Abstract 331: The Effects of the Rate of Post-resuscitation Rewarming following Hypothermia on Outcomes of CPR in a Rat Model

Abstract

Background: Both clinical and laboratory studies have demonstrated that mild therapeutic hypothermia(TH) following CPR improves myocardial and neurologic outcomes of cardiac arrest. However, the optimal rewarming strategy following TH remains to be explored. The present study investigated the optimal rewarming rate following TH in a rat model of CPR. Methods and Results: Four groups of SD rats were randomized: 1) Control; 2) rewarming rate at 2°C/h; 3) rewarming rate at 1°C/h ; 4) rewarming rate at 0.5°C/h. VF was induced and untreated for 8 mins; defibrillation was attempted after 8 mins of CPR. The body temperature of the controls were maintained at 37±0.2°C throughout the experiment; For 2°C/h, 1°C/h and 0.5°C/h groups, rapid cooling was started with the aid of ice packs at the beginning of CPR. Upon reaching the target temperature of 33±0.2°C, the temperature was maintained with the aid of a cooling blanket until 4hrs after resuscitation. The rewarming was then initiated at the rate of 2.0°C, 1.0 °C or 0.5 °C per hour respectively until the body temperature reached 37±0.2 °C,which maintained at 37±0.2 °C for 6 hrs in 2 °C/h group, 4 hrs in 1 °C/h group, and 0 hours in 0.5 °C/h group. Blood samples were drawn at baseline, PR4, 6, 8, 10 and 12hrs for the measurements . Blood temperature was significantly decreased in the TH groups from CPR to PR 4hrs. Significantly better CO, EF, MPI and reduced neurological scores, longer duration of survival were observed in 1°C/hr and 0.5°C/hr groups. The increased serum levels of TnI, IL-6 and TNF-α was partly attenuated in 1°C/hr and 0.5°C/hr groups when compared with control and 2°C/hr groups. Conclusions: The present study demonstrated that the severity of myocardial, cerebral injuries and inflammatory reaction after CPR were reduced when mild TH was applied. A rewarming rate at 0.5-1°C/h did not alter the beneficial effects of TH. However, a rapid rewarming rate at 2°C/h abolished the beneficial effects of TH.