Heart Rate Variability (HRV) as a digital biomarker for Mental Health and the promise of Apple Watch

Heart Rate Variability, or HRV, is becoming one of the most popular metrics available on a Smartwatch today.

HRV as a metric has far more wide-ranging applications beyond tracking your athletic recovery and the effects of training loads. It is also becoming one of the leading digital biomarkers for Mental Health.

Heart rate variability is a robust biomarker of emotional well-being, consistent with the shared brain networks regulating emotion regulation and heart rate. 

Related reading:

• Apple Watch readings help with Mental Health Care for postpartum depression
• Complete Guide to Apple Watch Heart Rate Zones
• Apple Watch guide understanding and use of HRV (Heart Rate Variability)
• HRV Measured via wearables could help with AUD (Alcohol use disorder) related problems

Heart Rate Variability (HRV) and emotional states

Researchers at the University of Southern California found that heart rate oscillatory activity not only provides a measure of the current state of regulatory brain systems but also changes emotion network coordination in the brain.

The researchers concluded that people whose breathing oscillates their heart rate more (leading to higher heart rate variability or HRV) generally have better-regulated emotions.

This is one of the reasons we have seen a sharp rise in the popularity of apps such as Headspace, Calm, and others over the last five years. 

In this short article, we highlight several studies that have been performed over the last few years evaluating the use of HRV as a digital biomarker for mental health-related use cases. 

Heart Rate Variability and understanding bipolar disorder

Bipolar disorder is characterized by cycles of depression and mania. This euphoric, high-energy state can result in heightened levels of creativity or output erratic or risky behavior. 

According to the National Institute of Mental Health (NIMH), an estimated 2.8% of U.S. adults had bipolar disorder in the past year, and an estimated 4.4% of U.S. adults experienced bipolar disorder at some time in their lives. 

Furthermore, 82.9% of people with bipolar disorder had a severe impairment; the highest percent severed impairment among mood disorders.

• The best food and cosmetic apps for a toxic-free lifestyle
• Set up medication reminders on your Apple Watch and iPhone with the Health app

In 2022, A team of scientists and researchers in Norway looked at using HRV as a digital biomarker for bipolar disorder. The Norwegian Research Council funded this study.

They found that Bipolar disorder is associated with autonomic nervous system dysfunction (ANS), indexed by heart rate variability (HRV).

The researchers used wrist-based wearables for this study and found statistically significant increases in HRV measures SDNN, RMSSD, LF, and Sample Entropy from mania to euthymia.

The findings reveal lower HRV in the manic state compared to the euthymic state. This indicates that HRV collected by wrist-worn PPG sensors is a possible biomarker for bipolar mood states.

Depression and its effects on Heart Rate Variability

Depression is one of the most common mental disorders in the United States. Some 15.5 million Americans have been taking Antidepressants for at least five years. The rate has almost doubled since 2010 and more than tripled since 2000.

A study published in 2020 by Italian researchers found that Reduced heart rate variability (HRV)  is associated with vulnerability to depression.

In numerous studies, participants who fulfilled DSM-IV criteria for an anxiety disorder displayed diminished HRs. Anxiety disorders that often develop into full-scale Panic attacks have shown a marked reduction in Heart Rate variability.

Patients with panic disorder are characterized by higher baseline heart rate (HR) and periods of tachycardia which coincide with panic symptoms.

Increased cardiac mortality and morbidity have been suggested in these patients, according to a study published in 2018. 

Standard Deviation of Normal to Normal intervals (SDNN) and High Frequency (HF) power of HRV were found to be reduced both in individuals with dysphoria (a state of unease or generalized dissatisfaction in life) and in those with past depression as compared with controls. 

Whereas psychological measures did not significantly differ among individuals with past depression and controls, HRV was capable of discriminating between the two groups.

The findings from this study suggest that reduced HRV is likely to indicate the risk of developing full-blown depression rather than being a mere correlate of a current depressive state. 

This would suggest that HRV tracked via wearables such as the Apple Watch may hopefully improve clinicians’ ability to early identify people at risk for depression who can benefit from targeted prevention by psychiatric and psychological interventions.

