Reproducibility of QT measurement by implantable loop recorders: impact of R-wave amplitude on accuracy

Background Continuous QT monitoring through implantable loop recorders (ILRs) may enable early detection of repolarization abnormalities, but the clinical value depends on the reproducibility of QT assessment and the quality of the sensed signal. Objective To evaluate the reproducibility of ILR-derived QT measurements versus surface ECG and identify predictors of accuracy, focusing on R-wave amplitude. Methods Consecutive patients with paired QT values from surface ECG and ILR were retrospectively analyzed. Reproducibility was assessed by Pearson correlation, Bland–Altman analysis, and intraclass correlation coefficient (ICC[A,1]). Absolute QT error (|QT_loop − QT_EKG|) was used as the dependent variable in multivariable regression including R-wave amplitude, QRS ≥120 ms, bundle branch block, age, and QT at EKG. Results Among 141 paired measurements (mean QT_EKG = 408 ms, QT_ILR = 416 ms), correlation between QT_loop and QT_ECG was r = 0.40 (95% CI 0.25–0.53; p < 10−6), with mean bias +7.7 ms (LoA −70.3 to +85.8 ms) and ICC = 0.39. In multivariable analysis (R² = 0.58), R-wave amplitude was the strongest independent predictor of accuracy (β = −71.8 ms/mV; p < 0.001). Wider QRS, lower QT_EKG, and younger age predicted larger errors. ROC analysis confirmed that R-wave amplitude discriminated accurate QT detection (|error| < 20 ms) with AUC = 0.95, comparable to the multivariable model (AUC = 0.96). Conclusions ILR-based QT analysis shows moderate agreement with surface ECG but is highly dependent on signal quality. QT monitoring may be unreliable when R-wave amplitude is low, highlighting the need for adequate sensing optimization before relying on QT trends from ILRs.