In patients with heart failure,left ventricular systolic dysfunction and prolonged QRS complex, cardiac resynchronization therapy(CRT)is a treatment method aimed at restoration of myocardial depolarization synchronicity.However,the extent of clinical and echocardiographic improvement depends on anatomical relations in individualpatients, on structural changes in the heart,on intrinsic electrical activation,and on the position of pacing leads.Many parameters of CRTdevices may be changed in order to tailor the function of CRT to the needs of a particular patient;the most important among them is AV and VV interval.