Effects of hypothermia for a short period on histologic outcome and extracellular glutamate concentration during and after cardiac arrest in rats*

Exploratory Feasibility Study of Cerebral Cooling by Transpulmonary Cooling During Cardiac Arrest in a Swine Cardiac Arrest Model

Studies on targeted temperature management for postcardiac arrest syndrome have shown no difference in outcomes between normothermia and hypothermia in patients with postcardiac arrest brain injury. Therefore, further development of therapeutic methods for temperature control in cardiac arrest patients is desirable. Although animal studies have shown that inducing hypothermia during cardiac arrest improves outcomes, no clinically effective method has yet been reported. We investigated whether intra-arrest lung cooling (IALC) effectively lowers brain temperature. A device capable of cooling oxygen was developed. The pigs were subjected to cardiac arrest using the device, ventilated, cooled during cardiopulmonary resuscitation, and resuscitated for 1 hour, with changes in brain temperature closely monitored. A device capable of cooling oxygen to -30°C was used to cool the lungs during cardiac arrest. Through this approach, IALC successfully reduced the brain temperature. Optimal cooling efficiency was observed when chest compressions and ventilation were synchronized at a ratio of 5:1, resulting in an approximate brain temperature reduction of 1.5°C/h. Our successful development of an oxygen-cooling device underscores the potential for lowering brain temperature through IALC using inhaled oxygen cooling.

Cardiopulmonary Resuscitation May Not Stop Glutamate Release in the Cerebral Cortex

BACKGROUND

Cardiopulmonary resuscitation (CPR) may not be sufficient to halt the progression of brain damage. Using extracellular glutamate concentration as a marker for neuronal damage, we quantitatively evaluated the degree of brain damage during resuscitation without return of spontaneous circulation.

MATERIALS AND METHODS

Extracellular cerebral glutamate concentration was measured with a microdialysis probe every 2 minutes for 40 minutes after electrical stimulation-induced cardiac arrest without return of spontaneous circulation in Sprague-Dawley rats. The rats were divided into 3 groups (7 per group) according to the treatment received during the 40 minutes observation period: mechanical ventilation without chest compression (group V); mechanical ventilation and chest compression (group VC) and; ventilation, chest compression and brain hypothermia (group VCH). Chest compression (20 min) and hypothermia (40 min) were initiated 6 minutes after the onset of cardiac arrest.

RESULTS

Glutamate concentration increased in all groups after cardiac arrest. Although after the onset of chest compression, glutamate concentration showed a significant difference at 2 min and reached the maximum at 6 min (VC group; 284±48 μmol/L vs. V group 398±126 μmol/L, P =0.003), there was no difference toward the end of chest compression (513±61 μmol/L vs. 588±103 μmol/L, P =0.051). In the VCH group, the initial increase in glutamate concentration was suddenly suppressed 2 minutes after the onset of brain hypothermia.

CONCLUSIONS

CPR alone reduced the progression of brain damage for a limited period but CPR in combination with brain cooling strongly suppressed increases in glutamate levels.

Recent developments and controversies in therapeutic hypothermia after cardiopulmonary resuscitation

Therapeutic hypothermia was recommended as the only neuroprotective treatment in comatose patients after return of spontaneous circulation (ROSC). With new evidence suggesting a similar neuroprotective effect of 36 °C and 33 °C, the term "therapeutic hypothermia" was substituted by "targeted temperature management" in 2011, which in turn was replaced by the term "temperature control" in 2022 because of new evidence of the similar effects of target normothermia and 33 °C. However, there is no clear consensus on the efficacy of therapeutic hypothermia. In this article, we provide an overview of the recent evidence from basic and clinical research related to therapeutic hypothermia and re-evaluate its application as a post-ROSC neuroprotective intervention in clinical settings.

Neuroprotection by the noble gases argon and xenon as treatments for acquired brain injury: a preclinical systematic review and meta-analysis

BACKGROUND

The noble gases argon and xenon are potential novel neuroprotective treatments for acquired brain injuries. Xenon has already undergone early-stage clinical trials in the treatment of ischaemic brain injuries, with mixed results. Argon has yet to progress to clinical trials as a treatment for brain injury. Here, we aim to synthesise the results of preclinical studies evaluating argon and xenon as neuroprotective therapies for brain injuries.

METHODS

After a systematic review of the MEDLINE and Embase databases, we carried out a pairwise and stratified meta-analysis. Heterogeneity was examined by subgroup analysis, funnel plot asymmetry, and Egger's regression.

RESULTS

A total of 32 studies were identified, 14 for argon and 18 for xenon, involving measurements from 1384 animals, including murine, rat, and porcine models. Brain injury models included ischaemic brain injury after cardiac arrest (CA), neurological injury after cardiopulmonary bypass (CPB), traumatic brain injury (TBI), and ischaemic stroke. Both argon and xenon had significant (P<0.001), positive neuroprotective effect sizes. The overall effect size for argon (CA, TBI, stroke) was 18.1% (95% confidence interval [CI], 8.1-28.1%), and for xenon (CA, TBI, stroke) was 34.1% (95% CI, 24.7-43.6%). Including the CPB model, only present for xenon, the xenon effect size (CPB, CA, TBI, stroke) was 27.4% (95% CI, 11.5-43.3%). Xenon, both with and without the CPB model, was significantly (P<0.001) more protective than argon.

CONCLUSIONS

These findings provide evidence to support the use of xenon and argon as neuroprotective treatments for acquired brain injuries. Current evidence suggests that xenon is more efficacious than argon overall.

Cryopreservation of Animals and Cryonics: Current Technical Progress, Difficulties and Possible Research Directions

The basis of cryonics or medical cryopreservation is to safely store a legally dead subject until a time in the future when technology and medicine will permit reanimation after eliminating the disease or cause of death. Death has been debunked as an event occurring after cardiac arrest to a process where interjecting its progression can allow for reversal when feasible. Cryonics technology artificially halts further damages and injury by restoring respiration and blood circulation, and rapidly reducing temperature. The body can then be preserved at this extremely low temperature until the need for reanimation. Presently, the area has attracted numerous scientific contributions and advancement but the practice is still flooded with challenges. This paper presents the current progression in cryonics research. We also discuss obstacles to success in the field, and identify the possible solutions and future research directions.

