a great number of patients with major injuries [ 13 ] suffer from accidental hypothermia ranging from 12 to 66% [ 4 , 5 ] . in the current literature ,
an increase of mortality has been demonstrated below a core temperature of 34c in patients with multiple injuries , this temperature threshold seems to be critical in trauma patients and therefore a modified definition is reasonable .
thereby , the extent of hypothermia correlates with the overall injury severity and is increased by pelvic or abdominal surgery ; furthermore , hypothermic polytrauma patients suffer from an increased incidence of posttraumatic complications [ 2 , 4 , 810 ] as well as a an increased mortality [ 2 , 8 , 11 ] . depending on its origin ,
endogenous hypothermia results from a metabolic dysfunction with a decreased heat production ( e.g. , hypothyreodism , hypoglycaemia , hypoadrenalism ) or a disturbed thermoregulation ( e.g. , intracranial tumor , degenerative neurologic disorders ) .
accidental hypothermia is characterized by an unintentional decrease of the core temperature due to exposure to a cold environment without a thermoregulative dysfunction .
the infusion of cold fluids as , for example , 2 l of crystalloids ( 18 ) decreases core temperatures about 0.6c .
in addition , a reduced oxygen supply with an anaerobic state decreases the amount of adenosine trisphosphate ( atp ) and subsequent heat production .
furthermore , the application of anaesthetics and skeletal muscle relaxants prevents shivering and vasoconstriction and therefore advances heat loss .
while the first two entities do not play a major role after trauma , accidental hypothermia is common in trauma patients as well as patients undergoing major surgery [ 4 , 5 ] .
in contrast to accidental hypothermia that needs to be addressed in the treatment of severely injured patients due to its detrimental effects , induced hypothermia is commonly used , that is , in elective cardiac surgery .
in addition there exists a strong recommendation for the induction of hypothermia after cardiopulmonary reanimation , and there exists a growing body of evidence that suggests the application of hypothermia after blunt brain injury .
some articles were published regarding the influence of hypothermia in context of elective surgery but also on the posttraumatic immune response in animal models . however , an overview about the influence of hypothermia on the humeral and cellular immune response with special focus on apoptosis is missing .
this paper outlines the molecular mechanisms by which hypothermia influences apoptosis as well as the immune response following severe trauma and major surgery .
in general , cell death following hemorrhage and ischemia occurs either as necrosis of affected cells or as a complex process of programmed cell death called apoptosis . in contrast
, necrosis represents the premature death of cells caused by external factors , that is , trauma , infections , or exposure to toxins .
apoptosis is associated with a cascade of enzymatic reactions in which proteolytic caspase enzymes play a major role .
apoptosis could be initiated either in an extrinsic or intrinsic way . the extrinsic way is characterized by the interaction of ligands ( e.g. , tnf- ) with  death receptors  on the cell surface ( e.g. , cd95 ) activating the enzymatic cascade by caspase-8 .
the intrinsic activation of apoptosis is triggered by tumor suppressing factors ( e.g. , p53 ) resulting in an increased expression of proapoptotic factors of the bcl-2 family ( e.g. , bax , bad ) and an increased mitochondrial release of cytochrome - c .
binding apaf-1 ( apoptotic protease activating factor ) cytochrome - c activates the apoptotic cascade via caspase-9 [ 63 , 64 ] .
besides the proteins of the bcl-2 family ( antiapoptotic bcl-2 ; proapoptotic bax and bad ) , the apoptotic process is regulated by mitogen - activated protein kinases ( map - kinases ) like extracellular - signal - regulated kinase 1/2 ( erk 1/2 ) , cjun n - terminal protein kinase 1/2 ( jnk 1/2 ) , and p38 map - kinase . additionally , other pathways involving phosphatidylinositol-3 kinases ( pi3k ) generating phosphatidylinositol-3,4,5-trisphosphate ( pi-3,4,5-p(3 ) ) and other phospholipids interacting with akt are known to play an important role in regulating cell survival .
furthermore , besides kinases the transcription factor nf - kappab regulates cellular apoptotic programs . in various studies
, hypothermia was shown to prevent additional tissue injury by interrupting both , the intrinsic , and extrinsic apoptotic pathway [ 15 , 6770 ] .
