Objective: Our study aims to evaluate the association between heart rate variability (HRV) and short and long-term prognosis in patients admitted to intensive care unit (ICU). Methods and Results: Adult patients continuously monitored for over 24h in ICUs from the MIMIC-IV Waveform Database were recruited in our study. Twenty HRV-related variables (8 time-domain, 6 frequency-domain; and 6 nonlinear variables) were calculated based on RR intervals. The association between HRV and 30-day all-cause mortality was assessed. Ninety-three patients met the inclusion criteria and were classified into 30-day survivor group and non-survivor groups based on their survival status. The 30-day all-cause mortality rate was 17.2%. NN50 and pNN50 were both significantly higher in non-survivors compared to survivors, whereas the rest of the time-domain, frequency domain and non-linear HRV parameters did not differ significantly between the two groups (all P >0.05). In addition, at 180 days after admission, non-survivors had significantly higher levels of NN50 and rMSSD than the survivors. However, NN50 was not an independent predictor of 30-day all-cause mortality in patients by multivariate COX regression analysis (HR, 1.0; 95% CI, 1.000 - 1.001; P =0.594). The Area Under the Curve (AUC), cut-off value, sensitivity and specificity of NN50 for predicting 30-day all-cause mortality using ROC were 0.67, 799, 0.813 and 0.584, respectively. Plotting Kaplan-Meier analysis using this cut-off value showed that patients with high NN50 had considerably greater 30-day all-cause mortality than those with low NN50 (P < 0.001). Conclusion: NN50 and pNN50 are associated with elevated 30-day all-cause mortality in ICU patients but are not independent predictors of all-cause mortality using multivariate COX regression analysis.

The authors have declared no competing interest.

Funding for this project were provided by the Guangdong Basic and Applied Basic Research Foundations (No. 2021A1515010402 and 2021A1515011433).

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Studies involving human participants were reviewed and approved by the Institutional Review Board of the Massachusetts Institute of Technology and the Beth Israel Deaconess Medical Center. Written informed consent for participation was not required for this study in accordance with the national laws and the institutional requirements. The study was also approved by the Ethics Committee of Sun Yat-sen Memorial Hospital, Sun Yat-sen University (approval ID: SYSKY-2023-089-01).

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