Safety and efficacy of endovascular cooling and rewarming for induction and reversal of hypothermia in human-sized pigs

Mild hypothermia therapy attenuates early BBB leakage in acute ischaemic stroke

Reperfusion therapy inevitably leads to brain-blood barrier (BBB) disruption and promotes damage despite its benefits for acute ischaemic stroke (AIS). An effective brain cytoprotective treatment is still needed as an adjunct to reperfusion therapy. Here, we explore the potential benefits of therapeutic hypothermia (HT) in attenuating early BBB leakage and improving neurological outcomes. Mild HT was induced during the early and peri-recanalization stages in a mouse model of transient middle cerebral artery occlusion and reperfusion (tMCAO/R). The results showed that mild HT attenuated early BBB leakage in AIS, decreased the infarction volume, and improved functional outcomes. RNA sequencing data of the microvessels indicated that HT decreased the transcription of the actin polymerization-related pathway. We further discovered that HT attenuated the ROCK1/MLC pathway, leading to a decrease in the polymerization of G-actin to F-actin. Arachidonic acid (AA), a known structural ROCK agonist, partially counteracted the protective effects of HT in the tMCAO/R model. Our study highlights the importance of early vascular protection during reperfusion and provides a new strategy for attenuating early BBB leakage by HT treatment for ischaemic stroke.

Research progress of selective brain cooling methods in the prehospital care for stroke patients: A narrative review

Over the past four decades, therapeutic hypothermia (TH) has long been suggested as a promising neuroprotective treatment of acute ischemic stroke (AIS). Much attention has focus on keeping the hypothermic benefits and removing side effects of systemic hypothermia. In the past few years, the advent of intravenous thrombolysis and endovascular thrombectomy has taken us into a reperfusion era of AIS treatment. With recent research emphasizing ways to plus neuroprotective treatments to reperfusion therapy, the spotlight is now shifting toward the study of how selective brain hypothermia can offset the drawbacks of systemic hypothermia and be applied in prehospital condition. This mini-review summarizes current brain cooling methods that can be used for inducing selective hypothermia in prehospital care. It will guide the future development of selective cooling methods, extend the application of TH in prehospital care, and provide insights into the prospects of selective hypothermia in AIS.

Hypothermic neuroprotection against acute ischemic stroke: The 2019 update

Acute ischemic stroke is a leading cause of death and disability worldwide. Therapeutic hypothermia has long been considered as one of the most robust neuroprotective strategies. Although the neuroprotective effects of hypothermia have only been confirmed in patients with global cerebral ischemia after cardiac arrest and in neonatal hypoxic ischemic encephalopathy, establishing standardized protocols and strictly controlling the key parameters may extend its application in other brain injuries, such as acute ischemic stroke. In this review, we discuss the potential neuroprotective effects of hypothermia, its drawbacks evidenced in previous studies, and its potential clinical application for acute ischemic stroke especially in the era of reperfusion. Based on the different conditions between bench and bedside settings, we demonstrate the importance of vascular recanalization for neuroprotection of hypothermia by analyzing numerous literatures regarding hypothermia in focal cerebral ischemia. Then, we make a thorough analysis of key parameters of hypothermia and introduce novel hypothermic therapies. We advocate in favor of the process of clinical translation of intra-arterial selective cooling infusion in the era of reperfusion and provide insights into the prospects of hypothermia in acute ischemic stroke.

Transient selective brain cooling confers neurovascular and functional protection from acute to chronic stages of ischemia/reperfusion brain injury

Ischemic injury can be alleviated by the judicious use of hypothermia. However, the optimal regimens and the temporal kinetics of post-stroke neurovascular responses to hypothermic intervention have not been systematically studied. These gaps slow the clinical translation of hypothermia as an anti-stroke therapy. Here, we characterized the effects of transient selective brain hypothermia (TSBH) from the hyperacute to chronic stages of focal ischemia/reperfusion brain injury induced by transient middle cerebral artery occlusion in mice. A simple cooling device was used to induce TSBH during cerebral ischemia. This treatment reduced mortality from 31.8% to 0% and improved neurological outcomes for at least 35 days post-injury. TSBH mitigated blood-brain barrier leakage during the hyperacute and acute injury stages (1-23 h post-reperfusion). This early protection of the blood-brain barrier was associated with anti-inflammatory phenotypic polarization of microglia/macrophages, reduced production of pro-inflammatory cytokines, and less brain infiltration of neutrophils and macrophages during the subacute injury stage (three days post-reperfusion). TSBH elicited enduring protective effects on both grey and white matter for at least 35 days post-injury and preserved the long-term electrophysiological function of fiber tracts. In conclusion, TSBH ameliorates ischemia/reperfusion injury in the neurovascular unit from hyperacute to chronic injury stages after experimental stroke.

Heating and Cooling Rates With an Esophageal Heat Exchange System

BACKGROUND

The Esophageal Cooling Device circulates warm or cool water through an esophageal heat exchanger, but warming and cooling efficacy in patients remains unknown. We therefore determined heat exchange rates during warming and cooling.

METHODS

Nineteen patients completed the trial. All had general endotracheal anesthesia for nonthoracic surgery. Intraoperative heat transfer was measured during cooling (exchanger fluid at 7°C) and warming (fluid at 42°C). Each was evaluated for 30 minutes, with the initial condition determined randomly, starting at least 40 minutes after induction of anesthesia. Heat transfer rate was estimated from fluid flow through the esophageal heat exchanger and inflow and outflow temperatures. Core temperature was estimated from a zero-heat-flux thermometer positioned on the forehead.

