Hypothermia Improves Defibrillation Success and Resuscitation Outcomes From Ventricular Fibrillation

Abstract

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.

Early Induction of Hypothermia During Cardiac Arrest Improves Neurological Outcomes in Patients With Out-of-Hospital Cardiac Arrest Who Undergo Emergency Cardiopulmonary Bypass and Percutaneous Coronary Intervention

“…Using a porcine model of VF cardiac arrest (no-flow) of 8 min, Boddicker et al showed that pre-existing hypothermia of 30°C or 33°C facilitated significantly improved resuscitation outcomes in comparison with normothermic resuscitation, but that a temperature of 35°C was not beneficial to the resuscitation outcome. 30 Wu et al showed that profound hypothermia (10-15°C for 1 h), followed by mild hypothermia (34°C for 36 h) produced a better intact survival rate than normothermic resuscitation or profound hypothermia (10-15°C for 1 h), followed by mild hypothermia (34°C for 12 h) in a canine model of hemorrhagic cardiac arrest. 31 Gunn et al reported that delayed onset brain edema following 30 min of cerebral ischemia was abolished by prolonged (3-day) hypothermia.…”

Moderate hypothermia increases the chance of spiral wave collision in favor of self-termination of ventricular tachycardia/fibrillation

“…subjected to defibrillation shocks and cardiopulmonary resuscitation following unsupported VF, Boddicker et al (3) showed that the defibrillation success and subsequent resuscitation outcome were improved by preexisting MH and SH, and the effect was most remarkable at MH. Some unidentified mechanical, metabolic, or electrophysiological properties were suggested to be involved in the beneficial effect of hypothermia in terms of prevention of refibrillation (3). In the present study, a certain modification of SW dynamics at MH could contribute to the in vivo prevention of refibrillation following DC shocks.…”

“…This has led to recent guideline recommendations that all unconscious adult patients with spontaneous circulation after out-of-hospital cardiac arrest should be cooled to 32-34°C for 12-24 h when the initial rhythm was VF (1). In experiments using swines with induced VF, Boddicker et al (3) demonstrated that defibrillation success of electrical shocks and resuscitation outcomes were significantly improved under moderate hypothermia (MH, 33°C) compared with normothermia (NR, 37°C) and that the benefit of MH was larger than severe hypothermia (SH, 30°C) (3). The underlying mechanisms of this amelioration are not well understood.…”

Mild hypothermia markedly reduces ischemia related coronary t-PA release

“…Taken together, it is possible that the hypothermia related reduction in t-PA release during ischemia/reperfusion may contribute to the tissue protective effects of hypothermia. It may also be part of the explanation to why hypothermia has been found to increase defibrillation success and resuscitation outcome at cardiac arrest in a porcine model [48].…”