Heart rate variability measurement in epilepsy and prediction of seizures

HRV has been studied in patients with epilepsy for more than 30 years, and overall, patients with epilepsy display altered interictal HRV, suggesting a shift in autonomic balance toward sympathetic dominance. 

Recent research has looked at methods that can be used to predict epilepsy seizures from HRV data.

Although refractory epileptic patients suffer from uncontrolled seizures, their quality of life (QoL) may be improved if an epileptic seizure can be predicted in advance.

A study in 2013 by a team of Japanese Researchers looked at this use case and found that it was feasible to predict the onset of seizures using HRV data. Numerous studies since 2013 have looked at this use case since then. 

A more recent 2018 study found that the average prediction time was relatively high, 13.7 min, meaning that a long period is available to trigger an intervention or to secure the patient before the upcoming seizure.

Apple Watch HRV improvements, mental health research, and state of digital health apps

The Apple Watch is the fastest-growing smartwatch today. According to a report by Counterpoint Research, the Apple Watch captured 36.1% market share in Q1 2022, more than three times that of Samsung’s market share.

Starting with iOS 13 and watchOS 6, Apple made several improvements to its HRV processes.

The company removed some limitations associated with capturing the HRV data via Apple Watch. This was the first watchOSto to provide RR intervals access via the Health app. Before that, you could only access the SDNN intervals.

Apple has been steadily improving upon the HRV features with recent watchOS upgrades. With watchOS 9, your Apple Watch’s HRV sampling frequency can sharply increase when you turn on the Afib history feature.

In countries such as China and India, human resources for professional mental health treatment are often low, and the stigma around mental health conditions is high.

These geographies also happen to be one of the fastest-growing wearables growth regions.

While the global market for smartwatch shipments grew at just 13% YoY in 2022, India saw a 347% year-on-year jump, making it the second biggest smartwatch market in Q2 of 2022. Apple happens to be the top brand.

Apple, on its part, has partnered with UCLA to study stress, anxiety, and depression, with Apple Watch and iPhone data. A total of  3,000 volunteers will be tracked in a study that started in 2021.

• Apple iPhone Features could help detect depression and cognitive decline

A pilot phase that began in 2020 recorded data from 150 participants. The three-year Apple collaborative study with UCLA was announced in August 2020.

The company is also partnering with Biogen to study if cognitive decline can be observed using accelerometer data and if there is a path to spot mild cognitive impairment.

The demand for Mental Health apps is also growing at a staggering rate.

There seems to be an app for everything these days, and mental health is no exception. Deloitte Global predicts that global spending on mobile mental health applications will reach nearly US$500 million in 2022.

Data shows that the COVID-19 pandemic has exacerbated mental health concerns and triggered declines in well-being, with a dramatic rise in the prevalence of problems such as depression, anxiety, post-traumatic stress symptoms, and stress.

Apple integrated mindful meditations as a core feature into its Fitness+ platform recently. There are currently more than 390Meditation modules on the Fitness+ platform.

With the latest iOS 16 and watchOS 9, you do not need to own an Apple Watch to take advantage of this platform.

• Apple Fitness+ subscription for iPhone v.s Apple Watch: what’s different?

Companies such as Happify Health have become pioneers in prescription digital therapeutics for mental health.

Last year the company announced the launch of Ensemble, a transdiagnostic prescription digital therapeutic for treating patients 22 years of age with major depressive disorder (MDD) or generalized anxiety disorder (GAD).

As more research is conducted around HRV as a potential digital biomarker for Mental health and the adoption of wearables such as the Apple Watch grows across the world, we will be able to offer better healthcare equity, particularly in regions where mental health is still a stigma and resources are scarce.

Sources and References:

1. How Meditation helps with Depression – https://www.health.harvard.edu/mind-and-mood/how-meditation-helps-with-depression
2. Antidepressant use in United States – NY Times https://www.nytimes.com/2018/04/07/health/antidepressants-withdrawal-prozac-cymbalta.html
3. https://bmcpsychology.biomedcentral.com/articles/10.1186/s40359-016-0138-z
4. Deloitte Insights on Mental Health Technology – https://www2.deloitte.com/xe/en/insights/industry/technology/technology-media-and-telecom-predictions/2022/mental-health-app-market.html