Techniques to induce and maintain hypothermia can be either non-invasive or invasive. Methods are continually evolving, and new techniques are presented each year at the American Heart Association Scientific Sessions. Let’s start with a description of current non-invasive methods. 

Non-Invasive Methods for Therapeutic Hypothermia

Surface cooling can be accomplished with circulating cold water blankets and cold air-forced blankets. These devices are usually already available in hospitals, and care providers know how to operate them, but it takes two to eight hours to reduce the core temperature to 32°-34°C and titration of temperature can be difficult. 

MTRE®'s Criticool System achieves precise management of patient body temperature in a non-invasive, effective manner. The CritiCool is a microprocessor-controlled temperature management unit with a proprietary control algorithm for patient temperature. The user sets the desired patient temperature and the system modifies water temperature flowing to a Patient Wrap using feedback from the patient’s core and skin temperature sensors. The system automatically cools the patient to the required temperature and re-warms gradually at the end of the procedure. The water-filled body shaped wrap covers over 85% of skin surface, without adhering to the patient's skin. The wrap can easily be fastened and refastened to the patient for effortless and secure patient care and handling.


CritiCool Therapy by MTRE 

Another new cooling blanket is the EMCOOLS® pad, which consists of multiple cooling elements that are filled with a combination of graphite/water and placed directly on the skin surface. In a study with eight cardiac arrest patients, the cooling-blanket decreased temperatures from 35.8°C at baseline to 34.0°C within an average of 37 (30-45) minutes, and to the target temperature of 33°C within an average of 56 (51-62) minutes after initiation of cooling, resulting in a cooling rate of 3.3 (2.6-3.5)°C/hour. Under investigation is use of the device in the pre-hospital setting since it is independent of an energy source.

(Uray, T. et al. Surface cooling with a new cooling blanket for rapid induction of mild hypothermia in humans after cardiac arrest: A feasibility trial. Circulation 2006;114:II-1190[Suppl])

EMCOOLS Cooling Blanket   


Ice packs can easily be applied to the armpits, head/neck and groin, but the rate of core temperature decrease is only 0.9°C/hour. Caps and helmets have been used to cool the surface of the head and neck to create selective cerebral hypothermia in infants. Immersion in cold water is not practical, but a new flexible surround suit system using this concept was recently described with domestic swine by Ohley. It provides a thin, 0.5 cm layer of circulating ice water in direct skin contact held between 0.5°C and 1.5°C. A pumping system was used to circulate the water volume of 20 liters at a rate of 15 liters/minute. The average time to decrease the temperature by 3°C was 13 minutes.

(Ohley, W. et al. Mild hypothermia induced rapidly in human sized animals. Circulation 2006;114:II-1204 [Suppl])

Medivance produces the non-invasive Arctic Sun® Temperature Management System, in which hydrogel-coated pads that circulate temperature-controlled water under negative pressure adhere to the patient’s abdomen, back and thighs. The Energy Transfer Pads™ provide direct thermal conduction through the skin. This is different than conventional water blankets or wraps, in which air is trapped between the cooling source and the skin, and the patient is cooled by cold air (convection). The cooling rate is reported to be 1.5°C/hour or better.  

Arctic Sun Temperature Management System by Medivance 



Invasive Methods for Therapeutic Hypothermia

Now to invasive means for inducing and maintaining hypothermia. Bernard first described inducing hypothermia in patients out-of-hospital resuscitation. In this method, 30 ml/kg lactated Ringers solution that had been chilled to 4°C was infused over 30 minutes using a pressure bag via either a peripheral cannula or femoral venous catheter. This dropped the median core temperature in his study patients from 35.5° to 33.8°C. There were significant improvements in mean arterial blood pressure, renal function and acid-base analysis and no adverse effects of the rapid infusion of this volume of IV crystalloid fluid. Thus, induction of hypothermia by means of cold IV infusion is fast, efficacious and safe. It should be followed by another method to maintain hypothermia, such as cooling blankets.

(Bernard, S. et al. Induced hypothermia using large volume, ice-cold intravenous fluid in comatose survivors of out-of-hospital cardiac arrest: A preliminary report. Resuscitation 2003;56:9-13)

External heat exchange-control devices are available that circulate chilled saline to an indwelling venous line placed percutaneously in the patient – a closed-loop design. Special femoral, subclavian and internal jugular catheters are used for this technique. The device remotely senses patient temperature through standard probes and compares it to a user-selected target temperature, adjusting the temperature of the circulating sterile saline appropriately. Two devices on the market are:

In a feasibility study with the CoolGard 3000® device and Icy catheter, cooling averaged 0.8°C/hour, and it took 3 hours and 39 minutes to reach 33°C. (Al-Senani, F.M. et al. A prospective, multicenter pilot study to evaluate the feasibility and safety of using the CoolGard™ System and Icy™ catheter following cardiac arrest. Resuscitation 2004;62:143-150). Temperature can be tightly maintained, and rewarming can be controlled actively. Patient access for care is facilitated with this catheter-based technology, but there is a risk of infection and of bleeding at the insertion site. It is claimed that there are no external skin injuries that appear after external cooling, and less or no shivering compared to external cooling. Patient and system data are continuously stored, allowing recall and graphical display. A major limitation is the high cost of the device.

CoolGard 3000 with Icy Femoral Catheter 



Another invasive type of cooling is the use of devices that require circulation of blood through an extracorporeal circuit, allowing for rapid infusion of cold fluids, oxygenation of blood during resuscitation and rapid delivery of intravenous drugs. Soga from Japan reported on the use of an extracorporeal cooling method (KTEK-3, Kawasumi Company) to cool post resuscitation patients to a target temperature of 34°C within two hours after ROSC. The target temperature was maintained precisely for 24 hours when collapse-to-ROSC interval was within 20 minutes, 48 hours when that interval was 20-30 minutes and 72 hours when that interval was more than 30 minutes. A total of 566 patients were enrolled; 30 were treated with the hypothermia and 539 were treated with normothermia.  Among patients with VF/VT as an initial cardiac rhythm, 17 of the 22 (77.3%) patients in the hypothermia group had a favorable neurological outcome, as compared with 80 of 207 (38.6%) patients in the normothermia group (p<0.001). In addition among patients with pulseless electrical activity or asystole, the hypothermia group was associated with an improved favorable neurological outcome compared with the normothermia group (25% vs 7%, p=0.05). This technology can only be employed in centers where the extracorporeal device and trained staff are available 24 hours a day.

(Soga, T. et al. Mild therapeutic hypothermia using extracorporeal cooling method in comatose survivors after out-of-hospital cardiac arrest. Circulation 2006;114:II-1190)


Retrograde jugular vein flush and intraventricular cerebral hypothermia have been used for selective brain cooling. Other methods for invasive cooling that are infrequently used include cold carotid infusions, single carotid artery perfusion with extracorporeal cooled blood, ice water nasal lavage, cold peritoneal and lung lavage and nasogastric and rectal lavage.