Effect of hypothermia on transthoracic defibrillation in a swine model.
BACKGROUND
Induced hypothermia (H) appears a promising intervention to protect the heart and brain after resuscitation from cardiac arrest. However, the influence of H on transthoracic defibrillation energy requirements is not well documented.


METHODS
In 39 swine (21.4+/-1.3(S.E.) kg) hypothermia was induced by surrounding the head, thorax and abdomen with ice. The swine were divided into four groups: (1) normothermia (N) followed by severe H (30 degrees C) (n=10), (2) severe H followed by N (n=10), (3) N followed by moderate H (33 degrees C) (n=10) and (4) moderate H followed by N (n=9). After 30s of electrically induced ventricular fibrillation (VF), the swine were defibrillated (biphasic waveform) at energies of 20J, 30J, 50J and 100J in random order in both N and H conditions.


RESULTS
For pigs in Group 1 (N followed by severe H), shock success in terminating VF was higher during hypothermia (odds ratio 4.09 (95% CI: 2.21, 5.58; p<0.0001), despite the fact that impedance rose from 39+/-3Omega (N) to 42+/-3Omega (H) (p<0.001) and current fell from 22+/-8 (N) to 21+/-7A (H) (p<0.001). There were no significant differences in the shock success between N and H for the other groups. Post-defibrillation ventricular asystole occurred less often during hypothermia compared to normothermia (p=0.0002).


CONCLUSIONS
Severe H facilitated transthoracic defibrillation in this swine model. Since impedance rose and current fell during H, the improved shock success must be due to a hypothermia-induced change in the mechanical or electrophysiologic properties of the myocardium. Moderate hypothermia did not alter the energy requirement for defibrillation.
Rhee
Benjamin J
BJ
The Cardiovascular Center, College of Medicine, University of Iowa, Iowa City, IA 52242, USA.
Zhang
Yi
Y
Boddicker
Kimberly A
KA
Davies
Loyd R
LR
Kerber
Richard E
RE
D000818:Animals; D004195:Disease Models, Animal; D004554:Electric Countershock; D006323:Heart Arrest / Q000150:complications / Q000503:physiopathology / Q000628:therapy*; D006439:Hemodynamics; D007036:Hypothermia, Induced; D012016:Reference Values; D012769:Shock / Q000209:etiology; D013552:Swine