# QTc measurement using Apple Watch electrocardiogram in congenital long QT syndrome

## TL;DR

The study evaluates whether Apple Watch electrocardiograms can accurately measure QT intervals in patients with congenital long QT syndrome.

## Contribution

The study provides empirical validation of Apple Watch mECGs for QTc measurement in cLQTS patients, comparing them to standard 12-lead ECGs.

## Key findings

- Apple Watch mECGs showed modest mean QTc differences compared to 12-lead ECGs.
- Wide limits of agreement suggest variability that limits unsupervised clinical use.
- mECGs may complement but not replace standard ECGs for QTc assessment in cLQTS.

## Abstract

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.

Graphical AbstractAccuracy of Apple Watch single-lead ECG for QTc assessment in congenital long QT syndrome. Study overview showing single-lead Apple Watch mECG and 12-lead ECG QTc measurements for both Leads I and II in patients with congenital long QT syndrome. Mean difference was modest, but wide limits of agreement limit suitability for unsupervised clinical use.A graphical abstract illustrating the study design and findings on the accuracy of Apple Watch single-lead ECG for QTc assessment in patients with congenital long QT syndrome (cLQTS). The image shows single-lead Apple Watch mECG recordings for both leads I (wrist) and lead II (ankle), alongside and a standard 12-lead ECG used to measure QTc. A Bland-Altman analysis demonstrated modest mean differences between smartwatch-derived and 12-lead QTc values, but with wide limits of agreement. A concluding panel summarises that smartwatch-derived QTc may complement QTc assessment in cLQTS patients, but the large measurement variability limits the suitability for unsupervised clinical use.

Accuracy of Apple Watch single-lead ECG for QTc assessment in congenital long QT syndrome. Study overview showing single-lead Apple Watch mECG and 12-lead ECG QTc measurements for both Leads I and II in patients with congenital long QT syndrome. Mean difference was modest, but wide limits of agreement limit suitability for unsupervised clinical use.