Extracorporeal Cardiopulmonary Resuscitation Guided by End-Tidal Carbon Dioxide-a Porcine Model

Extracorporeal membrane cardiopulmonary resuscitation (ECPR) during cardiopulmonary resuscitation (CPR) for selected cases and end-tidal carbon dioxide (ETCO₂) could be used to guide initiation of ECPR. Ventricular fibrillation was induced in 12 pigs and CPR was performed until ETCO₂ fell below 10 mmHg; then, ECPR was performed. Animals were divided into group short (G_Short) and group long (G_Long), according to time of CPR. Carotid blood flow was higher (p = 0.02) and mean arterial blood pressure lower in G_Long during CPR (p < 0.05). B-Lactate was lower and pH higher in G_Short (p < 0.01). In microdialysis lactate-pyruvate ratio, glycerol and glutamate increased in both groups during CPR, but considerably in G_Long (p < 0.01). No difference could be seen in histopathology of the brain or kidney post-ECPR. No apparent histological differences of tissue damage in brains or levels of S100B in plasma were detected between groups. This might suggest that ETCO₂ could be used as a marker for brain injury following ECPR.

CaMKII versus DAPK1 Binding to GluN2B in Ischemic Neuronal Cell Death after Resuscitation from Cardiac Arrest

DAPK1 binding to GluN2B was prominently reported to mediate ischemic cell death in vivo. DAPK1 and CaMKII bind to the same GluN2B region, and their binding is mutually exclusive. Here, we show that mutating the binding region on GluN2B (L1298A/ R1300Q) protected against neuronal cell death induced by cardiac arrest followed by resuscitation. Importantly, the GluN2B mutation selectively abolished only CaMKII, but not DAPK1, binding. During ischemic or excitotoxic insults, CaMKII further accumulated at excitatory synapses, and this accumulation was mediated by GluN2B binding. Interestingly, extra-synaptic GluN2B decreased after ischemia, but its relative association with DAPK1 increased. Thus, ischemic neuronal death requires CaMKII binding to synaptic GluN2B, whereas any potential role for DAPK1 binding is restricted to a different, likely extra-synaptic population of GluN2B.

Ischemic insults cause excitotoxic neuronal cell death via NMDA receptor overstimulation. Buonarati et al. find that excitotoxic insults cause DAPK1 movement to extra-synaptic NMDA receptors and CaMKII movement to synaptic NMDA receptors; importantly, preventing this CaMKII movement protects neurons from ischemic death.

Elimination of glutamate using CRRT for 72 h in patients with post-cardiac arrest syndrome: A randomized clinical pilot trial

AIM

Glutamine and glutamate are major mediators of secondary brain cell death during post-cardiac arrest syndrome. As there is an equilibrium between brain tissue and plasma concentrations of glutamine and glutamate, their elimination from systemic circulation by extracorporeal blood purification may ultimately lead to reduced secondary cell death in the brain. We hypothesized that systemic glutamine and glutamate can be significantly reduced by continuous venovenous hemodiafiltration (CVVHDF).

METHODS

This was a prospective, randomized clinical trial in post cardiac-arrest survivors evaluating standard of care or additional CVVHDF over 72 h immediately after admission. Glutamine and glutamate plasma concentrations were analyzed at eight time points in both groups. Primary endpoint was reduction of glutamine and glutamate plasma concentrations. The trial has been registered at clinical trial.gov (NCT02963298).

RESULTS

In total, 41 patients were randomized over a period of 12 months (control n = 21, CVVHDF n = 20). The primary aim reduction of glutamine and glutamate plasma concentrations by CVVHDF, was not achieved; both groups-maintained concentrations within a normal range over the study period (glutamate: 4.7-11.1 mg/dL; glutamine: 0.2-3.7 mg/dL). However, post-filter concentrations of glutamine and glutamate in CRRT patients were significantly decreased as compared to pre-filter concentrations (glutamate: pre-filter median 8.85 mg/dL IQR 7.1-9.6; post-filter 0.95 mg/dL IQR 0.5-2; p < 0.001; glutamine: pre-filter 0.7 mg/dL IQR 0.6-1; post-filter 0.2 mg/dL IQR 0-0.2; p < 0.001).

CONCLUSION

In this trial, CVVHDF was not able to statistically significantly lower systemic plasma glutamine and glutamate levels. Post-cardiac arrest patients had plasma glutamine and glutamate levels within the normal range.

A New Vision for Therapeutic Hypothermia in the Era of Targeted Temperature Management: A Speculative Synthesis

Three decades of animal studies have reproducibly shown that hypothermia is profoundly cerebroprotective during or after a central nervous system (CNS) insult. The success of hypothermia in preclinical acute brain injury has not only fostered continued interest in research on the classic secondary injury mechanisms that are prevented or blunted by hypothermia but has also sparked a surge of new interest in elucidating beneficial signaling molecules that are increased by cooling. Ironically, while research into cold-induced neuroprotection is enjoying newfound interest in chronic neurodegenerative disease, conversely, the scope of the utility of therapeutic hypothermia (TH) across the field of acute brain injury is somewhat controversial and remains to be fully defined. This has led to the era of Targeted Temperature Management, which emphasizes a wider range of temperatures (33–36°C) showing benefit in acute brain injury. In this comprehensive review, we focus on our current understandings of the novel neuroprotective mechanisms activated by TH, and discuss the critical importance of developmental age germane to its clinical efficacy. We review emerging data on four cold stress hormones and three cold shock proteins that have generated new interest in hypothermia in the field of CNS injury, to create a framework for new frontiers in TH research. We make the case that further elucidation of novel cold responsive pathways might lead to major breakthroughs in the treatment of acute brain injury, chronic neurological diseases, and have broad potential implications for medicines of the distant future, including scenarios such as the prevention of adverse effects of long-duration spaceflight, among others. Finally, we introduce several new phrases that readily summarize the essence of the major concepts outlined by this review—namely, Ultramild Hypothermia, the “Responsivity of Cold Stress Pathways,” and “Hypothermia in a Syringe.”

