['Cardiac resynchronization therapy (CRT) improves symptoms and survival in patients with chronic heart failure. The presence of electrocardiographic (ECG) signs of electrical dyssynchrony during ventricular contractions are an essential criterion for the implantation of CRT. Nevertheless, 1/3 proportion of patients who undergo CRT do not seem to respond favorably. Aim of study was to investigate the relevant ECG parameters prior to CRT (as well as their postimplantation changes) and to determine their relation to predicting favorable response to CRT.', '52 symptomatic patients (age 62.2 ± 10.5 years, 39 men) with severe left ventricular (LV) systolic dysfunction (ejection fraction 26.3% ± 6.9%) with QRS 120 ms underwent CRT implantation. In early pre- and postimplantation CRT period the following ECG parameters were recorded: QRS complex width and morphology (complete and atypical left bundle branch block, nonspecific intraventricular conduction delay), the size of R and S wave. After 6 months of CRT, responders were definied by improvement of LV ejection fraction > 5 % and/or reduction of end-systolic LV volume > 15 %.', 'The incidence of left bundle branch block (complete or atypical) prior to CRT implantation was higher in responders compared to non-responders group (64% vs 35%, p = 0.036). After implantation of CRT, responders showed significant narrowing of the QRS complex, while non-responder`s QRS complex width remained unchanged (QRS -18 ± 22 ms vs 1 ± 27 ms, p = 0.018). Furthermore QRS complex reduction led to increase in LV ejection fraction (r = 0.47, p = 0.001) and to reduction of LV end-systolic volume (r = 0.42, p = 0.004). R and S changes between responders and non-responders did not differ (p = 0.598 respectively, P = 0.685).'] Do [ Importance of EKG in patients wuth resynchronization therapy ]?