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Introduction Hypothermia is therapeutically used after cardiac arrest. The moderate hypothermia alters cardiac function, with reduced heart rate (HR), prolonged systolic duration and increased QT interval. The risk of arrhythmia increases with prolonged QT-interval, but clinical studies have not been able to demonstrate an increase of arrhythmic events during moderate hypothermia. The mechanism of this is not thoroughly described, but the concurrent increase in mechanical systolic duration may play a part of the explanation. Purpose QT prolongation and increased electromechanical window (EMW) negativity; the duration of left ventricle mechanical systole relatively to the duration of the electrical systole, are associated with arrhythmia. We hypothesized that moderate hypothermia would increase ECG intervals and systolic duration without an increase in EMW negativity. Methods In an open-chest porcine model (n=10), 2D echocardiography measurements and Doppler recordings were performed with concurrent 3-lead ECG registrations during normothermia (38°C) and moderate hypothermia (33°C) at spontaneous HR. Diastolic and systolic durations were assessed by valve opening to valve closure and Doppler assessed ejection time (ET). Isovolumetric contraction time (IVCT) and EMW, were measured. Systolic function was assessed by ejection fraction (EF), stroke volume (SV) and systolic velocity (s'). Results are presented as means ± SD. Two-sided T-test was used for comparison and P-value Results Moderate hypothermia reduced HR from 91±9 to 82±7 beats/min (p Conclusion Moderate hypothermia increased the QTc-interval but the mechanical systolic duration was even more prolongated. The EMW became positive demonstrating that the mechanical systole outlasted the electrical systole. As less EMW negativity is considered to be protective regarding arrhythmia, these findings may explain why arrhythmia is not frequently occurring during moderate hypothermia. Acknowledgement/Funding None