**3. Hypothermia at the cellular level**

Hypothermia is neuroprotective through several mechanisms. The effects of hypothermia include a wide range of biological processes which include decreasing excitatory amino acid release, reducing free radical formation, enhancing small ubiquitin-related modifier-related pathways, attenuating protein kinase C activity and slowing cellular metabolism.[11-13] Hypothermia has little effect on the core of infarcted tissue, but acts on tissue at risk in the penumbra by modulating different mechanisms that lead to cellular injury and death.[14] A marked reduction in the metabolic demand of penumbral tissue with induced hypothermia may prevent damage due to oxidative stress and energy failure. Cooling also results in reduced proteolysis and excitotoxic damage caused by glutamate toxicity, and reduction in neuronal calcium influx.[15, 16]

For every 1 °C reduction in brain temperature, the cerebral metabolic rate decreases in 6%. [17] Under stress conditions, hypothermia decreases high energy organic phosphates losses, slows the rates of metabolite consumption and lactic acid accumulation and reduces cerebral metabolic oxygen consumption, while improving glucose utilization.[11]

Hypothermia not only protects the brain by reducing cerebral metabolism during conditions of reduced substrate and shift to anaerobic glycolysis. Hypothermia also suppresses the accumulation and release of glutamate.[18] ATP loss during ischemia leads to ions flowing down their concentration gradients, and eventual efflux of potassium and influx of sodium and calcium.[19] Calcium influxes lead to direct neurotoxicity as well as extracellular accumulation of glutamate, which are neurotoxic. Experimental studies have shown that mild to moderate hypothermia attenuates the initial and delayed rise of extracellular potassium and prevents intracellular calcium accumulation, thus leading to decreased glutamate efflux and finally neuroprotection.

Numerous studies have shown that hypothermia reduces the generation of reactive oxygen species, decreases brain edema, and prevents blood-brain barrier breakdown.[18] One potential mechanism is that hypothermia inhibits matrix metalloproteinases and preserves basal lamina proteins after stroke.[20-22] Moreover, a clinical study of 10 patients with large strokes who underwent mild hypothermia demonstrated lower levels of matrix metalloproteinase than normothermic patients.[23] Serum metalloproteinases are a good marker of blood-brain barrier breakdown.[20]

Hypothermia has been documented by numerous investigators to alter gene expression normally observed after brain ischemia. Whereas a majority of genes are downregulated by hypothermia, a number of genes are also upregulated. [24] Interestingly, many proinflammatory and proapoptotic genes tend to be downregulated.[25-27] Whereas those genes that contribute to cell survival seem to be upregulated. [28-32]

Additionally, hypothermia has been shown to inhibit activation of the inflammatory transcription factor nuclear factor kappa B via temperature-dependent inhibition of its inhibitor protein's kinase. Other studies indicate that hypothermia has antiapoptotic effects such as reduction of cytochrome C release, and inhibition of caspases and proapoptotic genes.[33-37]

#### **4. Cooling temperatures**

52 Therapeutic Hypothermia in Brain Injury

treatment in these circumstances. Hypothermia has the potential to minimize secondary injury resulting from insufficient cerebral perfusion pressure or mechanical compression from herniation by ICH. Hypothermia preserves autoregulation of the cerebral vasculature

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

Hypothermia is neuroprotective through several mechanisms. The effects of hypothermia include a wide range of biological processes which include decreasing excitatory amino acid release, reducing free radical formation, enhancing small ubiquitin-related modifier-related pathways, attenuating protein kinase C activity and slowing cellular metabolism.[11-13] Hypothermia has little effect on the core of infarcted tissue, but acts on tissue at risk in the penumbra by modulating different mechanisms that lead to cellular injury and death.[14] A marked reduction in the metabolic demand of penumbral tissue with induced hypothermia may prevent damage due to oxidative stress and energy failure. Cooling also results in reduced proteolysis and excitotoxic damage caused by glutamate toxicity, and reduction in

For every 1 °C reduction in brain temperature, the cerebral metabolic rate decreases in 6%. [17] Under stress conditions, hypothermia decreases high energy organic phosphates losses,

and reduces cytotoxic edema around the hemorrhagic clot.[5]

**2. Pathophysiology of ischemic brain injury** 

hours until cellular ATP levels recover sufficiently.

**3. Hypothermia at the cellular level** 

neuronal calcium influx.[15, 16]

Therapeutic hypothermia is defined as an intentionally induced, controlled reduction of a patient's core temperature below 36**°**C. Further classification includes mild (34**°**C–35.9**°**C), moderate (32**°**C–33.9**°**C), moderate/deep (30**°**C–31.9**°**C), and deep (< 30**°**C) hypothermia. [38]

In general, hypothermia appears to be effective whether the brain is cooled to 33°C or 28°C, but temperatures on the lower end appeared to be most effective according to a recent metaanalysis of the experimental literature.[39] However, lower temperatures are associated with a higher incidence of complications, require more sedation and sometimes even induction of paralysis accompanied by intubation and ventilatory support.
