Validation of the Apple Watch for Heart Rate Variability Measurements during Relax and Mental Stress in Healthy Subjects

David Hernando, Surya Roca, Jorge Sancho, Álvaro Alesanco, Raquel Bailón, David Hernando, Surya Roca, Jorge Sancho, Álvaro Alesanco, Raquel Bailón

Abstract

Heart rate variability (HRV) analysis is a noninvasive tool widely used to assess autonomic nervous system state. The market for wearable devices that measure the heart rate has grown exponentially, as well as their potential use for healthcare and wellbeing applications. Still, there is a lack of validation of these devices. In particular, this work aims to validate the Apple Watch in terms of HRV derived from the RR interval series provided by the device, both in temporal (HRM (mean heart rate), SDNN, RMSSD and pNN50) and frequency (low and high frequency powers, LF and HF) domain. For this purpose, a database of 20 healthy volunteers subjected to relax and a mild cognitive stress was used. First, RR interval series provided by Apple Watch were validated using as reference the RR interval series provided by a Polar H7 using Bland-Altman plots and reliability and agreement coefficients. Then, HRV parameters derived from both RR interval series were compared and their ability to identify autonomic nervous system (ANS) response to mild cognitive stress was studied. Apple Watch measurements presented very good reliability and agreement (>0.9). RR interval series provided by Apple Watch contain gaps due to missing RR interval values (on average, 5 gaps per recording, lasting 6.5 s per gap). Temporal HRV indices were not significantly affected by the gaps. However, they produced a significant decrease in the LF and HF power. Despite these differences, HRV indices derived from the Apple Watch RR interval series were able to reflect changes induced by a mild mental stress, showing a significant decrease of HF power as well as RMSSD in stress with respect to relax, suggesting the potential use of HRV measurements derived from Apple Watch for stress monitoring.

Keywords: ANS assessment; Apple Watch; RR series; heart rate variability; validation; wearable device.

Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Figures

Figure 1
Figure 1
Example of the RR series for a subject in both stages: Polar H7 (blue) and Apple Watch (red). In the stress stage, there are gaps in the Apple Watch recording where no beats are detected.
Figure 2
Figure 2
Bland-Altman plot: RRH7g vs. RRAW. Mean of the difference of the RR series ±2*std values (limits of agreement, LOA).
Figure 3
Figure 3
Heart rate variability (HRV) parameters: time domain.
Figure 4
Figure 4
HRV parameters: frequency domain, derived from RRH7, RRH7g and RRAW. * denotes significant differences (p < 0.05) between adjacent boxplots. Adim refers to adimensional units.
Figure 5
Figure 5
HRV parameters: temporal domain (Relax vs. Stress). * denotes significant differences (p < 0.05) between adjacent boxplots.
Figure 6
Figure 6
HRV parameters: frequency domain (Relax vs. Stress). * denotes significant differences (p < 0.05) between adjacent boxplots. Adim refers to adimensional units.

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