Inducibility of ventricular fibrillation in a swine model of rapidly induced mild therapeutic hypothermia

Abstract

Purpose: Mild therapeutic hypothermia (MTH) is a routinely used procedure for its expected improvement in neurological outcome after cardiac arrest. Moreover, there are efforts to cool patients as early and as fast as possible. Safety of MTH in terms of malignant arrhythmias has not been satisfactorily explained. The aim of this study was to compare inducibility of ventricular fibrillation (VF) before and after initiation of MTH in a swine model. Methods: The veno-arterial extracorporeal membrane oxygenation (ECMO) with cooling circuit was introduced in fully anesthetized swine (n=5) in order to provide rapid MTH (32 degrees Celsius in the pulmonary artery). Inducibility of VF was studied by programmed stimulation (8 stimuli of cycle length 300-400ms plus up to 4 subsequent extrastimuli) from the apex of the right ventricle during normothermia, after reaching the target temperature and after another 60 minutes. Serum potassium level was continuously monitored and checked by laboratory tests. Basic electrophysiologic parameters including QTc interval were observed. Results: Starting at normothermia (38.8+/-0.8 degrees Celsius) the target temperature was achieved in 48+/-13 minutes and was further maintained constant (31.9+/-0.2 degrees Celsius). Serum potassium level decreased during the cooling phase (from 4.4+/-0.6 mmol/ l to 3.9+/-0.5 mmol/l, p=0.02). After reaching and maintaining the target temperature serum potassium level began to increase spontaneously and after 60 minutes was not significantly different to normothermia level (4.9+/-0.9 mmol/l, p=0.07). Inducibility of VF was 100% at normothermia, 80% after reaching the target temperature and 27% (p < 0.0001) after another 60 minutes. Number of extrastimuli after basic cycle necessary to initiate VF or achieve the refractoriness was 2.57+/-0.76, 2.60+/-0.51 and 3.13+/-0.52 ( p=0.01), respectively. The QTc interval was inversely proportional to the body temperature (25+/-1 ms per degree Celsius, r2=0.84) and extended from the initial 366+/-43 ms to 542+/-34 ms at the target temperature. Conclusions: Rapid induction of MTH in a swine model concerning VF is safe and MTH combined with normokalemia seems to have protective effects despite prolonged QTc interval. There is need for further studies to clarify the mechanism of these protective features at the cellular and molecular level.