Effects of Permissive Hypercapnia on Transient Global Cerebral Ischemia-Reperfusion Injury in Rats

Background: Permissive hypercapnia is a widely practiced protective ventilatory strategy that has significant protective effects on several models of in vitro and in vivo neuronal injury. However, conclusive effects of permissive hypercapnia on cerebral ischemia are still unknown. Methods: One hundred sixty male Wistar rats were divided into five groups: S group (control), ischemia-reperfusion (I/R) group, P1 group, P2 group, and P3 group. I/R was induced by bilateral occlusion of the common carotid arteries, combined with controlled hypotension for 15 min. In groups P1, P2, and P3, the rats inhaled carbon dioxide for 2 h during reperfusion to keep Paco(2) within the ranges of 60-80 mmHg, 80-100 mmHg, and 100-120 mmHg, respectively. After 24 and 72 h, neurologic deficit scores, ultrastructural changes, apoptotic neurons, and brain wet-to-dry weight ratios were observed. Caspase-3 and aquaporin-4 protein expression and caspase-3 activity were analyzed. Results: Compared with groups I/R and P3, groups P1 and P2 had better neurologic deficit scores and fewer ultrastructural histopathologic changes. I/R-induced cerebral apoptosis was also significantly reduced. The neuroprotective effect was significantly increased in the P2 group compared with the P1 group. There was a significant increase of brain water content and of aquaporin-4 levels in the P3 group. Conclusions: Mild to moderate hypercapnia (Paco(2) 60-100 mmHg) is neuroprotective after transient global cerebral I/R injury. Such a protection might be associated with apoptosis-regulating proteins. In contrast, severe hypercapnia (Paco(2) 100-120 mmHg) increased brain injury, which may be caused by increased brain edema.