².Yu. Burdà The significance of QRS complex duration for the effectiveness of atrial fibrillation therapy with beta-blockers The study has been held to investigate the significance of of normal (60—100 ms) and prolonged (more than 100 ms) QRS complex duration for the effectiveness of atrial fi- brillation (AF) therapy with beta-blockers in 75 patients aged (64 ± 9) years. Pati- ents were examined before, after 6 months and 1 year from the therapy start. In pati- ents with normal QRS complex duration beta-blockers had no significant effect on the frequency of AF clinical manifestations, with prolonged— led to increasing of lighter forms of arterial hypertension and stable angina pectoris frequency. In patients with normal QRS complex duration the most part of hemodynamic indicators were improved, with prolonged duration were worsened. Patients with AF and prolonged QRS complex re- quire more thorough control.