Rapid cooling for saving lives: a bioengineering opportunity

Intravascular Cooling Device Versus Esophageal Heat Exchanger for Mild Therapeutic Hypothermia in an Experimental Setting

BACKGROUND

Targeted temperature management is a standard therapy for unconscious survivors of cardiac arrest. To date, multiple cooling methods are available including invasive intravascular cooling devices (IVDs), which are widely used in the clinical setting. Recently, esophageal heat exchangers (EHEs) have been developed providing cooling via the esophagus that is located close to the aorta and inferior vena cava. The objective was to compare mean cooling rates, as well as differences, to target temperature during maintenance and the rewarming period of IVD and EHE.

METHODS

The study was conducted in 16 female domestic pigs. After randomization to either IVD or EHE (n = 8/group), core body temperature was reduced to 33°C. After 24 hours of maintenance (33°C), animals were rewarmed using a target rate of 0.25°C/h for 10 hours. All cooling phases were steered by a closed-loop feedback system between the internal jugular vein and the chiller. After euthanasia, laryngeal and esophageal tissue was harvested for histopathological examination.

RESULTS

Mean cooling rates (4.0°C/h ± 0.4°C/h for IVD and 2.4°C/h ± 0.3°C/h for EHE; P < .0008) and time to target temperature (85.1 ± 9.2 minutes for IVD and 142.0 ± 21.2 minutes for EHE; P = .0008) were different. Mean difference to target temperature during maintenance (0.07°C ± 0.05°C for IVD and 0.08°C ± 0.10°C for EHE; P = .496) and mean rewarming rates (0.2°C/h ± 0.1°C/h for IVD and 0.3°C/h ± 0.2°C/h for EHE; P = .226) were similar. Relevant laryngeal or esophageal tissue damage could not be detected. There were no significant differences in undesired side effects (eg, bradycardia or tachycardia, hypokalemia or hyperkalemia, hypoglycemia or hyperglycemia, hypotension, overcooling, or shivering).

CONCLUSIONS

After insertion, target temperatures could be reached faster by IVD compared to EHE. Cooling performance of IVD and EHE did not significantly differ in maintaining target temperature during a targeted temperature management process and in active rewarming protocols according to intensive care unit guidelines in this experimental setting.

Cannabinoid Receptor Agonist WIN55, 212-2 Adjusts Lipid Metabolism in a Rat Model of Cardiac Arrest

The objective of this study was to investigate the effects of pharmacologically induced hypothermia with WIN55, 212-2 (WIN)on postresuscitation myocardial function, microcirculation, and metabolism-specific lipids in a rat cardiac arrest (CA) model. Ventricular fibrillation was electrically induced and untreated for 6 minutes in 24 Sprague-Dawley rats weighing 450-550 g. Cardiopulmonary resuscitation including chest compression and mechanical ventilation was then initiated and continued for 8 minutes, followed by defibrillation. At 5 minutes after restoration of spontaneous circulation (ROSC), animals were randomized into four groups: (1) normothermia with vehicle (NT); (2) physical hypothermia with vehicle (PH); (3) WIN55, 212-2 with normothermia (WN); and (4) WIN55, 212-2 with hypothermia (WH). For groups of WN and WH, WIN was administered by continuous intravenous infusion with a syringe pump for 4 hours. PH started at 5 minutes after resuscitation. NT maintained core temperature at 37°C ± 0.2°C with the aid of a heating blanket. Hypothermia groups maintained temperature at 33°C ± 0.5°C for 4 hours after ROSC. There was a significant improvement in myocardial function as measured by ejection fraction, cardiac output, and myocardial performance index in animals treated with WH and PH beginning at 1 hour after start of infusion. In the WH and PH groups, buccal microcirculation was significantly improved compared with NT and WN. Plasma at pre-CA and ROSC 4 hours was harvested for lipid metabolism. The WH group appeared to be closer to baseline than the other groups in lipid metabolism. lysophosphatidylcholine (LPC) 18:2, free fatty acid (FFA) 22:6, and ceramide (CER) (24:0) changed significantly among the lipidomic data compared with NT (p < 0.05). Postresuscitation hypothermia improved myocardial function and microcirculation. WH-mediated lipid metabolism had the best metabolic outcome to bring back the animals to normal metabolism, which may be protective to improve outcomes of CA. LPC 18:2, FFA 22:6, and CER (24:0) may be important predictors of outcomes of CA.

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.

Improved Ease of Use Designs for Rapid Heart Cooling

Mild hypothermia has been shown to reduce heart tissue damage resulting from acute myocardial infarction (AMI). In previous work we developed a trilumen cooling catheter to deliver cooled blood rapidly to the heart during emergency angioplasty. This paper describes two alternative designs that seek to maintain tissue cooling capability and improve “ease of use.” The first design was an autoperfusion design that uses the natural pressure difference between the aorta and the coronary arteries to move blood through the trilumen catheter. The second design used an external cooling system, where blood was cooled externally before being pumped to the heart through a commercially available guide catheter. Heat transfer and pressure drop analyses were performed on each design. Both designs were fabricated and tested in both in vitro and in vivo settings. The autoperfusion design did not meet a cooling capacity target of 20 W. Animal tests, using swine with healthy hearts, showed that the available pressure difference to move blood through the trilumen catheter was approximately 5–10 mmHg. This differential pressure was too low to motivate sufficient blood flow rates and achieve the required cooling capacity. The external cooling system, however, had sufficient cooling capacity and reasonable scalability. Cooling capacity values varied from 14 to 56 W over a flow range of 30–90 ml/min. 20 W and 30 W were achieved at 38 ml/min and 50 ml/min, respectively. Animal testing showed that a cooling capacity of 30 W delivered to the left anterior descending (LAD) and left circumflex arteries (LCX) of a healthy 70 kg swine can reduce heart tissue temperatures rapidly, approximately 3 °C in 5 min in some locations. Core temperatures dropped by less than 0.5 °C during this cooling period. An autoperfusion design was unable to meet the target cooling capacity of 20 W. An external cooling design met the target cooling capacity, providing rapid (1 °C/min) localized heart tissue cooling in a large swine model. Future animal testing work, involving a heart attack model, will investigate if this external cooling design can save heart tissue.

