Cardiac resynchronization therapy with biventricular pacing is an established therapy for heart failure patients with sinus rhythm, reduced left ventricular ejection fraction and electrical ventricular desynchronization. The aim of the study was to evaluate electrical interventricular delay and left ventricular delay in right ventricular pacemaker pacing before upgrading to cardiac resynchronization therapy. Heart failure patients with right ventricular pacing, DDD pacemaker, DDD defibrillator and 24.5 ± 4.9 % left ventricular ejection fraction were measured by surface ECG and transesophageal bipolar left ventricular ECG before upgrading to cardiac resynchronization therapy. Interventricular and intraventricular desynchronization in right ventricular pacemaker pacing were 228.2 ± 44.8ms QRS duration, 86.5 ± 32.8ms interventricular delay and 94.4 ± 23.8ms left ventricular delay. Cardiac resynchronization therapy was optimized by impedance cardiography. Transesophageal electrical interventricular delay and left ventricular delay in right ventricular pacemaker pacing may be additional useful ventricular desynchronization parameters to improve patient selection for upgrading right ventricular pacemaker pacing to cardiac resynchronization therapy.