RESULTS

Mean heat transfer rate during warming was 18 (95% confidence interval, 16-20) W, which increased core temperature at a rate of 0.5°C/h ± 0.6°C/h (mean ± standard deviation). During cooling, mean heat transfer rate was -53 (-59 to -48) W, which decreased core temperature at a rate of 0.9°C/h ± 0.9°C/h.

CONCLUSIONS

Esophageal warming transferred 18 W which is considerably less than the 80 W reported with lower or upper body forced-air covers. However, esophageal warming can be used to supplement surface warming or provide warming in cases not amenable to surface warming. Esophageal cooling transferred more than twice as much heat as warming, consequent to the much larger difference between core and circulating fluid temperature with cooling (29°C) than warming (6°C). Esophageal cooling extracts less heat than endovascular catheters but can be used to supplement catheter-based cooling or possibly replace them in appropriate patients.

Hypothermia Used in Medical Applications for Brain and Spinal Cord Injury Patients

Despite more than 80 years of animal experiments and clinical practice, efficacy of hypothermia in improving treatment outcomes in patients suffering from cell and tissue damage caused by ischemia is still ongoing. This review will first describe the history of utilizing cooling in medical treatment, followed by chemical and biochemical mechanisms of cooling that can lead to neuroprotection often observed in animal studies and some clinical studies. The next sections will be focused on current cooling approaches/devices, as well as cooling parameters recommended by researchers and clinicians. Animal and clinical studies of implementing hypothermia to spinal cord and brain tissue injury patients are presented next. This section will review the latest outcomes of hypothermia in treating patients suffering from traumatic brain injury (TBI), spinal cord injury (SCI), stroke, cardiopulmonary surgery, and cardiac arrest, followed by a summary of available evidence regarding both demonstrated neuroprotection and potential risks of hypothermia. Contributions from bioengineers to the field of hypothermia in medical treatment will be discussed in the last section of this review. Overall, an accumulating body of clinical evidence along with several decades of animal research and mathematical simulations has documented that the efficacy of hypothermia is dependent on achieving a reduced temperature in the target tissue before or soon after the injury-precipitating event. Mild hypothermia with temperature reduction of several degrees Celsius is as effective as modest or deep hypothermia in providing therapeutic benefit without introducing collateral/systemic complications. It is widely demonstrated that the rewarming rate must be controlled to be lower than 0.5 °C/h to avoid mismatch between local blood perfusion and metabolism. In the past several decades, many different cooling methods and devices have been designed, tested, and used in medical treatments with mixed results. Accurately designing treatment protocols to achieve specific cooling outcomes requires collaboration among engineers, researchers, and clinicians. Although this problem is quite challenging, it presents a major opportunity for bioengineers to create methods and devices that quickly and safely produce hypothermia in targeted tissue regions without interfering with routine medical treatment.

Neuroprotective mechanisms and translational potential of therapeutic hypothermia in the treatment of ischemic stroke

Stroke is a leading cause of disability and death, yet effective treatments for acute stroke has been very limited. Thus far, tissue plasminogen activator has been the only FDA-approved drug for thrombolytic treatment of ischemic stroke patients, yet its application is only applicable to less than 4–5% of stroke patients due to the narrow therapeutic window (< 4.5 hours after the onset of stroke) and the high risk of hemorrhagic transformation. Emerging evidence from basic and clinical studies has shown that therapeutic hypothermia, also known as targeted temperature management, can be a promising therapy for patients with different types of stroke. Moreover, the success in animal models using pharmacologically induced hypothermia (PIH) has gained increasing momentum for clinical translation of hypothermic therapy. This review provides an updated overview of the mechanisms and protective effects of therapeutic hypothermia, as well as the recent development and findings behind PIH treatment. It is expected that a safe and effective hypothermic therapy has a high translational potential for clinical treatment of patients with stroke and other CNS injuries.

Regulation of therapeutic hypothermia on inflammatory cytokines, microglia polarization, migration and functional recovery after ischemic stroke in mice

Stroke is a leading threat to human life and health in the US and around the globe, while very few effective treatments are available for stroke patients. Preclinical and clinical studies have shown that therapeutic hypothermia (TH) is a potential treatment for stroke. Using novel neurotensin receptor 1 (NTR1) agonists, we have demonstrated pharmacologically induced hypothermia and protective effects against brain damages after ischemic stroke, hemorrhage stroke, and traumatic brain injury (TBI) in rodent models. To further characterize the mechanism of TH-induced brain protection, we examined the effect of TH (at ±33°C for 6h) induced by the NTR1 agonist HPI-201 or physical (ice/cold air) cooling on inflammatory responses after ischemic stroke in mice and oxygen glucose deprivation (OGD) in cortical neuronal cultures. Seven days after focal cortical ischemia, microglia activation in the penumbra reached a peak level, which was significantly attenuated by TH treatments commenced 30min after stroke. The TH treatment decreased the expression of M1 type reactive factors including tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), IL-12, IL-23, and inducible nitric oxide synthase (iNOS) measured by RT-PCR and Western blot analyses. Meanwhile, TH treatments increased the expression of M2 type reactive factors including IL-10, Fizz1, Ym1, and arginase-1. In the ischemic brain and in cortical neuronal/BV2 microglia cultures subjected to OGD, TH attenuated the expression of monocyte chemoattractant protein-1 (MCP-1) and macrophage inflammatory protein-1α (MIP-1α), two key chemokines in the regulation of microglia activation and infiltration. Consistently, physical cooling during OGD significantly decreased microglia migration 16h after OGD. Finally, TH improved functional recovery at 1, 3, and 7days after stroke. This study reveals the first evidence for hypothermia mediated regulation on inflammatory factor expression, microglia polarization, migration and indicates that the anti-inflammatory effect is an important mechanism underlying the brain protective effects of a TH therapy.

