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 (meanϮSE weight, 23.0Ϯ0.6 kg) were divided into 4 groups: normothermia (nϭ8), mild hypothermia (35°C) (nϭ8), moderate hypothermia (33°C) (nϭ8), and severe hypothermia (30°C) (nϭ8). 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 (Ն10 minutes). First-shock defibrillation success was higher in the moderate hypothermia group (6 of 8 hypothermia versus 1 of 8 normothermia; Pϭ0.04). The number of shocks needed for late defibrillation (Ն1 minute after initial shock) was less in all 3 hypothermia groups compared with normothermia (all PϽ0.05). None of the 8 animals in the normothermia group achieved resumption of spontaneous circulation compared with 3 of 8 mild hypothermia (PϭNS), 7 of 8 moderate hypothermia (Pϭ0.001), and 5 of 8 severe hypothermia (Pϭ0.03) 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.

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The reaction of brain parenchymal vessels in areas of no-reflow following ischemia in cats was evaluated. A method was devised by which brain biopsies following ischemia were quickly frozen at -170 degrees C, sections were cut and stained and vessel internal and external diameter measured. Vessels in the no-reflow areas had smaller internal and external diameters and thicker walls when compared to adjacent reflow areas as well as to normal control animals. By utilizing a 2-way analysis of variance in which reflow versus no-reflow vessel diameters were compared by region the differences were found to be statistically significant (p less than 0.05). The data raise the possibility that there may exist normal regional differences in the size of cerebral vessels.

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