Heart Rate Variability Measurement through a Smart Wearable Device: Another Breakthrough for Personal Health Monitoring?

Abstract

Heart rate variability (HRV) is a measurement of the fluctuation of time between each heartbeat and reflects the function of the autonomic nervous system. HRV is an important indicator for both physical and mental status and for broad-scope diseases. In this review, we discuss how wearable devices can be used to monitor HRV, and we compare the HRV monitoring function among different devices. In addition, we have reviewed the recent progress in HRV tracking with wearable devices and its value in health monitoring and disease diagnosis. Although many challenges remain, we believe HRV tracking with wearable devices is a promising tool that can be used to improve personal health.

Keywords: autonomic nervous system; heart rate variability; stress; wearable device.

Figure 7

(Left panel) shows the use of a smartphone camera to measure the heart rate variability (HRV). Source: [52]. (Right panel) shows real-time phone camera based HRV recording procedures using the Welltory app on the iPhone (photo taken by the authors).

Figure 1

Factors that affect heart rate variability (HRV). Source: [5]. * HRV decrease as a result of a physiological reaction to a physical stimulus. Provides a summary of the results referring to the factors and covers the four main areas, i.e., non-influenceable physiological factors, illnesses, influenceable lifestyle factors, and external factors.

Figure 2

PubMed publication records when using keywords: heart rate variability AND measurement.

Figure 3

A power spectral density analysis on a normal individual at supine rest and after a head-up tilt. Source: [7].

Figure 4

HRV analysis based on an Apple watch (series 7)-acquired ECG recording. The analysis included beat by beat NN intervals. The HRV is calculated as 47 ms using RMSDD. (This analysis was performed by the authors using the ECG HRV app on the iPhone 13).

Figure 5

The PPG sensors on the Apple Watch that uses infrared sensors, photodiodes, and green LEDs to measure heart rate. The Apple Watch uses green LED lights paired with light-sensitive photodiodes to detect the amount of blood flowing through the wrist at any given moment. When the wearer’s heart beats, the blood flow in the wrist and the green light absorption are greater. Between beats, the absorption is less. By flashing its LED lights hundreds of times per second and calculating the green light absorption variation, the Apple Watch can calculate the heart rate. Source: [26].

Figure 6

HRV analysis from the third-party app Welltory on iOS, which reads the raw data from a 5 min breath reading through an Apple Watch series 7. The (upper panel) shows the average heart rate and total heartbeats with a distribution of RR intervals. The (lower panel) shows the common time-domain measurements (left side) and the frequency-domain measurements (right side). (Analysis performed by the authors).

Figure 8

Heart rate variability data from Apple Watch automatic tracking in the Apple Health App (from authors’ data). Because the time points are randomly distributed, it is difficult to ascertain the wearer’s activity at the time of the recording.

Figure 9

Bland-Altman plots of ECG-derived and PPG-derived heart rate variability measurements (from six brands). The measurements took place simultaneously, and different brands presented different reading variations. Source: [60]. * means manufacturers provided raw data.

Figure 10

(Left panel): HRV4biofeed asks users to choose their current mental status before a manual session. (Right panel): after the manual session, the Welltory app asks users to choose the words that best reflect their mental status and activities (from the author’s iOS app).