Yu index of longitudinal tissue Doppler dyssynchrony is the standard deviation of time to peak systolic velocities from the onset of the QRS complex. This is calculated after determining the time to peak systolic velocities from 12 sites. A value of 32 or more milliseconds is considered to be significant dyssynchrony. Measurement of dyssynchrony is used to assess the possibility of improvement with cardiac resynchronization therapy in heart failure. Region of interest for evaluation of dyssynchrony are placed in basal and mid left ventricular segments in three apical views: four chamber, two chamber and long axis.