Cardiac Arrest Induces Ischemic Long-Term Potentiation of Hippocampal CA1 Neurons That Occludes Physiological Long-Term Potentiation

Ischemic long-term potentiation (iLTP) is a form of synaptic plasticity that occurs in acute brain slices following oxygen-glucose deprivation. In vitro, iLTP can occlude physiological LTP (pLTP) through saturation of plasticity mechanisms. We used our murine cardiac arrest and cardiopulmonary resuscitation (CA/CPR) model to produce global brain ischemia and assess whether iLTP is induced in vivo, contributing to the functionally relevant impairment of pLTP. Adult male mice were subjected to CA/CPR, and slice electrophysiology was performed in the hippocampal CA1 region 7 or 30 days later. We observed increased miniature excitatory postsynaptic current amplitudes, suggesting a potentiation of postsynaptic AMPA receptor function after CA/CPR. We also observed increased phosphorylated GluR1 in the postsynaptic density of hippocampi after CA/CPR. These data support the in vivo induction of ischemia-induced plasticity. Application of a low-frequency stimulus (LFS) to CA1 inputs reduced excitatory postsynaptic potentials in slices from mice subjected to CA/CPR, while having no effects in sham controls. These results are consistent with a reversal, or depotentiation, of iLTP. Further, depotentiation with LFS partially restored induction of pLTP with theta burst stimulation. These data provide evidence for iLTP following in vivo ischemia, which occludes pLTP and likely contributes to network disruptions that underlie memory impairments.

Determination of the Target Temperature Required to Block Increases in Extracellular Glutamate Levels During Intraischemic Hypothermia

This study aimed to determine a target temperature for intraischemic hypothermia that can block increases in extracellular glutamate levels. Two groups of 10 rats each formed the normothermia and intraischemic hypothermia groups. Extracellular glutamate levels, the extracellular potential, and the cerebral blood flow were measured at the adjacent site in the right parietal cerebral cortex. Cerebral ischemia was induced by occlusion of the bilateral common carotid arteries and hypotension. In the intraischemic hypothermia group, brain hypothermia was initiated immediately after the onset of membrane potential loss. In the normothermia group, extracellular glutamate levels began to increase simultaneously with the onset of membrane potential loss and reached a maximum level of 341.8 ± 153.1 μmol·L^-1. A decrease in extracellular glutamate levels was observed simultaneously with the onset of membrane potential recovery. In the intraischemic hypothermia group, extracellular glutamate levels initially began to increase, similarly to those in the normothermia group, but subsequently plateaued at 140.5 ± 105.4 μmol·L^-1, when the brain temperature had decreased to <32.6°C ± 0.9°C. A decrease in extracellular glutamate levels was observed simultaneously with the onset of membrane potential recovery, similarly to the findings in the normothermia group. The rate of decrease in extracellular glutamate levels was the same in both groups (-36.6 and -36.0 μmol·L^-1 in the normothermia and intraischemic hypothermia groups, respectively). In conclusion, the target temperature for blocking glutamate release during intraischemic hypothermia was found to be 32.6°C ± 0.9°C. Our results suggest that the induction of intraischemic hypothermia can maintain low glutamate levels without disrupting glutamate reuptake. Institutional protocol number: OKU-2016146.

Combined intra- and post-cardiac arrest hypothermic-targeted temperature management in a rat model of asphyxial cardiac arrest improves survival and neurologic outcome compared to either strategy alone

AIM

Post-cardiac arrest hypothermic-targeted temperature management (HTTM) improves outcomes in preclinical cardiac arrest studies. However, inadequate understanding of the mechanisms and therapeutic windows remains a barrier to optimization. We tested the hypothesis that combined intra- and post-cardiac arrest HTTM provides a synergistic outcome benefit compared to either strategy alone.

METHODS

Rats subjected to 8-min asphyxial cardiac arrest were block randomized to 4 treatment groups (n=12/group): NTTM) normothermic-targeted temperature management; 1-24 HTTM) HTTM initiated 1h post-ROSC and maintained for 24h; Intra-1 HTTM) HTTM initiated at CPR onset and maintained for 1h; and Intra-24 HTTM) HTTM initiated at CPR onset and maintained for 24h. HTTM was induced by nasopharyngeal cooling and maintained using an automated temperature regulation system. Target temperature range was 36.5-37.5°C for NTTM and 32.0-34.0°C for HTTM. Post-arrest neurologic function score (NFS) was measured daily, and rats surviving 72h were euthanized for histological analysis of neurodegeneration.

RESULTS

Target brain temperature was achieved 7.8±3.3min after initiating intra-arrest cooling. The survival rate was 42%, 50%, 50%, and 92% in the NTTM, 1-24 HTTM, Intra-1 HTTM, and Intra-24 HTTM groups, respectively (p<0.05, Intra-24 group vs. all other groups). The rate of survival with good neurologic function (NFS≥450) was 33% in the Intra-24 HTTM group vs. 0% in all other groups (mid p<0.05). Hippocampal CA1 sector neurodegeneration was significantly reduced in the Intra-24 HTTM group compared to all other groups (p<0.05).

CONCLUSION

Combined intra- and post-cardiac arrest HTTM has greater outcome benefits than either strategy alone.