Therapeutic Hypothermia for Acute Stroke

Hypothermia is the most potent neuroprotective therapy available. Clinical use of hypothermia is limited by technology and homeostatic mechanisms that maintain core body temperature. Recent advances in intravascular cooling catheters and successful trials of hypothermia for cardiac arrest revivified interest in hypothermia for stroke, resulting in Phase 1 clinical trials and plans for further development. Given the recent spate of neuroprotective therapy failures, we sought to clarify whether clinical trials of therapeutic hypothermia should be mounted in stroke patients. We reviewed the preclinical and early clinical trials of hypothermia for a variety of indications, the putative mechanisms for neuroprotection with hypothermia, and offer several hypotheses that remain to be tested in clinical trials. Therapeutic hypothermia is promising, but further Phase 1 and Phase 2 development efforts are needed to ensure that cooling of stroke patients is safe, before definitive efficacy trials.

Improved Therapeutic Benefits by Combining Physical Cooling With Pharmacological Hypothermia After Severe Stroke in Rats

Supplemental Digital Content is available in the text.

Background and Purpose—

Therapeutic hypothermia is a promising strategy for treatment of acute stroke. Clinical translation of therapeutic hypothermia, however, has been hindered because of the lack of efficiency and adverse effects. We sought to enhance the clinical potential of therapeutic hypothermia by combining physical cooling (PC) with pharmacologically induced hypothermia after ischemic stroke.

Methods—

Wistar rats were subjected to 90-minute middle cerebral artery occlusion by insertion of an intraluminal filament. Mild-to-moderate hypothermia was induced 120 minutes after the onset of stroke by PC alone, a neurotensin receptor 1 (NTR1) agonist HPI-201 (formally ABS-201) alone or the combination of both. The outcomes of stroke were evaluated at 3 and 21 days after stroke.

Results—

PC or HPI-201 each showed hypothermic effect and neuroprotection in stroke rats. The combination of PC and HPI-201 exhibited synergistic effects in cooling process, reduced infarct formation, cell death, and blood-brain barrier damages and improved functional recovery after stroke. Importantly, coapplied HPI-201 completely inhibited PC-associated shivering and tachycardia.

Conclusions—

The centrally acting hypothermic drug HPI-201 greatly enhanced the efficiency and efficacy of conventional PC; this combined cooling therapy may facilitate clinical translation of hypothermic treatment for stroke.

Endovascular Cooling Method for Hypothermia in Injured Swine

We evaluated an endovascular cooling method to modulate core temperature in trauma swine models with and without fluid support. Anesthetized swine (N = 80) were uninjured (SHAM) or injured through a bone fracture plus soft tissue injury or an uncontrolled hemorrhage and then subdivided to target body temperatures of 38°C (normothermia) or 33°C (hypothermia) by using a Thermogard endovascular cooling device (Zoll Medical). Temperature regulation began simultaneously at onset of injury (T0). Body temperatures were recorded from a rectal probe (Rec Temp) and from a central pulmonary artery catheter (PA Temp). At T15, swine received 500 mL IV Hextend over 30 minutes or no treatment (NONE) with continued monitoring until 3 hours from injury. Hypothermia was attained in 105 ± 39 minutes, at a cooling rate of -0.061°C ± 0.007°C/min for NONE injury groups. Postinjury Hextend administration resulted in faster cooling (-0.080°C ± 0.006°C/min); target temperature was reached in 83 ± 11 minutes (p < 0.05). During active cooling, body temperature measured by the PA Temp was significantly cooler than the Rec Temp due to the probe's closer proximity to the blood-cooling catheter balloons (p < 0.05). This difference was smaller in SHAM and fluid-supported injury groups (1.1°C ± 0.4°C) versus injured NONE groups (2.1°C ± 0.3°C). Target temperatures were correctly maintained thereafter in all groups. In normothermia groups, there was a small initial transient overshoot to maintain 38°C. Despite the noticeable difference between PA Temp and Rec Temp until target temperature was attained, this endovascular method can safely induce moderate hypothermia in anesthetized swine. However, likely due to their compromised hemodynamic state, cooling in hypovolemic and/or injured patients will be different from those without injury or those that also received fluids.

Therapeutic hypothermia for stroke: Where to go?

Ischemic stroke is a major cause of death and long-term disability worldwide. Thrombolysis with recombinant tissue plasminogen activator is the only proven and effective treatment for acute ischemic stroke; however, therapeutic hypothermia is increasingly recognized as having a tissue-protective function and positively influencing neurological outcome, especially in cases of ischemia caused by cardiac arrest or hypoxic-ischemic encephalopathy in newborns. Yet, many aspects of hypothermia as a treatment for ischemic stroke remain unknown. Large-scale studies examining the effects of hypothermia on stroke are currently underway. This review discusses the mechanisms underlying the effect of hypothermia, as well as trends in hypothermia induction methods, methods for achieving optimal protection, side effects, and therapeutic strategies combining hypothermia with other neuroprotective treatments. Finally, outstanding issues that must be addressed before hypothermia treatment is implemented at a clinical level are also presented.

