Circadian variations of heart rate and STIM-T interval: adaptation for nighttime pacing

In order to determine the optimal pacing rate for pacemaker patients at night, 150 normal subjects with regular sinus rhythm and free of manifest heart disease, were studied using 24-hour Holter monitoring. Minimum and average heart rates were analyzed on an hourly basis. The study group was divided into six age groups, 25 subjects each, ranging from 20-29 years to 60-69 years. The minimum heart rate during the night was found to be lower than 65 ppm for all groups. The youngest subjects showed the largest variation in the minimum heart rate. The results suggest that an automatic lowering of the pacing rate during the night would allow for longer periods of sinus rhythm, thereby improving hemodynamic performance and reducing pacemaker power consumption. Suitable sensors for automatic lowering of the pacing rate include inbuilt 24-hour clock systems and the QT interval that lengthens during sleep.