Microdialysis Assessment of Cerebral Perfusion during Cardiac Arrest, Extracorporeal Life Support and Cardiopulmonary Resuscitation in Rats - A Pilot Trial

Cerebral metabolic alterations during cardiac arrest, cardiopulmonary resuscitation (CPR) and extracorporeal cardiopulmonary life support (ECLS) are poorly explored. Markers are needed for a more personalized resuscitation and post-resuscitation care. Aim of this study was to investigate early metabolic changes in the hippocampal CA1 region during ventricular fibrillation cardiac arrest (VF-CA) and ECLS versus conventional CPR. Male Sprague-Dawley rats (350g) underwent 8min untreated VF-CA followed by ECLS (n = 8; bloodflow 100ml/kg), mechanical CPR (n = 18; 200/min) until return of spontaneous circulation (ROSC). Shams (n = 2) were included. Glucose, glutamate and lactate/pyruvate ratio were compared between treatment groups and animals with and without ROSC. Ten animals (39%) achieved ROSC (ECLS 5/8 vs. CPR 5/18; OR 4,3;CI:0.7-25;p = 0.189). During VF-CA central nervous glucose decreased (0.32±0.1mmol/l to 0.04±0.01mmol/l; p<0.001) and showed a significant rise (0.53±0.1;p<0.001) after resuscitation. Lactate/pyruvate (L/P) ratio showed a 5fold increase (31 to 164; p<0.001; maximum 8min post ROSC). Glutamate showed a 3.5-fold increase to (2.06±1.5 to 7.12±5.1μmol/L; p<0.001) after CA. All parameters normalized after ROSC with no significant differences between ECLS and CPR. Metabolic changes during ischemia and resuscitation can be displayed by cerebral microdialysis in our VF-CA CPR and ECLS rat model. We found similar microdialysate concentrations and patterns of normalization in both resuscitation methods used. Institutional Protocol Number: GZ0064.11/3b/2011.

Targeted temperature management for adult out-of-hospital cardiac arrest: current concepts and clinical applications

Targeted temperature management (TTM) (primarily therapeutic hypothermia (TH)) after out-of-hospital cardiac arrest (OHCA) has been considered effective, especially for adult-witnessed OHCA with a shockable initial rhythm, based on pathophysiology and on several clinical studies (especially two randomized controlled trials (RCTs) published in 2002). However, a recently published large RCT comparing TTM at 33 °C (TH) and TTM at 36 °C (normothermia) showed no advantage of 33 °C over 36 °C. Thus, this RCT has complicated the decision to perform TH after cardiac arrest. The results of this RCT are sometimes interpreted fever control alone is sufficient to improve outcomes after cardiac arrest because fever control was not strictly performed in the control groups of the previous two RCTs that showed an advantage for TH. Although this may be possible, another interpretation that the optimal target temperature for TH is much lower than 33 °C may be also possible. Additionally, there are many points other than target temperature that are unknown, such as the optimal timing to initiate TTM, the period between OHCA and initiating TTM, the period between OHCA and achieving the target temperature, the duration of maintaining the target temperature, the TTM technique, the rewarming method, and the management protocol after rewarming. RCTs are currently underway to shed light on several of these underexplored issues. In the present review, we examine how best to perform TTM after cardiac arrest based on the available evidence.

Therapeutic hypothermia applicable to cardiac surgery

OBJECTIVE

To review the beneficial and adverse effects of therapeutic hypothermia (TH) applicable to cardiac surgery with cardiopulmonary bypass (CPB) in the contexts of various temperature levels and techniques for achieving TH.

DATABASES USED

Multiple electronic literature searches were performed using PubMed and Google for articles published from June 2012 to December 2014. Relevant terms (e.g. 'hypothermia', 'cardiopulmonary bypass', 'cardiac surgery', 'neuroprotection') were used to search for original articles, letters and reviews without species limitation. Reviews were included despite potential publication bias. References from the studies identified were also searched to find other potentially relevant citations. Abstracts, case reports, conference presentations, editorials and expert opinions were excluded.

CONCLUSIONS

Therapeutic hypothermia is an essential measure of neuroprotection during cardiac surgery that may be achieved most effectively by intravascular cooling using hypothermic CPB. For most cardiac surgical procedures, mild to modest (32-36 °C) TH will be sufficient to assure neuroprotection and will avoid most of the adverse effects of hypothermia that occur at lower body core temperatures.

Prolonged cooling duration mitigates myocardial and cerebral damage in cardiac arrest

BACKGROUND

The purpose of this study was to investigate the effect of prolonged cooling on cardiac and cerebral injury in animals under cardiac arrest.

METHODS

Adult male Wistar rats were equally randomized to normothermia, 5H1, 5H2, 7H1, 7H2, and 7H4 groups. The first number in the group name indicated ventricular fibrillation duration (minutes), the middle H indicated hypothermia, and the last number signified hypothermia duration (hours). Ventricular fibrillation was induced and untreated for 5 minutes (normothermia, 5H1, and 5H2) or 7 minutes (7H1, 7H2, and 7H4) followed by 1 minute of cardiopulmonary resuscitation followed by electric shocks. Hypothermia was initiated simultaneously with cardiopulmonary resuscitation initiation and maintained for 1 hour (5H1 and 7H1), 2 hours (5H2 and 7H2) or 4 hours (7H4).

RESULTS

There were 12 rats in each group. Compared with the 7H1 group, the 7H4 group had significantly better systolic function (dp/dt40) and cardiac output within the early postcardiac arrest period. Histologic examination disclosed less myocardial and hippocampal damage in the 7H4 group than the 7H1 group and in the 5H2 group than the 5H1 group. Plasma troponin I, fatty acid-binding protein, and S-100β concentrations were significantly lower in the 7H4 and 5H2 groups. The 7H4 and 5H2 groups survived statistically longer than the groups with shorter cooling duration.

CONCLUSION

Slightly prolonging hypothermia may mitigate myocardial and cerebral damage and improve survival and neurologic outcomes in a rat model of ventricular fibrillation cardiac arrest.

Improving neurological outcome after cardiac arrest: Therapeutic hypothermia the best treatment

Cardiac arrest, irrespective of its etiology, has a high mortality. This event is often associated with brain anoxia which frequently causes severe neurological damage and persistent vegetative state. Only one out of every six patients survives to discharge following in-hospital cardiac arrest, whereas only 2-9% of patients who experience out of hospital cardiac arrest survive to go home. Functional outcomes of survival are variable, but poor quality survival is common, with only 3-7% able to return to their previous level of functioning. Therapeutic hypothermia is an important tool for the treatment of post-anoxic coma after cardiopulmonary resuscitation. It has been shown to reduce mortality and has improved neurological outcomes after cardiac arrest. Nevertheless, hypothermia is underused in critical care units. This manuscript aims to review the mechanism of hypothermia in cardiac arrest survivors and to propose a simple protocol, feasible to be implemented in any critical care unit.

