BACKGROUND The Apple Watch Series 4 (AW) can detect atrial fibrillation and perform a single-lead electrocardiogram (ECG), but the clinical accuracy of AW ECG waveforms compared to lead 1 of a 12-lead ECG is unclear. OBJECTIVE The purpose of this study was to assess the accuracy of interval measurements on AW ECG tracings in comparison to lead 1 on a 12-lead ECG. METHODS We obtained ECGs at a university hospital of healthy volunteers age .18 years. ECG waveforms were measured with calipers to the nearest 0.25 mm. When possible, 3 consecutive waveforms in lead 1 were measured. Waveform properties, including intervals, were recorded. Concordance correlation coefficients and Bland-Altman plots were used to assess level of agreement between devices. RESULTS Twelve-lead (n 5 113) and AW (n 5 129) ECG waveforms from 43 volunteers (mean age 31 years; 65% female) were analyzed. Sinus rhythm interpretation between devices was 100% concordant. No arrhythmias were recorded. Mean difference (d) for heart rate was 1.16 6 4.33 bpm (r 5 0.94); 3.83 6 113.54 ms for RR interval (r 5 0.79); 5.43 6 17 ms for PR interval (r 5 0.83);-6.89 6 14.81 ms for QRS interval (r 5 0.65);-11.27 6 22.9 ms for QT interval (r 5 0.79); and-11.67 6 27 ms for QTc interval (r 5 0.57). There was moderate (d ,40 ms) to strong (d ,20 ms or , 5 bpm) agreement between devices represented by Bland-Altman plots. CONCLUSION The AW produces accurate ECGs in healthy adults with moderate to strong agreement of basic ECG intervals.

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Thyrotoxic periodic paralysis is a life-threatening complication of hyperthyroidism characterized by transient episodes of muscle paralysis and hypokalemia, commonly seen in Asian men. We present a rare case of ventricular fibrillation as the initial presentation of thyrotoxic periodic paralysis. ( Level of Difficulty: Intermediate. )

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