Analysis of heart rate variability in critical septic patients using a smartphone application: a prospective cohort study

Introduction Heart rate variability (HRV) reflects autonomic nervous system modulation and has been associated with mortality in sepsis. However, in the intensive care unit (ICU), HRV assessment with traditional equipment limits its use. This study aimed to investigate the association between HRV measured by a mobile application and mortality in septic patients. Methods This is a prospective observational cohort study conducted in two ICUs in Curitiba, Brazil, between July 2024 and March 2026, including septic patients (≥18 years) within 24 h of sepsis diagnosis. HRV, assessed using the standard deviation of NN intervals (SDNN), was measured through a heart rate monitor strap for 5 min over 3 consecutive days; clinical and hemodynamic data were also collected. Pregnancy, patients in exclusive palliative care, skin lesions that could interfere with the cardiac sensor, and decompensated irregular heart rhythm signs during data collection were excluded. The primary outcome was 28-day mortality. Results A total of 112 septic patients were included, with an overall 28-day mortality of 49%. Non-survivors had a significantly higher APACHE II (30 vs. 22, p < 0,001), and SDNN values were also significantly lower in non-survivors at 72 h (9.4 ms vs 11.3 ms; p = 0,027). Additionally, in multivariable analysis, female sex, age, SOFA score, capillary refill time (CRT), and SDNN < 5 ms were independently associated with mortality. Conclusion HRV assessed by a mobile application demonstrated a significant association with 28-day mortality in septic patients, with lower SDNN values at 72 h. This method might facilitate the wider implementation of HRV monitoring in resource-limited ICUs to stratify mortality in septic patients.