Can Therapeutic Hypothermia Be Guided by Advanced Neuromonitoring in Neurocritical Care Patients? A Review

The impact of therapeutic hypothermia (TH) on long-term neurological outcome is still controversial. Data on the effects of TH on brain homeostasis are mostly derived from experimental research. Invasive multimodal neuromonitoring techniques may provide additional insight into pathophysiological changes associated with primary or secondary brain injury in humans. In this study we describe the principles of multimodal neuromonitoring and its potential in the clinical setting of TH. We call for more research using multimodal neuromonitoring techniques in patients undergoing TH to optimize cooling and rewarming strategies.

Inducibility of ventricular fibrillation during mild therapeutic hypothermia: electrophysiological study in a swine model

INTRODUCTION

Mild therapeutic hypothermia (MTH) is being used after cardiac arrest for its expected improvement in neurological outcome. Safety of MTH concerning inducibility of malignant arrhythmias has not been satisfactorily demonstrated. This study compares inducibility of ventricular fibrillation (VF) before and after induction of MTH in a whole body swine model and evaluates possible interaction with changing potassium plasma levels.

METHODS

The extracorporeal cooling was introduced in fully anesthetized swine (n = 6) to provide MTH. Inducibility of VF was studied by programmed ventricular stimulation three times in each animal under the following: during normothermia (NT), after reaching the core temperature of 32°C (HT) and after another 60 minutes of stable hypothermia (HT60). Inducibility of VF, effective refractory period of the ventricles (ERP), QTc interval and potassium plasma levels were measured.

RESULTS

Starting at normothermia of 38.7 (IQR 38.2; 39.8)°C, HT was achieved within 54 (39; 59) minutes and the core temperature was further maintained constant. Overall, the inducibility of VF was 100% (18/18 attempts) at NT, 83% (15/18) after reaching HT (P = 0.23) and 39% (7/18) at HT60 (P = 0.0001) using the same protocol. Similarly, ERP prolonged from 140 (130; 150) ms at NT to 206 (190; 220) ms when reaching HT (P < 0.001) and remained 206 (193; 220) ms at HT60. QTc interval was inversely proportional to the core temperature and extended from 376 (362; 395) at NT to 570 (545; 599) ms at HT. Potassium plasma level changed spontaneously: decreased during cooling from 4.1 (3.9; 4.8) to 3.7 (3.4; 4.1) mmol/L at HT (P < 0.01), then began to increase and returned to baseline level at HT60 (4.6 (4.4; 5.0) mmol/L, P = NS).

CONCLUSIONS

According to our swine model, MTH does not increase the risk of VF induction by ventricular pacing in healthy hearts. Moreover, when combined with normokalemia, MTH exerts an antiarrhythmic effect despite prolonged QTc interval.

Feasibility study of immediate pharyngeal cooling initiation in cardiac arrest patients after arrival at the emergency room

AIM

Cooling the pharynx and upper oesophagus would be more advantageous for rapid induction of therapeutic hypothermia since the carotid arteries run in their vicinity. The aim of this study was to determine the effects of pharyngeal cooling on brain temperature and the safety and feasibility for patients under resuscitation.

METHODS

Witnessed non-traumatic cardiac arrest patients (n=108) were randomized to receive standard care with (n=53) or without pharyngeal cooling (n=55). In the emergency room, pharyngeal cooling was initiated before or shortly after return of spontaneous circulation by perfusing physiological saline (5 °C) into a pharyngeal cuff for 120 min.

RESULTS

There was a significant decrease in tympanic temperature at 40 min after arrival (P=0.02) with a maximum difference between the groups at 120 min (32.9 ± 1.2°C, pharyngeal cooling group vs. 34.1 ± 1.3°C, control group; P<0.001). The return of spontaneous circulation (70% vs. 65%, P=0.63) and rearrest (38% vs. 47%, P=0.45) rates were not significantly different based on the initiation of pharyngeal cooling. No post-treatment mechanical or cold-related injury was observed on the pharyngeal epithelium by macroscopic observation. The thrombocytopaenia incidence was lower in the pharyngeal cooling group (P=0.001) during the 3-day period after arrival. The cumulative survival rate at 1 month was not significantly different between the two groups.

CONCLUSIONS

Initiation of pharyngeal cooling before or immediately after the return of spontaneous circulation is safe and feasible. Pharyngeal cooling can rapidly decrease tympanic temperature without adverse effects on circulation or the pharyngeal epithelium.

Therapeutic benefits of mild hypothermia in patients successfully resuscitated from cardiac arrest: A meta-analysis

BACKGROUND

Good neurological outcome after cardiac arrest (CA) is hard to achieve for clinicians. Experimental and clinical evidence suggests that therapeutic mild hypothermia is beneficial. This study aimed to assess the effectiveness and safety of therapeutic mild hypothermia in patients successfully resuscitated from CA using a meta-analysis.

METHODS

We searched the MEDLINE (1966 to April 2012), OVID (1980 to April 2012), EMBASE (1980 to April 2012), Chinese bio-medical literature & retrieval system (CBM) (1978 to April 2012), Chinese medical current contents (CMCC) (1995 to April 2012), and Chinese medical academic conference (CMAC) (1994 to April 2012). Studies were included if 1) the study design was a randomized controlled trial (RCT); 2) the study population included patients successfully resuscitated from CA, and received either standard post-resuscitation care with normothermia or mild hypothermia; 3) the study provided data on good neurologic outcome and survival to hospital discharge. Relative risk (RR) and 95% confidence interval (CI) were used to pool the effect.

