post-CA/CPR. This effect was no longer significant at 7 days post-CA/CPR (*p=0.04, mean±SEM, n=15 for day 3 results, left. n=11/g for day 7, p>0.05).