interestingly , hypothermia seems to affect the very early steps in the apoptotic process including inhibition of activation of caspase enzymes , preserving mitochondrial function and decreased overload of excitatory transmitters .
in contrast , apoptosis occurs relatively late following tissue challenge but it was shown that the process continues for up to 3 days [ 7072 ] . due to the delay of the apoptotic process
, modulation of the apoptotic cascade could serve as a therapeutic target in early stages of polytrauma management after the initial resuscitation process in which the patient is stabilized with the aim to prevent additional damage . in this context , it is of special importance that the rate of apoptosis in neutrophils is dramatically decreased in multiple trauma patients .
cultured hepatocytes showed suppressed fas - mediated apoptosis detected by a decreased mitochondrial damage following moderate hypothermia . besides an attenuated cytochrome - c release , hypothermia suppressed the activation of caspase-7 and -9 .
this data suggests potential organ protective effects of hypothermia regarding apoptosis , which were confirmed in various animal models . on the other hand
, murine neutrophils revealed a reduced spontaneous and tnf-induced apoptosis under mild hypothermia of 35c .
this fact could result in a prolonged exposure to activated neutrophils after trauma resulting in secondary organ damage .
profound hypothermia was not only shown to preserve akt in cardiomyocytes and inhibit caspase-3 activation but also activate antiapoptotic proteins such as bcl-2 in an experimental model of hemorrhagic shock   and ischemic insult . in ischemia
/ reperfusion injury , hypothermia reversed activation of apoptosis stimulating fragment ( fas)/caspase-8 , the increase of bax ( an proapoptotic protein ) , and decrease of bcl-2 in endothelial cells .
following ischemia and reperfusion , isolated cardiomyocytes showed increased phospho - akt levels associated with attenuation of reactive oxygen species production , which was blocked by akt but not cgmp inhibition .
additionally , hypothermia was associated with downregulation of the tnf receptor ( tnfr)1 and its proapoptotic ligand fas in rat cerebral cortices following a moderate fluid percussion model of traumatic brain injury .
most of the clinical studies regarding the influence of hypothermia on apoptosis are limited to ischemic injuries following cardiac arrest or brain ischemia [ 78 , 79 ] . in summary , there is clear evidence that hypothermia reduces ischemic neuronal apoptosis in global cerebral ischemia as a result of attenuated p53 expression and increased bcl-2 release .
immune response following major surgery or trauma consists of a complex set of pro- and anti - inflammatory reactions in order to restore homeostasis . the balance or imbalance of the pro- and anti - inflammatory immune response in part influences the clinical course . whereas predominance of the pro - inflammatory response leads to the systemic inflammatory response syndrome ( sirs ) ,
the anti - inflammatory reaction also named as compensatory anti - inflammatory response syndrome ( cars ) might result in immune suppression with an enhanced risk of infectious complications .
sirs as well as immune suppression plays a decisive role in the development of sepsis and the multiple organ dysfunction syndrome ( mods ) after trauma .
cytokines , released from various cell types including immunocompetent and intrinsic cells , regulate the specific and unspecific immune response .
these mediators are detectable in the peripheral blood and several compartments like the lung and the liver .
they serve not only as a marker of injury severity or outcome predictors but also as a tool for decision - making regarding timing of elective surgery during the clinical course [ 80 , 81 ] .
the most important cytokines in this regard include tnf- , il-1 , il-6 , il-8 , and il-10 ( table 2 ) . as another essential step of the systemic immune response , chemotactic cytokines , so - called chemokines
( il-8 , mcp-1 , mip-1 , or mip-1 ) mediate neutrophil infiltration into the affected tissue .
the initial neutrophil - endothelial interaction , so - called rolling , is mediated by members of the selectin family of adhesion molecules .