A Head and Neck Support Device for Inducing Local Hypothermia

The present work describes the design of a device/system intended to induce local mild hypothermia by simultaneously cooling a patient's head and neck. The therapeutic goal is to lower the head and neck temperatures to 33-35 °C, while leaving the core body temperature unchanged. The device works by circulating a cold fluid around the exterior of the head and neck. The head surface area is separated into five different cooling zones. Each zone has a cooling coil and can be independently controlled. The cooling coils are tightly wrapped concentric circles of tubing. This design allows for a dense packing of tubes in a limited space, while preventing crimping of the tubing and minimizing the fluid pressure head loss. The design in the neck region also has multiple tubes wrapping around the circumference of the patient's neck in a helix. Preliminary testing indicates that this approach is capable of achieving the design goal of cooling the brain tissue (at a depth of 2.5 cm from the scalp) to 35 °C within 30- 40 min, without any pharmacologic or circulatory manipulation. In a comparison with examples of current technology, the device has shown the potential for improved cooling capability.

Rapid induction of brain hypothermia by endovascular intra-arterial perfusion

OBJECTIVES

Achieving rapid, brain cooling has potentially important clinical implications. To investigate potential practicalities, we induced brain hypothermia in canines by perfusing cooled crystalloid solution into the carotid artery using an extracorporeal cooling-filtration system.

METHODS

Ringer's solution cooled to approximately 6.5 degrees C was infused at a rate of 3 ml/kg/min for 30 minutes into the right common carotid artery through an angiographic catheter via the right femoral artery in six adult canines (13.81 +/- 0.60 kg). Excessive fluid was ultrafiltrated through a venovenous extracorporeal circuit via the right femoral vein. Temperature was monitored in the cerebral hemispheres, the rectum and the vena cava. The extracellular lactate concentrations were measured by microdialysis in the frontal lobes.

RESULTS

Right brain temperature decreased to 33.6 +/- 2.0 degrees C from 37.7 +/- 1.1 degrees C 30 minutes after initiation of perfusion, while left brain and rectal temperatures were 34.3 +/- 1.7 and 34.1 +/- 1.3 degrees C, respectively. The cooling rate of the right cerebral hemisphere was 4.2 +/- 1.1 degrees C/ 30 minutes and advanced compared with the rectum (p<0.01), the left cerebral hemisphere and the vena cava (both p<0.05). There was no significant increase in the extracellular lactate concentrations in the cerebral hemispheres. Hemoglobin, hematocrit and cardiac function significantly changed during perfusion (p<0.05).

CONCLUSIONS

Brain hypothermia was rapidly and safely induced using an intra-arterial crystalloid infusion and an extracorporeal cooing-filtration system. With refinement and further assessment of metabolic and physiologic parameters, the method holds a potential for clinical utility.

The small chill: mild hypothermia for cardioprotection?

Reducing the heart's temperature by 2-5°C is a potent cardioprotective treatment in animal models of coronary artery occlusion. The anti-infarct benefit depends upon the target temperature and the time at which cooling is instituted. Protection primarily results from cooling during the ischaemic period, whereas cooling during reperfusion or beyond offers little protection. In animal studies, protection is proportional to both the depth and duration of cooling. An optimal cooling protocol must appreciably shorten the normothermic ischaemic time to effectively salvage myocardium. Patients presenting with acute myocardial infarction could be candidates for mild hypothermia since the current door-to-balloon time is typically 90 min. But they would have to be cooled quickly shortly after their arrival. Several strategies have been proposed for ultra-fast cooling, but most like liquid ventilation and pericardial perfusion are too invasive. More feasible strategies might include cutaneous cooling, peritoneal lavage with cold solutions, and endovascular cooling with intravenous thermodes. This last option has been investigated clinically, but the results have been disappointing possibly because the devices lacked capacity to cool the patient quickly or cooling was not implemented soon enough. The mechanism of hypothermia's protection has been assumed to be energy conservation. However, whereas deep hypothermia clearly preserves ATP, mild hypothermia has only a modest effect on ATP depletion during ischaemia. Some evidence suggests that intracellular signalling pathways might be responsible for the protection. It is unknown how cooling could trigger these pathways, but, if true, then it might be possible to duplicate cooling's protection pharmacologically.

Design of a Cooling Guide Catheter for Rapid Heart Cooling

Cardiovascular disease is the leading cause of death in the United States. Despite decades of care path improvements approximately 30% of heart attack victims die within 1 year after their first heart attack. Animal testing has shown that mild hypothermia, reducing tissue temperatures by 2–4°C, has the potential to save heart tissue that is not adequately perfused with blood. This paper describes the design of a cooling guide catheter that can provide rapid, local cooling to heart tissue during emergency angioplasty. Using standard materials and dimensions found in typical angioplasty guide catheters, a closed-loop cooling guide catheter was developed. Thermal fluid modeling guided the interior geometric design. After careful fabrication and leak testing, a mock circulatory system was used to measure catheter cooling capacity. At blood analog flow rates ranging from 20 ml/min to 70 ml/min, the corresponding cooling capacity varied almost linearly from 20 W to 45 W. Animal testing showed 18 W of cooling delivered by the catheter can reduce heart tissue temperatures rapidly, approximately 3° in 5 min in some locations. Future animal testing work is needed to investigate if this cooling effect can save heart tissue.