RESULTS

The study included four RCTs with a total of 417 patients successfully resuscitated from CA. Compared to standard post-resuscitation care with normothermia, patients in the hypothermia group were more likely to have good neurologic outcome (RR=1.43, 95% CI 1.14-1.80, P=0.002) and were more likely to survive to hospital discharge (RR=1.32, 95% CI 1.08-1.63, P=0.008). There was no significant difference in adverse events between the normothermia and hypothermia groups (P>0.05), nor heterogeneity and publication bias.

CONCLUSION

Therapeutic mild hypothermia improves neurologic outcome and survival in patients successfully resuscitated from CA.

Extracranial hypothermia during cardiac arrest and cardiopulmonary resuscitation is neuroprotective in vivo

There is increasing evidence that ischemic brain injury is modulated by peripheral signaling. Peripheral organ ischemia can induce brain inflammation and injury. We therefore hypothesized that brain injury sustained after cardiac arrest (CA) is influenced by peripheral organ ischemia and that peripheral organ protection can reduce brain injury after CA and cardiopulmonary resuscitation (CPR). Male C57Bl/6 mice were subjected to CA/CPR. Brain temperature was maintained at 37.5°C ± 0.0°C in all animals. Body temperature was maintained at 35.1°C ± 0.1°C (normothermia) or 28.8°C ± 1.5°C (extracranial hypothermia [ExHy]) during CA. Body temperature after resuscitation was maintained at 35°C in all animals. Behavioral testing was performed at 1, 3, 5, and 7 days after CA/CPR. Either 3 or 7 days after CA/CPR, blood was analyzed for serum urea nitrogen, creatinine, alanine aminotransferase, aspartate aminotransferase, and interleukin-1β; mice were euthanized; and brains were sectioned. CA/CPR caused peripheral organ and brain injury. ExHy animals experienced transient reduction in brain temperature after resuscitation (2.1°C ± 0.5°C for 4 minutes). Surprisingly, ExHy did not change peripheral organ damage. In contrast, hippocampal injury was reduced at 3 days after CA/CPR in ExHy animals (22.4% ± 6.2% vs. 45.7% ± 9.1%, p=0.04, n=15/group). This study has two main findings. Hypothermia limited to CA does not reduce peripheral organ injury. This unexpected finding suggests that after brief ischemia, such as during CA/CPR, signaling or events after reperfusion may be more injurious than those during the ischemic period. Second, peripheral organ hypothermia during CA reduces hippocampal injury independent of peripheral organ protection. While it is possible that this protection is due to subtle differences in brain temperature during early reperfusion, we speculate that additional mechanisms may be involved. Our findings add to the growing understanding of brain-body cross-talk by suggesting that peripheral interventions can protect the brain even if peripheral organ injury is not altered.

Induction of cooling with a passive head and neck cooling device: effects on brain temperature after stroke

BACKGROUND AND PURPOSE

Therapeutic hypothermia improves clinical outcome after cardiac arrest and appears beneficial in other cerebrovascular diseases. We conducted this study to investigate the relationship between surface head/neck cooling and brain temperature.

METHODS

Prospective observational study enrolling consecutive patients with severe ischemic or hemorrhagic stroke undergoing intracranial pressure (ICP) and brain temperature monitoring. Arterial pressure, ICP, cerebral perfusion pressure, heart rate, brain, tympanic, and bladder temperature were continuously registered. Fifty-one applications of the Sovika cooling device were analyzed in 11 individual patients.

RESULTS

Sovika application led to a significant decrease of brain temperature compared with baseline with a maximum of -0.36°C (SD, 0.22) after 49 minutes (SD, 17). During cooling, dynamics of brain temperature differed significantly from bladder (-0.25°C [SD, 0.15] after 48 minutes [SD, 19]) and tympanic temperature (-1.79°C [SD, 1.19] after 37 minutes [SD, 16]). Treatment led to an increase in systolic arterial pressure by >20 mm Hg in 14 applications (n=7 patients) resulting in severe hypertension (>180 mm Hg) in 4 applications (n=3). ICP increased by >10 mm Hg in 7 applications (n=3), led to ICP crisis >20 mm Hg in 6 applications (n=3), and a drop of cerebral perfusion pressure <50 mm Hg in 1 application.

CONCLUSIONS

Although the decrease of brain temperature after Sovika cooling device application was statistically significant, we doubt clinical relevance of this rather limited effect (-0.36°C). Moreover, the transient increases of blood pressure and ICP warrant caution.

Therapeutic hypothermia for the management of intracranial hypertension in severe traumatic brain injury: a systematic review

BACKGROUND

Traumatic brain injury (TBI) is a major source of death and severe disability worldwide. Raised Intracranial pressure (ICP) is an important predictor of mortality in patients with severe TBI and aggressive treatment of elevated ICP has been shown to reduce mortality and improve outcome. The acute post-injury period in TBI is characterized by several pathophysiologic processes that start in the minutes to hours following injury. All of these processes are temperature-dependent; they are all aggravated by fever and inhibited by hypothermia.

METHODS

This study reviewed the current clinical evidence in support of the use of therapeutic hypothermia (TH) for the treatment of intracranial hypertension (ICH) in patients with severe TBI.

RESULTS

This study identified a total of 18 studies involving hypothermia for control of ICP; 13 were randomized controlled trials (RCT) and five were observational studies. TH (32-34°C) was effective in controlling ICH in all studies. In the 13 RCT, ICP in the TH group was always significantly lower than ICP in the normothermia group. In the five observational studies, ICP during TH was always significantly lower than prior to inducing TH.

CONCLUSIONS

Pending results from large multi-centre studies evaluating the effect of TH on ICH and outcome, TH should be included as a therapeutic option to control ICP in patients with severe TBI.

Therapeutic applications of hypothermia in cerebral ischaemia

There is considerable experimental evidence that hypothermia is neuroprotective and can reduce the severity of brain damage after global or focal cerebral ischaemia. However, despite successful clinical trials for cardiac arrest and perinatal hypoxia-ischaemia and a number of trials demonstrating the safety of moderate and mild hypothermia in stroke, there are still no established guidelines for its use clinically. Based upon a review of the experimental studies we discuss the clinical implications for the use of hypothermia as an adjunctive therapy in global cerebral ischaemia and stroke and make some suggestions for its use in these situations.

