As the availability of smartphone-paired devices and smartwatches increases, clinicians will increasingly encounter data generated from consumer-grade devices. This expert analysis reviews several common consumer-grade devices, their specificity for detecting atrial fibrillation (AF), and the limitations of their use.

The Basics

Most consumer-grade devices for detecting arrhythmias use photoplethysmography (PPG). This technology measures volumetric changes in blood flow on the basis of the intensity of reflected light measured from the skin's surface. This signal generates peaks proportional to pulsatile blood flow, with the peak-to-peak interval proportional to the R-R interval. Given this method's ease of detecting irregularity, PPG technology has primarily been validated for detecting AF.

The broadened availability of consumer-grade devices may increase the detection of AF in the subclinical phase. With stroke as the initial manifestation of AF in almost one-quarter of cases, early AF detection may help relieve its burden as a leading cause of disability in the United States.¹

The Accuracy

• Apple Watch. The Apple Watch (Apple Inc., Cupertino, California) monitors heart rate and rhythm using PPG. Newer models can also record an on-demand single-lead electrocardiogram (ECG). The Apple Heart Study investigators enrolled >400,000 participants without AF. People who received an irregular pulse notification had telemedicine visits with a clinician and received an ambulatory ECG monitor. Of the 2,064 patients with irregular pulse notifications, the positive predictive value (PPV) for AF was 84%.²
  
  Notably, Apple recently received a cease-and-desist order on some Apple Watches after the United States International Trade Commission (USITC) ruled that Apple Watch technology infringes on oxygen saturation patents held by Masimo Corporation (Irvine, California). The cease-and-desist order on relevant Apple Watches is scheduled to take effect on December 26. This order may significantly reduce the number of Apple Watches that are available for purchase until patent issues have been resolved.
• Kardiamobile. Kardiamobile (AliveCor, Mountain View, California) is a small handheld device that can provide a 30-sec single-lead ECG. One study included monitoring participants three times daily and whenever they felt palpitations, with findings of a higher rate of AF detection with the Kardiamobile device than with 24-hour ECGs (9.4% vs. 2%).³ In another study, >1,000 patients without a history of AF were randomized to standard care or twice-weekly monitoring with Kardiamobile, with findings of a 3.8% detection rate for AF in the Kardiamobile arm compared with <1% in the standard-care arm.⁴
• Fitbit. Fitbit (Google, Mountain View, California) is a wrist-worn device with PPG technology with 37 million monthly users as of 2022.⁵ Similar to the Apple Watch, some newer models also incorporate the ability to perform a single-lead ECG. The Fitbit Heart Study had a similar design to the Apple Heart Study; >400,000 participants enrolled. Routine ambulatory ECG monitoring occurred in patients with irregular rhythm notifications. Among 1,057 participants with an irregular heart rate notification and an analyzable confirmatory ambulatory ECG, the PPV of irregular rhythms for AF when using consumer-grade screening with reflex to medical-grade confirmation was 98.2%.⁶

Overall Efficacy

The findings of two meta-analyses included high specificity (94%) and sensitivity (96%) for AF detection with smartphones and noninferiority of smartwatches compared with medical-grade devices.^7,8

Benefits Compared With Medical-Grade Monitoring

• They are more widely available.⁴
• They do not require a prescription.
• They can detect arrhythmias independently of ECG checks when the patient is free of symptoms.
• They have very high sensitivity and specificity for detecting AF.^7,8
• They can help monitor patients with established asymptomatic AF to assess AF rate and burden.

Limitations Compared With Medical-Grade Devices

• They are not worn continuously and need to be removed for charging. Therefore, their sensitivity for infrequent paroxysmal arrhythmias is lower.
• The specificity for arrhythmias with regular R-R intervals is very poor.⁹
• Monitoring in patients with a low pretest probability of arrhythmias increases the false-positive rate.
• There are no guideline recommendations on what to do with information from consumer-grade devices.
• The large volume of data obtained from consumer-grade devices may further contribute to an already strained clinician workforce.

Future Directions

Given the higher false-positive rate in patients with a low pretest probability of AF, future studies need to determine the patients most likely to benefit from ambulatory monitoring. Further, it is unknown whether increased AF detection on consumer-grade heart rhythm monitoring increases appropriate anticoagulation prescription or lowers cardioembolic stroke risk.

The ongoing Heartline Study will assess the impact of AF detection with the Apple Watch on clinical outcomes. The primary endpoint is the time from randomization to the detection of AF. Secondary endpoints include health resource utilization, cost-effectiveness, and a composite including stroke, heart failure hospitalization, and all-cause death.¹⁰

References

1. Freedman B, Potpara TS, Lip GYH. Stroke prevention in atrial fibrillation. Lancet 2016;388:806-17.
2. Perez MV, Mahaffey KW, Hedlin H, et al.; Apple Heart Study Investigators. Large-scale assessment of a smartwatch to identify atrial fibrillation. N Engl J Med 2019;381:1909-17.
3. Koh KT, Law WC, Zaw WM, et al. Smartphone electrocardiogram for detecting atrial fibrillation after a cerebral ischaemic event: a multicentre randomized controlled trial. Europace 2021;23:1016-23.
4. Halcox JPJ, Wareham K, Cardew A, et al. Assessment of remote heart rhythm sampling using the AliveCor heart monitor to screen for atrial fibrillation: the REHEARSE-AF study. Circulation 2017;136:1784-94.
5. Statista. Number of active users of Fitbit from 2012 to 2022 (in millions) (Statista website). 2023. Available at: https://www.statista.com/statistics/472600/fitbit-active-users/. Accessed 05/15/2024.
6. Lubitz SA, Faranesh AZ, Selvaggi C, et al. Detection of atrial fibrillation in a large population using wearable devices: the Fitbit Heart Study. Circulation 2022;146:1415-24.
7. Prasitlumkum N, Cheungpasitporn W, Chokesuwattanaskul A, et al. Diagnostic accuracy of smart gadgets/wearable devices in detecting atrial fibrillation: a systematic review and meta-analysis. Arch Cardiovasc Dis 2021;114:4-16.
8. Elbey MA, Young D, Kanuri SH, et al. Diagnostic utility of smartwatch technology for atrial fibrillation detection - a systematic analysis. J Atr Fibrillation 2021;13:[ePub ahead of print].
9. Rajakariar K, Koshy AN, Sajeev JK, Nair S, Roberts L, Teh AW. Modified positioning of a smartphone based single-lead electrocardiogram device improves detection of atrial flutter. J Electrocardiol 2018;51:884-8.
10. Gibson CM, Steinhubl S, Lakkireddy D, et al.; Heartline Steering Committee. Does early detection of atrial fibrillation reduce the risk of thromboembolic events? Rationale and design of the Heartline study. Am Heart J 2023;259:30-41.