Impact of Atrio-ventricular Optimization With His Bundle Pacing on Treatment of Atrio-ventricular Dromotropathy

Inclusion Criteria: * First OR second degree Mobitz type 1 atrioventricular block with a PR interval \> 250 ms * left ventricular ejection fraction \> 50% * echocardiographic criteria of atrioventricular dyssynchrony (diastolic filling time/RR interval ratio \< 0,4 OR fusion of E and A waves OR diastolic mitral regurgitation) * symptoms on exertion (dyspnea, palpitations) * insufficient shortening of PR interval during exercise (PR interval \> 200 ms at heart rate of 100 beats per minute) Exclusion Criteria: * left ventricular ejection fraction \< 50% * third degree atrioventricular block * atrial fibrillation * sinus node disease * left bundle branch block * right bundle branch block * ventricular arrhythmia that indicates implantation of cardioverter defibrillator * consumption of drugs that influence atrioventricular conduction * active bacterial infection * inability to undergo cardiopulmonary exercise test * anaemia (haemoglobin concentration \< 100 g/L) * pregnancy