Therapeutic hypothermia: benefits, mechanisms and potential clinical applications in neurological, cardiac and kidney injury

Therapeutic hypothermia involves the controlled reduction of core temperature to attenuate the secondary organ damage which occurs following a primary injury. Clinicians have been increasingly using therapeutic hypothermia to prevent or ameliorate various types of neurological injury and more recently for some forms of cardiac injury. In addition, some recent evidence suggests that therapeutic hypothermia may also provide benefit following acute kidney injury. In this review we will examine the potential mechanisms of action and current clinical evidence surrounding the use of therapeutic hypothermia. We will discuss the ideal methodological attributes of future studies using hypothermia to optimise outcomes following organ injury, in particular neurological injury. We will assess the importance of target hypothermic temperature, time to achieve target temperature, duration of cooling, and re-warming rate on outcomes following neurological injury to gain insights into important factors which may also influence the success of hypothermia in other organ injuries, such as the heart and the kidney. Finally, we will examine the potential of therapeutic hypothermia as a future kidney protective therapy.

Impact of therapeutic hypothermia onset and duration on survival, neurologic function, and neurodegeneration after cardiac arrest

OBJECTIVE

Post-cardiac-arrest therapeutic hypothermia improves outcomes in comatose cardiac arrest survivors. This study tests the hypothesis that the efficacy of post-cardiac-arrest therapeutic hypothermia is dependent on the onset and duration of therapy.

DESIGN

Prospective randomized laboratory investigation.

SETTING

University research laboratory.

SUBJECTS

A total of 268 male Long Evans rats.

INTERVENTIONS

Post-cardiac-arrest therapeutic hypothermia.

MEASUREMENTS AND MAIN RESULTS

Adult male Long Evans rats that achieved return of spontaneous circulation after a 10-min asphyxial cardiac arrest were block randomized to normothermia (37°C ± 1°C) or therapeutic hypothermia (33°C ± 1°C) initiated 0, 1, 4, or 8 hrs after return of spontaneous circulation and maintained for 24 or 48 hrs. Therapeutic hypothermia initiated 0, 1, 4, and 8 hrs after return of spontaneous circulation resulted in 7-day survival rates of 45%*, 36%*, 36%*, and 14%, respectively, compared to 17% for normothermic controls and survival with good neurologic function rates of 24%*, 24%*, 19%*, and 0%, respectively, compared to 2% for normothermic controls (*p < .05 vs. normothermia). These outcomes were not different when therapeutic hypothermia was maintained for 24 vs. 48 hrs. In contrast, hippocampal CA1 pyramidal neuron counts were 53% ± 27%*, 53% ± 19%*, 51% ± 24%*, and 65% ± 16%* of normal, respectively, when therapeutic hypothermia was initiated 0, 1, 4, or 8 hrs after return of spontaneous circulation compared to 9% in normothermic controls (*p < .01 vs. normothermia). Furthermore, surviving neuron counts were greater when therapeutic hypothermia was maintained for 48 hrs compared to 24 hrs (68% ± 15%* vs. 42% ± 22%, *p < .0001).

CONCLUSIONS

In this study, post-cardiac-arrest therapeutic hypothermia resulted in comparable improvement of survival and survival with good neurologic function when initiated within 4 hrs after return of spontaneous circulation. However, histologic assessment of neuronal survival revealed a potentially broader therapeutic window and greater neuroprotection when therapeutic hypothermia was maintained for 48 vs. 24 hrs.

Glutamate release predicts ongoing myocardial ischemia of rat hearts

BACKGROUND

Glutamate metabolism is associated with myocardial ischemia-reperfusion, but it is not clear whether glutamate reveals ongoing ischemia (OI). We evaluated whether microdialysis would detect OI induced by coronary artery ligation in a rat cardiac transplantation model.

MATERIAL AND METHODS

A total of 24 Fischer 344 rats underwent syngeneic heterotopic cardiac transplantation. Of these, 16 rats underwent ligation of the left anterior coronary artery (LAD) of the heart to induce ongoing ischemia (OI), of which eight grafts received intra-aortally Gabapentin (12 mg/graft), a glutamate-release inhibitor and eight grafts with transplantation only served as the control. With a microdialysis catheter samples for glucose, lactate, pyruvate, glutamate, and glycerol were analysed spectrophotometrically. Histology and aquaporin 7 evaluations were performed after graft harvesting.

RESULTS

Glutamate was elevated after 15 min of reperfusion in OI as compared with Control (14.31 +/- 5.03 microM vs 6.75 +/- 2.21 microM, p = 0.05), respectively. Glycerol remained high in OI (61.89 +/- 46.13 microM to 15.84 +/- 0.85 microM, p = ns) and low in Control (12.33 +/- 3.36 microM to 5.52 +/- 0.25 microM, p = ns). Gabapentin decreased glutamate release from 7.32 +/- 1.57 microM to 2.71 +/- 0.64 microM, (p < 0.05) and resulted in decrease of glycerol levels from 24.64 +/- 4.03 microM to 10.43 +/- 2.49 microM, (p < 0.05) in OI. The expression of aquaporin 7 and histology confirmed OI.

CONCLUSIONS

We suggest that glutamate release may be used as an early indicator of OI after cardiac arrest.

Augmentation of the cooling capacity of refrigerated fluid by minimizing heat gain of the fluid using a simple method of cold insulation

OBJECTIVES

This study was undertaken to determine how rapidly refrigerated fluids gain heat during bolus infusion and to determine whether the refrigerated fluids could be kept cold by a simple cold-insulation method.

METHODS

One liter of refrigerated fluid was run through either a 16-gauge catheter (16G(-) and 16G(+) groups) or an 18-gauge catheter (18G(-) and 18G(+) groups) while monitoring the temperature in the fluid bag and the outflow site. In the 16G(+) and the 18G(+) groups, the fluid bag was placed with an ice pack inside an insulating sleeve during the fluid run.

