**2. Pathophysiology of ischemic brain injury**

Ischemic brain injury is composed by the initial ischemic cascade and reperfusion injury.[6] During cerebral ischemia, cessation of blood flow and hypoxia trigger a complex set of metabolic and biochemical processes that comprise the ischemic cascade. An initial event in the ischemic cascade is the depletion of adenosine triphosphate (ATP), which is generated by oxygen-dependent phosphorylation in the central nervous system. ATP depletion leads to neurolemma depolarization secondary to derangement of Na+ and K+ gradients and, consequently, inappropriate accumulation of intracellular Ca2+ resulting from both Ca2+ influx and release from intracellular Ca2+ stores.[7] Increased intracellular Ca2+ concentration causes promiscuous activation of multiple intracellular enzyme systems, including protein kinase C, protein kinase B, calcium/calmodulin-dependent protein kinase II, mitogen-activated protein kinases, and phospholipases A2, C, and D. Prolonged elevations in intracellular Ca2+ concentration trigger the release of neurotrasmitters, which couples with the activation of multiple enzyme systems, inevitably leading to necrotic cell death through membrane dissolution if ischemia continues. In dogs, when ischemic brain is reperfused within 3 to 12 minutes, neuronal ATP production appears to recover rapidly, with replenishment of baseline cellular levels within 6 minutes.[8] However, after 30 minutes of ischemia, the replenishment of ATP to baseline levels takes significantly longer (~36 minutes).[9] Furthermore, even after 3 hours of reperfusion after intracranial thrombus injection, brain ATP levels still may not return to baseline levels.[10] Therefore, timely reperfusion is paramount, and after reperfusion is established, the direct cytotoxic effects of the ischemic cascade likely continue for minutes to hours until cellular ATP levels recover sufficiently.