Intra-arrest transnasal evaporative cooling: a randomized, prehospital, multicenter study (PRINCE: Pre-ROSC IntraNasal Cooling Effectiveness)

BACKGROUND

Transnasal evaporative cooling has sufficient heat transfer capacity for effective intra-arrest cooling and improves survival in swine. The aim of this study was to determine the safety, feasibility, and cooling efficacy of prehospital transnasal cooling in humans and to explore its effects on neurologically intact survival to hospital discharge.

METHODS AND RESULTS

Witnessed cardiac arrest patients with a treatment interval <or=20 minutes were randomized to intra-arrest cooling with a RhinoChill device (treatment group, n=96) versus standard care (control group, n=104). The final analysis included 93 versus 101 patients, respectively. Both groups were cooled after hospital arrival. The patients had similar demographics, initial rhythms, rates of bystander cardiopulmonary resuscitation, and intervals to cardiopulmonary resuscitation and arrival of advanced life support personnel. Eighteen device-related adverse events (1 periorbital emphysema, 3 epistaxis, 1 perioral bleed, and 13 nasal discolorations) were reported. Time to target temperature of 34 degrees C was shorter in the treatment group for both tympanic (102 versus 282 minutes, P=0.03) and core (155 versus 284 minutes, P=0.13) temperature. There were no significant differences in rates of return of spontaneous circulation between the groups (38% in treated subjects versus 43% in control subjects, P=0.48), in overall survival of those admitted alive (44% versus 31%, respectively, P=0.26), or in neurologically intact survival to discharge (Pittsburgh cerebral performance category scale 1 to 2, 34% versus 21%, P=0.21), although the study was not adequately powered to detect changes in these outcomes.

CONCLUSIONS

Prehospital intra-arrest transnasal cooling is safe and feasible and is associated with a significant improvement in the time intervals required to cool patients.

Current and future role of therapeutic hypothermia

Therapeutic moderate hypothermia has been advocated for use in traumatic brain injury, stroke, cardiac arrest-induced encephalopathy, neonatal hypoxic-ischemic encephalopathy, hepatic encephalopathy, and spinal cord injury, and as an adjunct to aneurysm surgery. In this review, we address the trials that have been performed for each of these indications, and review the strength of the evidence to support treatment with mild/moderate hypothermia. We review the data to support an optimal target temperature for each indication, as well as the duration of the cooling, and the rate at which cooling is induced and rewarming instituted. Evidence is strongest for prehospital cardiac arrest and neonatal hypoxic-ischemic encephalopathy. For traumatic brain injury, a recent meta-analysis suggests that cooling may increase the likelihood of a good outcome, but does not change mortality rates. For many of the other indications, such as stroke and spinal cord injury, trials are ongoing, but the data are insufficient to recommend routine use of hypothermia at this time.

[Endovascular or surface cooling?: therapeutic hypothermia after cardiac arrest]

INTRODUCTION

Time course, time necessary to achieve the target temperature and stable maintenance, as well as a controlled rewarming period are important factors influencing the outcome of patients after successful cardiopulmonary resuscitation.

METHODS

After successful cardiopulmonary resuscitation a total of 49 patients were cooled via an endovascular or external cooling device to a target temperature of 33 degrees C. Relevant cooling parameters, such as time between admission and initiation of cooling, achievement of target temperature and stable maintenance of cooling therapy, were compared between both groups.

RESULTS

In the endovascular cooling group the target temperature was reached significantly faster (154 +/- 97 min vs. 268 +/- 95 min, p = 0.0002) and showed stable and controlled maintenance of cooling therapy (deviation from target temperature: 0.189 +/- 0.23 degrees C vs 0.596 +/- 0.61 degrees C, p = 0.00006). The rewarming phase was better controlled and length of ICU stay was shorter in the group with endovascular cooling (8.8 +/- 3 vs. 12.9 +/- 6 days).

CONCLUSION

Endovascular cooling offers the possibility to reach the target temperature significantly faster and a stable maintenance of therapeutic hypothermia. It is capable of a more controlled rewarming period and shortens the length of ICU stay.

Use of induced hypothermia for neuroprotection: indications and application

Therapeutic temperature regulation has become an exciting field of interest. Mild-to-moderate hypothermia is a safe and feasible management strategy for neuroprotection and control of intracranial pressure in neurological catastrophies such as traumatic brain injury, subarachnoid and intracerebral hemorrhage, and large hemispheric stroke. Fever is associated with worse neurological outcome in patients with brain injury, normothermia may be of benefit in this patient population. The efficacy of mild-to-moderate hypothermia has been proven for neuroprotection after cardiac arrest with ventricular fibrillation as initial rhythm, and after neonatal asphyxia. Application of hypothermia and fever control in neurocritical care, available cooling technologies and systemic effects and complications of hypothermia will be discussed.

Effect of endovascular hypothermia on acute ischemic edema: morphometric analysis of the ICTuS trial

INTRODUCTION

Pilot studies of hypothermia for stroke suggest a potential benefit in humans. We sought to test whether hypothermia decreases post-ischemic edema using CT scans from a pilot trial of endovascular hypothermia for stroke.

