Inter-Lead RRI Agreement Between Lead I and Lead II Across Diagnostic ECG Categories: Pilot Evidence for Wearable HRV Monitoring

Background: Wearable electrocardiogram (ECG) devices capable of recording a single-lead signal—typically Lead I—have proliferated rapidly, enabling long-term ambulatory cardiac monitoring outside clinical settings. However, the extent to which Lead I-derived heart rate variability (HRV) metrics are concordant with those obtained from the clinical gold standards—12-lead ECG and 3-channel Holter monitoring—remains incompletely characterised. Methods: We analysed RR-interval series extracted from (i) the PTB-XL large-scale publicly available 12-lead ECG dataset (n = 18,869 patients; 100 Hz, 10 s recordings) and (ii) ambulatory 3-channel Holter ECG recordings (Suzuken Pico303) from four participants (two elderly adults and two young adults). Pan–Tompkins R-wave detection was applied uniformly. Time-domain (SDNN, RMSSD, and pNN50), Poincaré (SD1 and SD2), and Bland–Altman agreement metrics were computed for Lead I versus Lead II comparisons. Results: Analysis of 300 PTB-XL records (a contiguous pilot subset: records 00040–00350) stratified by diagnostic superclass revealed strong Lead I–Lead II RRI concordance for normal ECGs (NORM: r = 0.844, ICC(2,1) = 0.841, mean bias = −1.4 ms, 95% LoA [−52.4, +49.7] ms; n = 179 records) and progressively lower agreement in pathological categories (MI: r = 0.659, ICC = 0.663; CD: r = 0.433, ICC = 0.484; HYP: r = 0.556, ICC = 0.574; STTC: r = 0.635, ICC = 0.634). Holter Pico303 recordings demonstrated significant inter-individual variability; SDNN ranged from 37.0 ms (elderly male) to 80.8 ms (elderly female) and 80.4 ms (young male), while RMSSD ranged from 27.0 to 68.0 ms. R-wave amplitude was systematically higher in Lead II (0.734 ± 0.413 mV) than Lead I (0.667 ± 0.339 mV) across the 300-record cohort (overall mean). Conclusions: Single-lead (Lead I) ECG provides HRV information comparable to Lead II for time-domain metrics, supporting the utility of wearable single-lead ECG for RRI estimation and time-domain HRV monitoring; however, systematic differences in R-wave morphology warrant lead-specific calibration of automated detection algorithms.