integrins ( cd11/cd18 ) and immunoglobulin superfamily receptors ( icam-1 , vcam-1 , elam-1 ) are important for the following firm adhesion and cell diapedesis [ 84 , 85 ] . in various experimental as well as clinical studies , an effect of hypothermia on the inflammatory response by altering the expression of pro- and anti - inflammatory cytokines ,
chemokines and adhesion molecules has been shown [ 38 , 8688 ] . in brain injury
there is profound evidence that an inadequate or disproportionate posttraumatic immune response not only increases the risk for brain cell injury but also the extent of damage [ 8995 ] . in this process
elevated levels of il-1 as well as an increased expression of il-1 mrna were detected following experimental brain injury in rodents , respectively [ 96100 ] . while il-1 does not cause brain damage itself , injection of il-1 increased cell death following various brain damage models [ 101103 ] .
the hypothesis that il-1 increases brain damage is supported by experiments in which an il-1 antagonist prevented cell death in experimental brain injury [ 102105 ] .
similar results were observed following treatment with an il-1 converting enzyme ( ice ) inhibitor in cerebral ischemia .
thus , modulation of cytokine release by hypothermia may serve as a therapeutic approach following major injury . peripheral blood mononuclear cells stimulated with lipopolysaccharide ( lps ) from healthy volunteers showed decreased tnf- release , while release of il-1 and il-6 was delayed when incubated at 33c as compared to incubation at 37c . in a similar study with human macrophages , early secretion of tnf- and il-6
additionally , a shift towards anti - inflammatory cytokines was detected in microglia cells following lps treatment .
in contrast , hypothermia of 33c raised the levels of il-1 , il-6 , and tnf- produced by monocytes from healthy volunteers stimulated with lps .
this controversial findings suggest diverse effects of hypothermia on different cell types . in a rat model of acute hemorrhage ,
gundersen et al . evaluated the effect of hypothermia on immune response and corresponding organ damage .
moderate hypothermia had an organ protective effect in liver and kidney , which was associated with a decreased release of il-6 as well as a reduction of reactive oxygen species .
in contrast , mild hypothermia did not affect systemic levels of il-1 , il-6 , and il-10 , while serum tnf- levels were even increased following hemorrhagic shock suggesting different responses of cytokines or their respective sources . in a study using a swine model of uncontrolled lethal hemorrhage ,
the authors were able to detect a decreased pro - inflammatory ( il-6 ) and an increased anti - inflammatory ( il-10 ) immune response following profound hypothermia .
furthermore , the potentially protective chaperone heat shock protein-70 ( hsp 70 ) was preserved .
the authors , therefore , concluded a beneficial modulation of the immune system due to hypothermia in this hemorrhage model .
only a few publications investigated the anti - inflammatory effects of hypothermia in a combined trauma - hemorrhage setting . in a two - hit model consisting of a femoral fracture and hemorrhage ,
systemic il-10 levels were elevated following mild hypothermia   confirming results from other experimental studies [ 35 , 112 , 113 ] . the increased anti - inflammatory response induced by hypothermia
was also associated with a conversion from th-1 to th-2 cytokine pattern . in a nonbacterial - driven sepsis model using intraperitoneal lipopolysaccharide injection , hypothermia also induced elevated plasma il-10 levels .
the effects of hypothermia on the release of inflammatory cytokines are presented in table 3 .
experimental studies of traumatic brain injury , in which hypothermia decreased systemic cytokine levels , were confirmed by a clinical study showing a suppression in cytokine release associated with an improved outcome [ 38 , 88 ] . in cerebral trauma patients ,
hypothermia of 32 - 33c suppressed systemic il-6 levels , which was associated with an increased glasgow outcome scale 6 months postinjury as compared to patients treated under normothermic conditions .
the effects of hypothermia on chemokine levels and the expression of adhesion molecules were investigated in experimental as well as clinical studies ( tables 4 and 5 ) . in vitro studies of human umbilical vein endothelial cells showed decreased mcp-1 as well as il-8 levels under hypothermic conditions .
in contrast , no effect of hypothermia on icam-1 expression in human cerebral endothelial cells was shown under stress conditions .
in cultured human umbilical vein cells , hypothermia inhibited the transcription and expression but not the induction of e - selectin . in a model of cerebral ischemia and reperfusion , mild hypothermia reduced local expression of mcp-1 .