METHODS

Eighteen patients with acute ischemic stroke underwent therapeutic hypothermia (target = 33 degrees C) for 12 or 24 h followed by a 12-h controlled re-warm using an endovascular system. CT scans obtained at baseline, 36-48 h (right after cooling and re-warming) and 30 days were digitized, intracranial compartment volumes measured using a validated stereological technique, and the calculated change in CSF volume between the three time-points were used as an estimate of edema formation in each patient. Patients were grouped retrospectively for analysis based on whether they cooled effectively (i.e., to a temperature nadir of less than 34.5 degrees C within 8 h) or not.

RESULTS

Eleven patients were cooled partially or not at all, and seven were effectively cooled. Baseline demographics and compartment volumes and densities were similar in both groups. At 36-48 h, the total CSF volume had significantly decreased in the not-cooled group compared to the cooled group (P < 0.05), with no significant difference in mean volume of ischemia between them (73 +/- 73 ml vs. 54 +/- 59 ml, respectively), suggesting an ameliorative effect of hypothermia on acute edema formation. At 30 days, the difference in CSF volumes had resolved, and infarct volumes (73 +/- 71 ml vs. 84 +/- 102 ml, respectively) and functional outcomes were comparable.

CONCLUSIONS

Endovascular hypothermia decreases acute post-ischemic cerebral edema. A larger trial is warranted to determine if it affects final infarct volume and outcome in stroke.

Rapid non-invasive external cooling to induce mild therapeutic hypothermia in adult human-sized swine

AIM OF THE STUDY

Mild therapeutic hypothermia is a promising new therapy for patients resuscitated from cardiac arrest. Early and fast induction of hypothermia seems to be crucial for best results. The aim of the study was to investigate the feasibility and safety of a new surface cooling method using cold metal plates.

SUBJECTS AND METHODS

Twelve adult human-sized swine (79+/-9 kg) were cooled from 38 to 33 degrees C brain temperature. The skin surface was covered with -20 degrees C metal plates (M), as compared to ice packs, alcohol rubs, and fans used in a control group (C). Each method was tested during spontaneous circulation and, after re-warming, during cardiac arrest. Temperatures were recorded continuously. Data are given as mean+/-standard deviation or as median (interquartile range), if not normally distributed. Comparisons between the treatment groups were performed with the independent samples t-test, or the Mann-Whitney rank-sum test.

RESULTS

During spontaneous circulation, cooling rates were 9.3+/-1.4 degrees C/h (M), and 6.1+/-1.4 degrees C/h (C) (p=0.003); no skin lesions were observed. During cardiac arrest, cooling rates were 4.1 degrees C/h (1.8-4.8) (M), and 3.7 degrees C/h (3.1-5.3) (C) (p=0.9); no skin lesions were observed.

CONCLUSION

Cooling with cold metal plates was an effective method for rapid induction of mild therapeutic hypothermia in adult human-sized swine during spontaneous circulation, without any signs of skin damage. This new surface-cooling device, independent of energy supply during use, should be further investigated.

Rapid endovascular warming for profound hypothermia

Profound hypothermia is associated with high mortality and morbidity. Optimal outcomes have been reported with invasive extracorporeal warming techniques not readily available in most hospitals. Endovascular warming devices may provide a less invasive alternative. A 68-year-old woman developed profound hypothermia after environmental exposure. On arrival, she was comatose, severely bradycardic, without palpable pulses, and with a core body temperature of 23.0 degrees C (72 degrees F). Attempts to warm her with traditional methods during 2 hours were ineffective. An endovascular temperature control system was placed and effectively warmed the patient at about 3 degrees C (4.5 degrees F) per hour, with return of hemodynamic stability. When hypothermia is profound, surface warming works poorly and invasive strategies, including cardiopulmonary bypass, are recommended. Rapid warming from profound hypothermia can be accomplished with endovascular systems, and these may be an effective alternative to more invasive extracorporeal methods.

Targeted brain hypothermia induced by an interstitial cooling device in human neck: theoretical analyses

In this study, the feasibility of a newly developed interstitial cooling device inserted into the neck muscle and placed on the surface of the common carotid artery is evaluated. A combination of vascular model and continuum model is developed to simulate the temperature fields in both the neck and brain regions. Parametric studies are conducted to test the sensitivity of various factors on the temperature distribution. It has been shown that the length of the device, temperature of the device, and the tissue gap between the device and the blood vessel are the dominant factors that determine the effectiveness of this cooling approach. Under the current design parameters, the device is capable of inducing a temperature drop of 2.8 degrees C along the common carotid artery and it results in a total of 90 W of heat carried away from the arterial blood. Although the degree of the cooling in the arterial blood is inversely proportional to the blood flow rate of the arteries, the total heat loss from the arterial blood does not vary significantly if the blood flow rate changes during the cooling. After the cold arterial blood is supplied to the brain hemisphere, temperature reduction in the brain tissue is almost uniform and up to 3.1 degrees C temperature drop is achieved within 1 hour. In addition to the possible benefits of brain hypothermia for stroke or head injury patients, the device has the potential to control fever as well as to improve patients' outcome during open neck and head surgery.