RESULTS

In the 16G(-) and the 18G(-) groups, the outflow temperature increased to 10-12 degrees C during the fluid run. Meanwhile, outflow temperatures in the 16G(+) and the 18G(+) groups remained below 4.6 and 6.8 degrees C, respectively. The temperatures differed significantly between the 16G(-) and the 16G(+) groups (p < 0.001) and between the 18G(-) and the 18G(+) groups (p < 0.001), respectively.

CONCLUSIONS

Substantial heat gain occurred in the refrigerated fluid even during the relatively short duration of bolus infusion. The heat gain could, however, be easily minimized by cold insulation of the fluid bag.

Survival and neurological outcomes after nasopharyngeal cooling or peripheral vein cold saline infusion initiated during cardiopulmonary resuscitation in a porcine model of prolonged cardiac arrest

OBJECTIVE

We have previously demonstrated that nasopharyngeal cooling initiated during cardiopulmonary resuscitation improves the success of resuscitation. In this study, we compared the effects of nasopharyngeal cooling with cold saline infusion initiated during cardiopulmonary resuscitation on resuscitation outcome in a porcine model of prolonged cardiac arrest. We hypothesized that nasopharyngeal cooling initiated during cardiopulmonary resuscitation would yield better resuscitation outcome when compared with cold saline infusion.

DESIGN

Randomized, prospective animal study.

SETTING

University-affiliated research laboratory.

SUBJECTS

Yorkshire-X domestic pigs (Sus scrofa).

INTERVENTIONS

Ventricular fibrillation was induced in 14 pigs weighing 38 +/- 2 kg. After 15 mins of untreated ventricular fibrillation, cardiopulmonary resuscitation was performed for 5 mins before defibrillation. Coincident with the start of cardiopulmonary resuscitation, animals were randomly assigned to receive nasopharyngeal cooling with the aid of the RhinoChill Device (BeneChill, San Diego, CA) or cold saline infusion with 30 mL/kg 4 degrees C saline. One hour after restoration of spontaneous circulation, surface cooling was begun with the aid of a water blanket in both groups and maintained for 4 hrs.

MEASUREMENTS AND MAIN RESULTS

Jugular vein temperature significantly decreased in animals subjected to nasopharyngeal cooling in comparison with those receiving cold saline infusion (p < .01). Core temperature, however, decreased only in animals receiving cold saline infusion (p < .01). Coronary perfusion pressure was significantly higher in the animals treated with nasopharyngeal cooling (p = .02). All seven animals treated with nasopharyngeal cooling were successfully resuscitated in contrast to only two animals resuscitated in the cold saline infusion group (p = .02).

CONCLUSION

In this model, nasopharyngeal cooling initiated during cardiopulmonary resuscitation improved the success of resuscitation compared to cooling with cold saline infusion.

Out-of-hospital therapeutic hypothermia in cardiac arrest victims

Despite many years of research, outcome after cardiac arrest is dismal. Since 2005, the European Resuscitation Council recommends in its guidelines the use of mild therapeutic hypothermia (32-34 degrees) for 12 to 24 hours in patients successfully resuscitated from cardiac arrest. The benefit of resuscitative mild hypothermia (induced after resuscitation) is well established, while the benefit of preservative mild to moderate hypothermia (induced during cardiac arrest) needs further investigation before recommending it for clinical routine. Animal data and limited human data suggest that early and fast cooling might be essential for the beneficial effect of resuscitative mild hypothermia. Out-of-hospital cooling has been shown to be feasible and safe by means of intravenous infusion with cold fluids or non-invasively with cooling pads. A combination of these cooling methods might further improve cooling efficacy. If out-of-hospital cooling will further improve functional outcome as compared with in-hospital cooling needs to be determined in a prospective, randomised, sufficiently powered clinical trial.

Mechanisms of action, physiological effects, and complications of hypothermia

BACKGROUND

Mild to moderate hypothermia (32-35 degrees C) is the first treatment with proven efficacy for postischemic neurological injury. In recent years important insights have been gained into the mechanisms underlying hypothermia's protective effects; in addition, physiological and pathophysiological changes associated with cooling have become better understood.

OBJECTIVE

To discuss hypothermia's mechanisms of action, to review (patho)physiological changes associated with cooling, and to discuss potential side effects.

DESIGN

Review article.

INTERVENTIONS

None.

MAIN RESULTS

A myriad of destructive processes unfold in injured tissue following ischemia-reperfusion. These include excitotoxicty, neuroinflammation, apoptosis, free radical production, seizure activity, blood-brain barrier disruption, blood vessel leakage, cerebral thermopooling, and numerous others. The severity of this destructive cascade determines whether injured cells will survive or die. Hypothermia can inhibit or mitigate all of these mechanisms, while stimulating protective systems such as early gene activation. Hypothermia is also effective in mitigating intracranial hypertension and reducing brain edema. Side effects include immunosuppression with increased infection risk, cold diuresis and hypovolemia, electrolyte disorders, insulin resistance, impaired drug clearance, and mild coagulopathy. Targeted interventions are required to effectively manage these side effects. Hypothermia does not decrease myocardial contractility or induce hypotension if hypovolemia is corrected, and preliminary evidence suggests that it can be safely used in patients with cardiac shock. Cardiac output will decrease due to hypothermia-induced bradycardia, but given that metabolic rate also decreases the balance between supply and demand, is usually maintained or improved. In contrast to deep hypothermia (<or=30 degrees C), moderate hypothermia does not induce arrhythmias; indeed, the evidence suggests that arrhythmias can be prevented and/or more easily treated under hypothermic conditions.

CONCLUSIONS

Therapeutic hypothermia is a highly promising treatment, but the potential side effects need to be properly managed particularly if prolonged treatment periods are required. Understanding the underlying mechanisms, awareness of physiological changes associated with cooling, and prevention of potential side effects are all key factors for its effective clinical usage.