A medical device (10; 90) for determining a dyssynchronicity measure indicating a degree of mechanical dyssynchronicity of a heart of a patient, said medical device (10; 90) being connectable to at least two medical leads (25, 30; 92, 93) each including electrodes (26, 27, 28, 29, 32, 34, 36, 38) adapted to be positioned within or at said heart, said medical device (10; 90) comprising: an impedance measuring unit (41; 97) adapted to measure: a first intracardiac impedance set over at least one cardiac cycle using a first electrode vector/array including at least two electrodes (26, 27, 32, 34) placed such that said first intracardiac impedance set substantially reflects a mechanical activity of said left side of said heart; and a second intracardiac impedance set over at least one cardiac cycle using a second electrode vector/array including at least two electrodes (32, 34) placed such that said second intracardiac impedance set substantially reflects a mechanical activity of said right side of said heart; a calculating module (47, 99) adapted to: calculate a difference value parameter set as a difference between said first and second intracardiac impedance sets and related to a morphology of said first and second intracardiac impedance sets, said difference value parameter set forming a waveform over a part of said at least one cardiac cycle; integrate said waveform over a predetermined period of time of said part of said at least one cardiac cycle to obtain an integral value, said predetermined period of time corresponding to a predetermined part of mechanical systole; and calculate a dyssynchronicity_measure to correspond to said integral value; and a cardiac event detector (50, 102) connectable to said electrodes (26, 27, 28, 29, 32, 34, 36, 38) and adapted to detect occurrence of cardiac events in said at least one cardiac cycle, characterized by: an impedance processing module (46, 98) adapted to synchronize said first and second intracardiac impedance sets in relation to a predetermined reference cardiac event detected by said cardiac event detector (50, 102), wherein said calculating module (47, 99) is adapted to integrate said waveform over said predetermined period of time corresponding to a range between 20 and 50 % of said at least one cardiac cycle following activation of a primary ventricle as detected by said cardiac event detector (50, 102). an impedance measuring unit (41; 97) adapted to measure: a first intracardiac impedance set over at least one cardiac cycle using a first electrode vector/array including at least two electrodes (26, 27, 32, 34) placed such that said first intracardiac impedance set substantially reflects a mechanical activity of said left side of said heart; and a second intracardiac impedance set over at least one cardiac cycle using a second electrode vector/array including at least two electrodes (32, 34) placed such that said second intracardiac impedance set substantially reflects a mechanical activity of said right side of said heart; a first intracardiac impedance set over at least one cardiac cycle using a first electrode vector/array including at least two electrodes (26, 27, 32, 34) placed such that said first intracardiac impedance set substantially reflects a mechanical activity of said left side of said heart; and a second intracardiac impedance set over at least one cardiac cycle using a second electrode vector/array including at least two electrodes (32, 34) placed such that said second intracardiac impedance set substantially reflects a mechanical activity of said right side of said heart; a calculating module (47, 99) adapted to: calculate a difference value parameter set as a difference between said first and second intracardiac impedance sets and related to a morphology of said first and second intracardiac impedance sets, said difference value parameter set forming a waveform over a part of said at least one cardiac cycle; integrate said waveform over a predetermined period of time of said part of said at least one cardiac cycle to obtain an integral value, said predetermined period of time corresponding to a predetermined part of mechanical systole; and calculate a difference value parameter set as a difference between said first and second intracardiac impedance sets and related to a morphology of said first and second intracardiac impedance sets, said difference value parameter set forming a waveform over a part of said at least one cardiac cycle; integrate said waveform over a predetermined period of time of said part of said at least one cardiac cycle to obtain an integral value, said predetermined period of time corresponding to a predetermined part of mechanical systole; and calculate a dyssynchronicity_measure to correspond to said integral value; and a cardiac event detector (50, 102) connectable to said electrodes (26, 27, 28, 29, 32, 34, 36, 38) and adapted to detect occurrence of cardiac events in said at least one cardiac cycle, an impedance processing module (46, 98) adapted to synchronize said first and second intracardiac impedance sets in relation to a predetermined reference cardiac event detected by said cardiac event detector (50, 102), wherein said calculating module (47, 99) is adapted to integrate said waveform over said predetermined period of time corresponding to a range between 20 and 50 % of said at least one cardiac cycle following activation of a primary ventricle as detected by said cardiac event detector (50, 102).