Background: Hypothermia has been shown to improve survival and neurological outcomes for ventricular fibrillation (VF) cardiac arrest. The electrophysiological mechanisms of hypothermia are not well-understood, nor are the effects of beginning cooling during the resuscitation.
Methods and results: We hypothesized that inducing hypothermia prior to the onset of VF would slow the deleterious changes seen in the ECG during VF and that inducing hypothermia at the start of resuscitation would increase the rates of ROSC and short-term survival in a porcine model of prolonged VF. We randomly assigned 42 domestic swine (27.2+/-2.3 kg) to either pretreatment with hypothermia before induction of VF (PRE), normothermic resuscitation (NORM) or intra-resuscitation hypothermia (IRH). During anesthesia, animals were instrumented via femoral cutdown. Lead II ECG was recorded continuously. PRE animals were cooled before the induction of VF, with a rapid infusion of 4 degrees normal saline (30mL/kg). VF was induced electrically, left untreated for 8min, then mechanical CPR began. During CPR the NORM animals got 30mL/kg body-temperature saline and the IRH animals got 30mL/kg 4 degrees saline. In all groups first rescue shocks were delivered after 13min of VF. We calculated the VF scaling exponent (ScE) for the entire 8min period (compared using GEE). ROSC and survival were compared with Fisher's exact test. Mean temperature in degrees C at the onset of VF was PRE=34.7 degrees (+/-0.8), NORM=37.8 (+/-0.9), and IRH=37.9 (+/-0.9). The ScE values over time were significantly lower after 8min in the PRE group (p=0.02). ROSC: PRE=10/14 (71%), NORM=6/14 (43%) and IRH=12/14 (86%); p for IRH vs. NORM=0.02. Survival: PRE=9/14 (64%), NORM=5/14 (36%), IRH 8/14 (57%).
Conclusion: Hypothermia slowed the decay of the ECG waveform during prolonged VF. IRH improved ROSC but not short-term survival compared to NORM. It is possible to rapidly induce mild hypothermia during CPR using an IV infusion of ice-cold saline.