QTc measurement using Apple Watch electrocardiogram in congenital long QT syndrome

In congenital long QT syndrome (cLQTS), monitoring of the heart rate–corrected QT interval (QTc) is essential as even transient prolongation can significantly increase the risk of torsades de pointes and sudden cardiac death. Apple Watch (AW) offers a single-lead mobile electrocardiogram (mECG), but its accuracy for QTc monitoring remains uncertain. The objective is to analytically validate AW mECGs for QTc measurement in paediatric and adult cLQTS patients, assessing agreement, systematic bias, and lead-specific feasibility compared with standard 12-lead ECG.

In this cross-sectional, dual-centre study, patients with cLQTS underwent consecutive 12-lead ECG, followed by mECG recordings of Leads I and II. QT intervals were measured by two blinded investigators, and accuracy was evaluated using Bland–Altman analysis. The study was deemed exempt from formal ethical approval by the Medical Ethics Committee of the Amsterdam UMC. Of 101 patients enrolled, 99 had ECGs suitable for QTc analysis; 15 (15.2%) were younger than 18 years and 62 (62.6%) were female. On 12-lead ECG, the mean QTc was 444.9 ± 30.2 ms (Lead I) and 449.0 ± 29.8 ms (Lead II), compared with 466.6 ± 28.9 ms (Lead I) and 470.0 ± 29.8 ms (Lead II) on mECG. The mean QTc difference (12-lead—AW) was −21.7 ms (95% limits of agreement: −53.1–9.7) for Lead I and −21.0 ms (−59.5–17.5) for Lead II.

In patients with cLQTS, AW-derived mECGs may complement, but not replace, standard 12-lead ECGs for QTc assessment, pending further validation in longitudinal and unsupervised settings.