Heart rate persistence index (HRPI): a threshold-free wearable metric for sustained HR elevation

Wearable devices generate dense longitudinal heart rate (HR) data, but summarizing sustained heart rate elevation in a single daily metric remains challenging. We developed the heart rate persistence index (HRPI), defined as the largest integer k such that at least k minutes in a day have HR [≥] k bpm. For example, an HRPI of 105 means daily HR was [≥]105 bpm for at least 105 minutes. HRPI is threshold-free and integrates magnitude and duration of elevated HR into a single interpretable value. Using multi-day wearable recordings from a PhysioNet dataset, we show that HRPI captures structure beyond mean HR, reflects variability-related features, and exhibits robust day-to-day stability. In an independent healthy cohort, HRPI declines strongly with age, supporting physiological relevance. HRPI offers a compact, interpretable, and robust summary of sustained HR elevation for longitudinal wearable studies, providing information easily accessible to both specialists and nonspecialists.