Lower Is Better for Cooling After Cardiac Arrest

November 07, 2012

LOS ANGELES — A pilot trial examining which of two temperatures to use when cooling patients following a sudden cardiac arrest suggests that the lower one, 32°C (89.6°F), may improve outcomes compared with the higher, 34°C [1]. Dr Esteban López-de-Sá (La Paz University Hospital, Madrid, Spain) reported the findings at a late-breaking trials-session here today at the American Heart Association 2012 Scientific Sessions. The results are also published online today in Circulation.

Dr Esteban López-de-Sá

Instituting hypothermia following resuscitation after a cardiac arrest is known to improve neurological outcomes in patients who survive. AHA and International Liaison Committee on Resuscitation (ILCOR) guidelines recommend therapeutic cooling to 32°C to 34°C, but the optimal temperature remains unclear.

Discussant of the study, Dr Graham Nichol (University of Washington, Seattle), told heartwire : "This small trial from Spain shows that patients with a shockable rhythm who are cooled more do better than those who are cooled less, and that's very important." However, "we still do not know how long we should cool, we still do not know what medications we should use to help cool, and we still don't know whether a surface method of cooling, such as ice packs--which are easier to apply--is as good, or better, or not, as a catheter, which requires a procedure," he added.

Patients with a shockable rhythm who are cooled more do better than those who are cooled less, and that's very important.

During a media briefing to discuss the results, Dr Elliott Antman (Brigham and Women's Hospital, Boston, MA) said: "This is a very challenging set of patients to have to deal with. Those who have had an out-of-hospital cardiac arrest [OHCA] often have these horrible neurological outcomes despite a successful resuscitation. When the [AHA] Committee on Scientific Sessions Programs saw this paper, our faces lit up, because we knew that there is very little in the way of randomized trials in the field of resuscitation science. We were very interested in a piece of evidence that would tell us whether 32°C or 34°C was better. This is a very important study that sheds some light on that."

Dr Graham Nichol

Antman applauded López-de-Sá and his team, noting how difficult it is to do such a study "in the hectic circumstances" of a cardiac arrest. "I look forward to having more trials in this area, because I think it's going to help refine this important therapy for our patients who suffer these very serious events."

Maintenance of Stable Temperature Now Possible With Devices

López-de-Sá and colleagues explain in their paper that, with classic cooling methods, it was very difficult to maintain a stable temperature at a particular level, but this is now possible with the use of devices that have automatic temperature-feedback control.

In their study, 36 adult patients with a witnessed out-of-hospital cardiac arrest, an interval of <60 minutes from collapse to return of spontaneous circulation, and an initial registered shockable rhythm (VF or pulseless VT) or asystole were randomized to be cooled to 32°C or 34°C.

All patients were cooled by intravenous infusion of <8°C cold saline followed by the implantation of the Icy 9.3F 38-cm catheter (Zoll Medical, Chelmsford, MA) placed in the inferior vena cava through a femoral vein connected to the Thermogard XP Temperature Management System (Zoll Medical). The system consists of a pump that circulates refrigerated sterile saline from the external device through balloons coaxially mounted on the catheter, enabling direct cooling of the blood.

Cooling was set at a maximum rate with a target temperature of 32°C or 34°C, according to randomization. The target temperature was maintained during 24 hours followed by 12 to 24 hours of controlled rewarming.

Significant Difference in Those With a Shockable Rhythm

The primary outcome was survival free from severe dependence (Barthel Index score >60) at six months. Eight of 18 patients in the 32°C group (44.4%) met the primary end point compared with two of 18 in the 34°C group (11.1%), which was not a significant difference (p=0.12).

However, when patients were analyzed by subgroup into those with an initial rhythm of asystole and those with a shockable rhythm, "we can see that patients with asystole all die within 30 days," said López-de-Sá. But eight of 13 patients with initial shockable rhythm assigned to 32°C (61.5%) were alive free from severe dependence at six months compared with two of 13 (15.4%) assigned to 34°C (p=0.029).

The incidence of complications was similar in both groups except for clinical seizures, which were lower in patients assigned to 32°C. Conversely, there was a trend toward a higher incidence of bradycardia in patients assigned to 32°C.

López-de-Sá noted some limitations of the study, including small sample size, absence of blinding of the target temperature, and the fact that there was a longer duration of cooling in patients assigned to 32°C due to longer induction and longer rewarming.

Nevertheless, the findings suggest that a cooling target of 32°C may improve outcomes of OHCA secondary to ventricular fibrillation or pulseless ventricular tachycardia, he said, but added, "Further investigation is needed to address the optimal target level of hypothermia in this challenging clinical setting."

López-de-Sáandcolleagues have noconflicts of interest. Nichol reports being the principal investigator for Medtronic's CASCADE HeartRescue Program and receiving funding from Zoll, Physio-Control Inc, Cardiac Science Corp, and the Dynamic AED registry.