Low-frequency heart rate variability: reproducibility in cardiac transplant recipients and normal subjects

Heart rate variability is a measure of autonomic nervous influence on the h eart. It has been suggested that it could be used to detect autonomic reinn ervation to the transplanted heart, but the reproducibility of the measurem ent is unknown. In the present study, 21 cardiac transplant recipients and 21 normal subjects were recruited. Three measurements of heart rate variabi lity were performed during the day: in the morning, in the early afternoon and in the late afternoon. These tests were then repeated I week later and then again 1 week after that, making nine tests in all. The within-subject S.D. was 0.49 log units in normal subjects and 0.79 log units in transplant recipients. In both cases, this is about 15% of the population range. Ther e was significant variation in heart rate variability between different tim es of day in both groups, and from day to day in transplant recipients. It was concluded that the reproducibility of measurements of heart rate variab ility is low, and that differences between measurements performed at differ ent times of day should be interpreted with caution.