Readdressing the issue of thermally significant blood vessels using a countercurrent vessel network

A physiologically realistic arterio-venous countercurrent vessel network model consisting of ten branching vessel generations, where the diameter of each generation of vessels is smaller than the previous ones, has been created and used to determine the thermal significance of different vessel generations by investigating their ability to exchange thermal energy with the tissue. The temperature distribution in the 3D network (8178 vessels; diameters from 10 to 1000 microm) is obtained by solving the conduction equation in the tissue and the convective energy equation with a specified Nusselt number in the vessels. The sensitivity of the exchange of energy between the vessels and the tissue to changes in the network parameters is studied for two cases; a high temperature thermal therapy case when tissue is heated by a uniformly distributed source term and the network cools the tissue, and a hypothermia related case, when tissue is cooled from the surface and the blood heats the tissue. Results show that first, the relative roles of vessels of different diameters are strongly determined by the inlet temperatures to those vessels (e.g., as affected by changing mass flow rates), and the surrounding tissue temperature, but not by their diameter. Second, changes in the following do not significantly affect the heat transfer rates between tissue and vessels; (a) the ratio of arterial to venous vessel diameter, (b) the diameter reduction coefficient (the ratio of diameters of successive vessel generations), and (c) the Nusselt number. Third, both arteries and veins play significant roles in the exchange of energy between tissue and vessels, with arteries playing a more significant role. These results suggest that the determination of which diameter vessels are thermally important should be performed on a case-by-case, problem dependent basis. And, that in the development of site-specific vessel network models, reasonable predictions of the relative roles of different vessel diameters can be obtained by using any physiologically realistic values of Nusselt number and the diameter reduction coefficient.

Extracorporeal venovenous cooling for induction of mild hypothermia in human-sized swine*

OBJECTIVE

Several cooling methods have been investigated for inducing mild hypothermia (33-36 degrees C) after cardiac arrest, brain trauma, or stroke. To achieve its best effect, therapeutic hypothermia has to be applied very early after the ischemic insult; otherwise, the beneficial effect would be diminished or even abrogated. The aim of this study was to investigate the effectiveness and safety of extracorporeal venovenous cooling as compared with endovascular cooling.

DESIGN

Swine were cooled in a randomized crossover design from 38 degrees C to 33 degrees C brain temperature, either with extracorporeal venovenous cooling or with endovascular cooling.

SETTING

Laboratory investigation.

SUBJECTS

Six swine of human size (85 to 101 kg).

INTERVENTIONS

Swine were randomly cooled with the first device, and after achieving the target brain temperature, re-warmed via the same technique and with heating lamps to baseline temperature. Then the other catheter was inserted and cooling was performed with the second device.

MEASUREMENTS

Brain, pulmonary artery and tympanic temperature, blood pressure, and heart rate were recorded continuously. Laboratory samples, including free hemoglobin, were taken at predefined temperature points during cooling. Comparisons between and within (baseline vs. 33 degrees C) the treatment groups were performed with the paired Student's t-test.

MAIN RESULTS

The time needed to reduce brain temperature from 38.0 degrees C to 33.0 degrees C was 41 +/- 17 mins with venovenous cooling and 126 +/- 37 mins with endovascular cooling (p = .001). Heart rate and mean arterial pressure decreased moderately during cooling and were significantly lower at 33 degrees C than at baseline in both groups, without differences between groups. None of the swine developed significant hemolysis, arrhythmias, or bleeding.

CONCLUSIONS

Extracorporeal venovenous cooling was an effective and safe method to rapidly induce therapeutic mild hypothermia in human-sized swine. It seems to be promising for further application and investigation in patients.

[Controlled mild-to-moderate hypothermia in the intensive care unit]

Controlled hypothermia is used as a therapeutic intervention to provide neuroprotection and (more recently) cardioprotection. The growing insight into the underlying pathophysiology of apoptosis and destructive processes at the cellular level, and the mechanisms underlying the protective effects of hypothermia, have led to improved application and to a widening of the range of potential indications. In many centres hypothermia has now become part of the standard therapy for post-anoxic coma in certain patients, but for other indications its use still remains controversial. The negative findings of some studies may be partly explained by inadequate protocols for the application of hypothermia and insufficient attention to the prevention of potential side effects. This review deals with some of the concepts underlying hypothermia-associated neuroprotection and cardioprotection, and discusses some potential clinical indications as well as reasons why some clinical trials may have produced conflicting results. Practical aspects such as methods to induce hypothermia, as well as the side effects of cooling are also discussed.

[Therapeutic hypothermia after cardiac arrest]

The use of therapeutic hypothermia following different hypoxic-ischaemic insults has played an important role in various concepts of non-specific protection of cells for a long time. Although the use of deep therapeutic hypothermia after cardiac arrest in the last century did not lead to an improved outcome, recent data have demonstrated very positive effects of mild therapeutic hypothermia. The data from the European multicenter trial as well as those from Australia have clearly demonstrated a decrease in mortality and a better neurological outcome for patients being cooled to 32-34 degrees C for 12 or 24 h. In 2003, this led to the implementation of mild therapeutic hypothermia (32-34 degrees C) into the International Liaison Committee on Resuscitation (ILCOR) recommendations and guidelines for the treatment of unconscious patients after prehospital cardiac arrest. This article gives an overview on existing concepts and future perspectives of therapeutic mild hypothermia.

Hypothermia – it's more than a toy

PURPOSE OF REVIEW

Perioperative hypothermia triples the incidence of adverse myocardial outcomes in high-risk patients; it significantly increases blood loss and augments allogeneic transfusion requirements. Even mild hypothermia increases the incidence of surgical wound infection following colon resection and therefore the duration of hospitalization. Hypothermia adversely affects antibody- and cell-mediated immune defenses, as well as the oxygen availability in the peripheral wound tissues. Mild perioperative hypothermia changes the kinetics and action of various anesthetic and paralyzing agents, increases thermal discomfort, and is associated with delayed postanesthetic recovery.

