Keep Your Cool

     There's little dispute anymore that controlled hypothermia can benefit victims of cardiac arrest. But how is it done most effectively? And, asks the prehospital world, when? There's still a lot to learn, but new developments are making the former easier and hinting at answers for the latter.

     External cooling has traditionally relied on methods like submersion or relatively straightforward devices like ice packs and blankets/pads. A pair of new products for hospital use give those ideas a technological twist.

     As reported in the August 2007 issue of EMS Magazine, the Arctic Sun system from Medivance (www.medivance.com) utilizes three-layer pads that circulate water atop a layer of gel. The pads provide direct thermal conduction to the skin, which is more effective than cooling by air. Life Recovery Systems' ThermoSuit (www.life-recovery.com) goes even farther, surrounding the patient with a disposable bodysuit through which ice water is pumped across the skin.

     "Air, in contact with skin, is much less efficient at putting heat into or taking it out of the body as is liquid, particularly water," explains Joe Ornato, MD, professor and chair of the Department of Emergency Medicine at Virginia Commonwealth University's Medical College of Virginia. "So in order to rapidly cool the body externally, you have to make good contact with the skin, and optimally have some kind of liquid interface."

     Applying the same idea internally, the Alsius IV system (www.alsius.com) circulates cooled saline through balloons in a percutaneous catheter into the central venous system. This quickly cools the patient as venous blood passes over each balloon, exchanging heat without infusing the saline.

     San Diego-based BeneChill (www.benechill.com) has developed a system called RhinoChill, currently for investigational use only, that delivers a perfluorocarbon cooling spray intranasally. With the upper airways designed for heat exchange, this works very rapidly. This may eventually be available for field use.

     Interesting though these methods are, —they're relatively expensive, and the Alsius product requires sterile conditions. But increasingly, the field is where experts are looking to begin cooling. A number of EMS systems have begun cooling cardiac arrest patients who achieve return of spontaneous circulation. And providers in Richmond, where Ornato is medical director, have gone even farther, and recently begun cooling some patients before ROSC.

     "We're actually not waiting for ROSC with patients who are initially in VF, had witnessed arrests, or for whom medics have any reason to believe it might have been a short downtime," says Ornato. "We have our supervisor unit respond with two liters of normal saline refrigerated on their vehicle, and get it started rapidly upon their arrival."

     Animal studies seem to support this, and no human data demonstrates harm. Of the first 12 patients treated under this new protocol in Richmond, eight left the hospital neurologically intact, which is certainly fuel for further study.

     "The bottom line is, we really don't know how early is going to give you better therapy," Ornato says. "We don't know how long a duration of cooling is optimal; most programs cool for 24–36 hours after hospital arrival, but it's not been really rigorously studied. And it's not clear whether if you cool very early, you might need a shorter duration of cooling than if you can't start for several hours. All of that remains to be sorted out."

     As with things like CPR and AEDs, the thing to remember about therapeutic hypothermia and cooling of cardiac arrest victims is that it's not a cure-all. These are very sick people, and they benefit most from concerted, experienced care that quickly and effectively delivers a range of needed resources.

     "It isn't just cooling that makes a difference," notes Ornato. "We're coming to realize that things like experience, technology and rate all matter. It's critical that once a patient's delivered to a hospital, staff there not only know what they're doing with respect to cooling, but really have a functional, well-thought-out, 24/7 program. There's a lot that goes into the care of these patients, and what this last 3–4 years has taught us is that to really see better results, we have to consolidate the care."

Paralytics and Prehospital Cooling
     Do medics need to use paralytics on patients they're cooling in the field? Likely not, Ornato says.

     Paralytics are typically used as cooling continues at the hospital because a patient who begins shivering will generate enough heat to oppose the cooling effect. That can delay getting them to the target range of 32°C–34°C. But because most patients don't start shivering until their temperature drops below 35°C, that's not a problem in the field.

     "You may get some difference of opinion on that, but you really don't have to complicate things in the field by adding paralytics," Ornato says. "To me, it makes much more sense to get the patient to the hospital, where a more definitive cooling method can be continued, rather than spend time adding extra drugs in the field."