the same finding could be confirmed in a murine multiple trauma model . in another cerebral ischemia model ,
intraischemic as well as delayed hypothermia decreased icam-1 expression as well as intracerebral neutrophil infiltration . in pediatric cardiopulmonary bypass ( cpb ) surgery , hypothermia reduced systemic levels of the chemokine regulated on activation normal t cell expressed and secreted ( rantes ) and mcp-1 . in contrast , no effect of hypothermia on mcp-1 levels was detected in a model of cardiac arrest in rats . in transient focal cerebral ischemia , mild hypothermia reduced icam-1 expression , which was associated with reduced neutrophil and monocyte infiltration .
in contrast to experimental studies , circulating adhesion molecules were not altered by hypothermia in aortocoronary artery bypass grafting .
, there is evidence that induced hypothermia decreases pro - inflammatory cytokines as well as chemokines and adhesion molecules . besides this , an increased anti - inflammatory cytokine response
hypothermia influences the cellular immune response , which was especially studied following brain injury . in various animal models ,
neutrophil and macrophage function was attenuated leading to a decreased extent of secondary brain injury and infarction size .
furthermore , posttraumatic hypothermia decreased early and prolonged accumulation of neutrophils and myeloperoxidase activity suggesting hypothermia as a potential mechanism to modulate outcome .
these findings confirmed an earlier study of toyoda et al . showing a decreased neutrophil infiltration following intraischemic hypothermia in a focal ischemia reperfusion injury , which was also shown after delayed hypothermia in another cerebral ischemia model .
additionally , posttraumatic hypothermia reduced neutrophil accumulation on the injury site at 24  h in a model of spinal cord injury .
although most studies were conducted in traumatic brain injury , similar findings were shown in other organs . already in the 1980s , it was shown that hypothermia reduced local neutrophil infiltration in an experimental pleuritis model , while the number of circulating neutrophils was not affected .
other studies suggest a hypothermia - induced decrease of circulating neutrophils after soft - tissue injury in piglets , supporting an older study with prolonged hypothermia of 29c in pigs . the reduced infiltration can be explained by a decrease of adhesion molecules due to hypothermia .
however , phagocytosis of opsonised bacteria is even increased at a lower temperature suggesting a temperature - dependent activation of neutrophils . following major injuries , infiltrated neutrophils release proteolytic enzymes as well as free radicals causing tissue damage which may subsequently lead to organ dysfunction and failure .
in contrast , hypothermia did not affect the formation of free radicals in a rat model of hemorrhagic shock [ 120122 ] .
the reduced number of free radicals is of great benefit since the capacity of antioxidative mechanisms is limited .
most of free radicals following brain injury are synthesized by nitric oxide synthase and by deregulated mitochondrial electron transporters [ 124127 ] .
thus , it was speculated that the prevention of release or synthesis of free radicals may be induced by preserved mitochondrial function .
interestingly , mitochondrial function plays also a pivotal role in the development of apoptosis through inhibition of the caspase cascade activation .
the essential role of the nitric oxide synthase was supported by another experiment , in which attenuation of acute lung injury by induced hypothermia following hemorrhagic shock was associated with less myeloperoxidase activity and decreased gene expression of inos .
furthermore , gene expression of heat shock protein ( hsp-72 ) , a molecular chaperone known to exert protective effects in ischemia - reperfusion injury , was detected in hypothermic but not in normothermic rats .
in contrast to these results , another study using a model of pressure - controlled hemorrhagic shock revealed no differences in serum - free 8-isoprostane ( a marker of lipid peroxidation by free radicals ) between the two groups at either baseline or resuscitation time 1  hr . in a forebrain ischemia and recirculation model ,
postischemic leukotriene production a well as edema development was reduced 2  h but not 10  min following reperfusion .
controlled mild hypothermia had no effect on the number of circulating t lymphocytes in patients with severe brain injury . a clinical study including infants and children with severe traumatic brain injury showed a preserved antioxidant reserve in cerebrospinal fluid , suggesting an attenuation of oxidative stress following hypothermia in severe brain injury .