RECENT FINDINGS

On the other hand however, therapeutic hypothermia may be an interesting approach in various settings. Lowering core temperature to 32-34 degrees C may reduce cell injury by suppressing excitotoxins and oxygen radicals, stabilizing cell membranes, and reducing the number of abnormal electrical depolarizations. Evidence in animals indicates that even mild hypothermia provides substantial protection against cerebral ischemia and myocardial infarction. Mild hypothermia has been shown to improve outcome after cardiac arrest in humans. Randomized trials are in progress to evaluate the potential benefits of mild hypothermia during aneurysm clipping and after stroke or acute myocardial infarction.

SUMMARY

This article reviews recent publications in the field of accidental as well as therapeutic hypothermia, and tries to assess what evidence is available at the present time.

Induced hypothermia by central venous infusion: Saline ice slurry versus chilled saline

OBJECTIVE

Surface cooling improves outcome in selected comatose survivors of cardiac arrest. Internal cooling with considerable volumes of intravenous cold saline may accelerate hypothermia induction. This study compares core temperatures in swine after central catheter infusions of saline ice slurry (saline with smoothed 100-microm-size ice particles) vs. an equal volume of chilled saline. We hypothesized that slurry would achieve core hypothermia (32-34 degrees C) more consistently and at a faster rate.

DESIGN

A total of 11 swine were randomized to receive microparticulate ice slurry, chilled saline infusion, or anesthesia alone in a monitored laboratory setting.

INTERVENTIONS

Intravenous bolus (50 mL/kg) of slurry or chilled 1.5% NaCl saline. Slurry was composed of a 1:1 mixture of ice and distilled H2O plus NaCl.

MEASUREMENTS

Cerebral cortex, tympanic membrane, inferior vena cava, rectal temperatures, electrocardiogram, arterial blood pressure, and arterial oxygen saturation were recorded for 1 hr after bolus.

MAIN RESULTS

Compared with anesthetized controls, core brain temperatures of the saline and slurry groups dropped by 3.4 +/- 0.4 degrees C and 5.3 +/- 0.7 degrees C (p = .009), respectively. With an infusion rate of 120 mL/min, cooling rates for the saline and slurry groups were -11.6 +/- 1.8 degrees C/hr and -18.2 +/- 2.9 degrees C/hr, respectively, during the first 20 mins. Four of four animals in the slurry group vs. zero of four animals in the saline group achieved target cortical temperatures of <34 degrees C.

CONCLUSIONS

Cold intravenous fluids rapidly induce hypothermia in swine with intact circulation. A two-phase (liquid plus ice) saline slurry cools more rapidly than an equal volume of cold saline at 0 degrees C. Ice-slurry could be a significant improvement over other cooling methods when rate of cooling and limited infusion volumes are important to the clinician.

Therapeutic hypothermia for acute ischemic stroke: what do laboratory studies teach us?

BACKGROUND AND PURPOSE

The significance of brain temperature to outcome in cerebral ischemia is recognized. Numerous variations of depth, duration, and delay of cooling have been studied in animal models. It is important to become familiar with these studies to design appropriate clinical trials. With that in mind, a critical review of the pertinent literature is presented, taking into consideration potential limitations in translating such laboratory work to the clinical level.

METHODS

Hypothermia is an especially robust neuroprotectant in the laboratory and has been shown to alter many of the damaging effects of cerebral ischemia. Most laboratory research on therapeutic cooling in cerebral ischemia has been conducted in rodent models of temporary and permanent middle cerebral artery occlusion and report the effects of mild or moderate hypothermia arranged during or after ischemia.

RESULTS

Intraischemic cooling vastly reduces infarct size in most occlusion models. Tissue salvage with delayed onset of cooling is less dramatic but is commonly observed when cooling is begun within 60 minutes of stroke onset in permanent and 180 minutes of stroke onset in temporary occlusion models. Prolonged postischemic cooling further enhances efficacy.

CONCLUSIONS

Laboratory studies have shown that intraischemic hypothermia is more protective than postischemic hypothermia and more benefit is conferred with temporary occlusion than permanent occlusion models. The efficacy of postischemic hypothermia is critically dependent on the duration and depth of hypothermia and its timing relative to ischemia.

Therapeutic hypothermia

Hypothermia is common during anaesthesia and surgery owing to anaesthetic-induced inhibition of thermoregulatory control. Perioperative hypothermia is associated with numerous complications. However, for certain patient populations, and under specific clinical conditions, hypothermia can provide substantial benefits. Lowering core temperature to 32-34 degrees C may reduce cell injury by suppressing excitotoxins and oxygen radicals, stabilizing cell membranes, and reducing the number of abnormal electrical depolarizations. Evidence from animal studies indicates that even mild hypothermia provides substantial protection against cerebral ischaemia and myocardial infarction. Mild hypothermia has been shown to improve outcome after cardiac arrest in humans. Randomized trials are in progress to evaluate the potential benefits of mild hypothermia during aneurysm clipping and after stroke or acute myocardial infraction. However, as hypothermia can cause unwanted side-effects, further research is needed to better quantify the risks and benefits of therapeutic hypothermia.