Induction of delayed repolarization during chronic beta-receptor blockade.

The development of delayed ventricular repolarization was studied in eight patients with clinical indication for chronic beta blockade. Their mean age was 74 years (range 69-81 years) and they all had permanent pacemakers for syncope. The ventricular repolarization time was assessed by means of the Q-T top interval during ventricular stimulation at different paced heart rates. In a group of seven other patients there was a close relationship between the right ventricular monophasic action potential duration at 90% repolarization (RV MAPD90) and the paced Q-T top interval (r = 0.90). After three weeks of treatment with 200 mg metoprolol daily, the paced Q-T top interval increased significantly (3-4%, P less than 0.001) at all paced heart rates, while after about two weeks of treatment a significant increase was seen only at the paced heart rate of 130 beats min-1 (P less than 0.01). Thus, the present study confirmed the development of delayed ventricular repolarization within three weeks of treatment with 200 mg metoprolol daily. The onset of this effect was first noticeable after about two weeks of treatment. Furthermore, the close relationship between the RV MAPD90 and the paced Q-T top interval implies that the paced Q-T top interval may be a reliable and useful feature to detect changes in the ventricular repolarization time.