Cannabinoid 1 (CB1) receptor mediates WIN55, 212-2 induced hypothermia and improved survival in a rat post-cardiac arrest model

AIM

The nonselective Cannabinoid (CB) receptor agonist, WIN55, 212-2, was demonstrated to induce hypothermia and improve post-resuscitation outcomes in a rat post-cardiac arrest model. The present study was to explore the potential mechanisms of WIN55, 212-2 on thermoregulation following resuscitation and to investigate which class of CB receptors was involved in WIN55, 212-2-induced hypothermia.

METHODS

Ventricular fibrillation (VF) was induced and untreated for 6 min in 20 male Sprague-Dawley rats. Defibrillation was attempted after 8min of Cardiopulmonary resuscitation (CPR). Five min post-resuscitation, resuscitated animals were randomized to receive an intramuscular injection of selective CB1 receptors antagonist, SR141716A (5 mg kg(-1)); selective CB2 receptors antagonist SR144528 (5 mg kg(-1)); or placebo. Thirty min after injection, animals received continuous intravenous infusion of WIN55, 212-2 (1.0 mgkg(-1) h(-1)) for 4h while control animals received placebo. The identical temperature environment was maintained in all animals.

RESULTS

In animals treated with WIN55, 212-2, blood temperatures decreased progressively from 37 °C to 34 °C within 4h. This hypothermic effect was completely blocked by CB1 but not CB2 antagonist. Accordingly, significantly better cardiac output, ejection fraction and myocardial performance index, reduced neurological deficit scores, improved microcirculation and longer duration of survival were observed in WIN55, 212-2-treated animals, which were also completely abolished by pretreatment with CB1 antagonist.

CONCLUSIONS

Pharmacologically induced hypothermia with WIN55, 212-2 improved post-resuscitation myocardial and cerebral function, associated with a significantly increased duration of survival in a rat post-cardiac arrest model. The hypothermic and resulted beneficial effects of WIN55, 212-2 were mediated through CB1 receptors.

State of the art in therapeutic hypothermia

Historically, hypothermia was induced prior to surgery to enable procedures with prolonged ischemia, such as open heart surgery and organ transplant. Within the past decade, the efficacy of hypothermia to treat emergency cases of ongoing ischemia such as stroke, myocardial infarction, and cardiac arrest has been studied. Although the exact role of ischemia/reperfusion is unclear clinically, hypothermia holds significant promise for improving outcomes for patients suffering from reperfusion after ischemia. Research has elucidated two distinct windows of opportunity for clinical use of hypothermia. In the early intra-ischemia window, hypothermia modulates abnormal cellular free radical production, poor calcium management, and poor pH management. In the more delayed post-reperfusion window, hypothermia modulates the downstream necrotic, apoptotic, and inflammatory pathways that cause delayed cell death. Improved cooling and monitoring technologies are required to realize the full potential of this therapy. Herein we discuss the current state of clinical practice, clinical trials, recommendations for cooling, and ongoing research on therapeutic hypothermia.

Pharmacologically induced hypothermia with cannabinoid receptor agonist WIN55, 212-2 after cardiopulmonary resuscitation

OBJECTIVE

To investigate whether hypothermia could be induced pharmacologically after resuscitation with the cannabinoid CB1/CB2 receptor agonist in a rat model and its effects on outcomes of cardiopulmonary resuscitation.

DESIGN

Prospective, randomized, placebo-controlled experimental study.

SETTING

University-affiliated animal research laboratory.

SUBJECTS

Ten healthy male Sprague-Dawley rats.

INTERVENTIONS

Ventricular fibrillation was induced and untreated for 6 mins. Defibrillation was attempted after 8 mins of cardiopulmonary resuscitation. Thirty minutes after resuscitation, animals were randomized to receive either WIN55, 212-2 (1.0 mg/kg/hr) or vehicle placebo (1.4 mL/kg/hr) for 6 hrs. Before infusion, the temperature was maintained at 37°C in all the animals with the help of a heating lamp. The same temperature environment was maintained for both groups after infusion.

MEASUREMENTS AND MAIN RESULTS

Hemodynamic measurements and cardiac output, ejection fraction, and myocardial performance index were measured at baseline and hourly for 6 hrs after resuscitation. Survival time up to 72 hrs was observed.

RESULTS

Blood temperature decreased progressively after infusion of WIN55, 212-2 from 37°C to 34°C 4 hrs after resuscitation. There was no significant change in blood temperature after 6 hrs of placebo infusion of the same volume and same infusate temperature. Significantly better postresuscitation myocardial function and longer durations of survival were observed in WIN55, 212-2-treated animals.

CONCLUSIONS

The selective cannabinoid agonist, WIN55, 212-2, produced a significant reduction in blood temperature and improved postresuscitation myocardial functions and survival after cardiopulmonary resuscitation. The study results may provide a further option for early and effective induction of therapeutic hypothermia in settings of cardiopulmonary resuscitation.