less neuron damage was detected at a temperature of 34c following brain ischemia   confirming another study of thoracic aortic ischemia - reperfusion injury in which hypothermia prevented and delayed paralysis by preserving cells of the central nervous system . in another study ,
a beneficial long - term effect of mild ( 35c ) and moderate ( 32c ) hypothermia was detected after spinal cord ischemia and reperfusion until 28 days following the injury . in a rat model of ischemia / reperfusion of the lower extremity , local hypothermia protected skeletal muscle from capillary leakage , which was prevented after treatment with heme oxygenase and nitric oxide synthase inhibitors .
hypothermia not only prevents damage on site of injury but also distant organ damage . in mesenteric ischemia - reperfusion injury ,
remote lung injury was prevented as measured by leukocyte trafficking , alveolar hemorrhage , and increased bal protein and wet / dry ratios .
these results confirmed another study using a hepatic model of ischemia / reperfusion injury , in which hypothermia reduced the associated lung injury . in a rat hemorrhagic shock model , mild hypothermia ( 34c ) improved survival associated with modulation of the immune response .
clinical studies are mainly focused on patients with traumatic brain injury . in general , there is a gap between experimental studies and clinical experience . in a multicenter study of 392 patients with severe head injuries , hypothermia of 33c did not influence outcome . in 5 meta - analyses ,
conflicting results regarding outcome were shown [ 138142 ] . only 2 of them [ 141 , 142 ] described a marginal benefit regarding mortality and neurological outcome .
patients with elevated intracranial pressure seem to be the cohort with the most benefit of hypothermia . regarding these controversial results , in current guidelines hypothermia is recommended as a level iii treatment option .
hypothermia at time of occlusion decreased infarct size in a myocardial ischemia - reperfusion injury supporting the results showing an increased cellular tolerance to ischemia under hypothermic conditions .
these results suggest starting hypothermia as soon as the hemodynamic and hemostaseological parameters were stabilized .
in contrast , there is consistent evidence that rapid rewarming reverses the beneficial effects of induced hypothermia in traumatic brain injury .
furthermore , gradual rewarming improved survival and attenuated remote acute lung injury after intestinal ischemia and reperfusion injury as compared to speed rewarming .
considering the current literature , it seems likely that abrogation of the beneficial effects is associated with atp depletion , energy failure , and consecutive mitochondrial dysfunction . to date , detailed knowledge about exact mechanisms in different rewarming strategies is lacking .
the european resuscitation council recommends early hypothermia ( 3234c ) for 1224 hours following cardiac arrest and a slow rewarming procedure with 0.250.5c per hour avoiding hyperthermia . in a clinical study in 57 hypothermic patients ,
continuous arteriovenous rewarming resulted in less fluid requirement and less mortality as compared to standard rewarming procedures . in summary
, there is only minor understanding of the rewarming process but slow rewarming is recommended based on mostly experimental studies .
before induced hypothermia can be introduced in the clinical management of patients several limitations of the presented studies need to be considered .
a potential limitation of all studies in which hypothermia is induced by a cardiopulmonary bypass machine is the fact that increase of cytokines may be due to the bypass procedure itself
. while numerous experimental as well as clinical studies regarding cardiac surgery , brain injury , or cardiac arrest are available information regarding hypothermia following major injuries is spare . since there are still divergent results in experimental studies that are mostly limited to traumatic brain injury , the mechanisms by which hypothermia influences the posttraumatic immune response after multiple trauma need to be elucidated .
 numerous multiple trauma and surgical patients suffer from accidental hypothermia . 
 while induced hypothermia is commonly used in elective cardiac surgery due to its protective effects , accidental hypothermia is associated with increased posttraumatic complications and even mortality in severely injured patients . 
 this paper focuses on protective molecular mechanisms of hypothermia on apoptosis and the posttraumatic immune response . 
 although information regarding severe trauma is limited , there is evidence that induced hypothermia may have beneficial effects on the posttraumatic immune response as well as apoptosis in animal studies and certain clinical situations . 
 however , more profound knowledge of mechanisms is necessary before randomized clinical trials in trauma patients can be initiated . 
1. Introduction
2. Apoptosis
3. Immune System
4. Functional Parameters
5. Cooling Rewarming Procedure
6. Perspective