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Mild to moderate hypercapnia (Paco2 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 (Paco2 100-120 mmHg) increased brain injury, which may be caused by increased brain edema.
Zhou et al. (Sat,) studied this question.