Published Online
on June 13, 2005

A more recent version of this article appeared on June 21, 2005

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Submitted on July 14, 2004
Revised on March 4, 2005
Accepted on March 21, 2005

Hypothermia Improves Defibrillation Success and Resuscitation Outcomes From Ventricular Fibrillation

Kimberly A. Boddicker MD, Yi Zhang MD, PhD, M. Bridget Zimmerman PhD, Loyd R. Davies BS, and Richard E. Kerber MD^*

From the Cardiovascular Center, College of Medicine, University of Iowa, Iowa City.

^* To whom correspondence should be addressed. E-mail: richard-kerber{at}uiowa.edu.

Background--Induced hypothermia is recommended to improve neurological outcomes in unconscious survivors of out-of-hospital ventricular fibrillation (VF) cardiac arrest. Patients resuscitated from a VF arrest are at risk of refibrillation, but there are few data on the effects of already existing hypothermia on defibrillation and resuscitation.

Methods and Results--Thirty-two swine (mean±SE weight, 23.0±0.6 kg) were divided into 4 groups: normothermia (n=8), mild hypothermia (35°C) (n=8), moderate hypothermia (33°C) (n=8), and severe hypothermia (30°C) (n=8). Hypothermia was induced by surrounding the animal with ice, and VF was electrically induced. After 8 minutes of unsupported VF (no CPR), the swine were defibrillated (biphasic waveform) with successive shocks as needed and underwent CPR until resumption of spontaneous circulation or no response (10 minutes). First-shock defibrillation success was higher in the moderate hypothermia group (6 of 8 hypothermia versus 1 of 8 normothermia; P=0.04). The number of shocks needed for late defibrillation (1 minute after initial shock) was less in all 3 hypothermia groups compared with normothermia (all P<0.05). None of the 8 animals in the normothermia group achieved resumption of spontaneous circulation compared with 3 of 8 mild hypothermia (P=NS), 7 of 8 moderate hypothermia (P=0.001), and 5 of 8 severe hypothermia (P=0.03) animals. Coronary perfusion pressure during CPR was not different between the groups.

Conclusions--When VF was induced in the setting of moderate or severe hypothermia, resuscitative measures were facilitated with significantly improved defibrillation success and resuscitation outcome. The beneficial effect of hypothermia was not due to alteration of coronary perfusion pressure, which suggests that changes in the mechanical, metabolic, or electrophysiological properties of the myocardium may be responsible.

Key words: cardiopulmonary resuscitation • defibrillation • hypothermia • ventricular fibrillation

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