The development of technology enables the construction and programming of more advanced devices that stimulate the heart, which is also associated with increased demands on doctors, on the knowledge of pacemakers and interpretation of the electrocardiography ( ECG). The description refers to 77-year-old patient after pacemaker implantation (according to the available documentation - DDDR), due to sick sinus syndrome, admitted to the clinic, because of an episode of fainting and feeling of general weakness, for diagnosis and treatment. ECG recorded an image of atrioventricular block third degree (AVB IIIᵒ), with no data on the presence of ventricular pacing (no ventricular spikes). Preclinical diagnosis was ventricular dysfunction or impaired control in ventricular channel (oversensing in the course of noise?, crosstalk?). ECG with magnet allowed us to exclude ovesrensing. Chest X-ray and programmer control revealed the presence of the one-chamber pacemaker cardiac (AAI). Finally, dual-chamber pacemaker (DDDR), with ventricular electrode was implanted – resulted in obtaining relief of patient symptoms.