HRV & Your Mind

So you’ve begun to grasp how heart rate variability (HRV) is associated with the stress response, especially the stress of physical training…    

Believe it or not, the science of HRV is not a recent field of study that’s been tapped into to create fitness apps.  Its origin dates back to the 1950’s and 60’s with the launch of the Russian space program, but that’s material for another day.

No, the science of HRV that’s developed over the past 60+ years is much more vast and complex than it’s often represented.  Here are just a few of the areas that HRV has been used as research tool:

Diet

Post-traumatic stress disorder (PTSD)

Cardiovascular disease

Epilepsy

Will power

Sleep quality

Gum disease

Emotion/mood

Diabetes

Effects of air pollution

Allergies

The list goes on.

One of the fields that has received more attention recently is how HRV relates to psychiatric conditions: what is the association between the brain and autonomic control?

How can we look at HRV to better understand psychiatric conditions such as PTSD, clinical depression, and anxiety disorders?

This area was discussed by Gary G. Berntson, PhD and John T. Cacioppo, PhD in their chapter of the book “Dynamic Electrocardiography.”

Before digging into the findings behind this chapter, we first have to construct a new understanding of the autonomic nervous system (ANS)...

The ANS: Not So Linear After All

Briefly, the ANS is comprised of two branches:

1. The sympathetic nervous system (think “fight or flight”)

2. The parasympathetic nervous system (think “rest and recover”)

These branches are often misrepresented as being mutually exclusive: when there is an increase in sympathetic control, parasympathetic control decreases and vice versa, all in the interested in maintaining a homeostatic internal environment.

This tug-of-war representation of sympathetic and parasympathetic control has been termed “the Bipolar-Continuum,” as shown here: 

(Berntson et al 2004)

 As you can infer from the model, autonomic regulation that is closer to one side (either sympathetic or parasympathetic) is further from the other side.  However, this is not always the case…

A better model of the autonomic nervous system is called “Bivariate Autonomic Plane:” 

(Berntson et al 2004)

By understanding autonomic control as existing in a bivariate plane, we can see that sympathetic activity may increase or remain the same as parasympathetic activity decreases, and vice versa.  A change in the contribution of one autonomic branch does not necessarily equate to an inverse change in the other branch.

Thus, HRV could potentially increase in three ways:

1. Through a decrease in sympathetic activity

2. Through an increase in parasympathetic activity

3. Through a combination of 1 & 2

Mental vs. Physical Stress: It's Personal

When Berntson and Cacioppo examined how an orthostatic challenge (physical stress; moving from sitting to standing) and a psychological challenge (mental stress; speech tasks, mental math, reaction time) affected the autonomic nervous system, their findings were very different at the group versus individual level:

At the group level, autonomic responses to the physical challenges were similar to responses mental challenges: there was a decrease in parasympathetic control and an increase in sympathetic control. 

On the individual level there was little difference between ANS responses to the physical challenge; everyone’s autonomic nervous systems reacted in almost identical manners. 

However, when it came to the mental challenges autonomic responses varied much more drastically from person-to-person:

Some individuals experienced little-to-no decrease in parasympathetic control and an increase in sympathetic control.  Others exhibited large decreases in parasympathetic control with only small increases in sympathetic control. 

Why were these individual responses so different when the physical stress elicited almost identical changes in autonomic control?

The answer is simple- you knew it already:

People differ in the way they respond to mental stress.  

But why?

The Brain: Different Regions, Different Responses

When our body carries out physical challenges, the autonomic nervous system maintains homeostasis through reflexive activity signaled by the brainstem regions. 

 

(National Institute on Drug Abuse 2012)

These lower brain regions trigger seemingly automatic changes in measures such as heart rate and blood pressure. 

However, higher processing regions of the brain implicated in behavior, such as the limbic and forebrain areas, have been shown to communicate directly with the brain stem and spinal cord to override more reflexive autonomic control. 

Thus, mental math and other cognitive tasks impact our autonomic nervous system differently depending on how stressful these higher order brain regions perceive them, and the variations in perceived stress will be highly personal.  

HRV in PTSD & Anxiety Disorder

Given that mental processes affect our autonomic nervous system in a highly individualistic manner, we would expect psychiatric disorders such as PTSD and anxiety to be manifested in a complex and personal way. 

However, there are some characteristics of how both PTSD and anxiety impact the ANS and HRV which are largely universal. 

In the case of anxiety disorders, it is common to see diminished capability of the ANS to change in response to perceived environmental cues, threatening or non-threatening. 

Basically, the autonomic nervous system loses its ability to seamlessly adapt by adjusting sympathetic and parasympathetic contributions. 

Another common finding associated with anxiety is an overall reduction of HRV through a decrease in parasympathetic activity; the body’s ability to rest and recover is compromised. 

This reduction in the parasympathetic component produces what’s called a “loss of autonomic flexibility,” meaning the ANS can only increase or decrease the sympathetic contribution to adapt rather than both the parasympathetic and sympathetic components. 

In the case of depression, it is also common to see an overall decrease in HRV through a reduction in parasympathetic activity. 

Beyond the problems that anxiety and depression represent, a decrease in HRV is concerning in that it is a negative predictor for cardiovascular disease and survival post-heart attack.  

Future Direction

The field of research focusing on autonomic control and mental processes would benefit from further interdisciplinary study.  While the nervous system, emotional processes, and psychiatric disorders have been studied extensively, the amount of research examining their association is still small and requires further development. 

Given that psychiatric conditions are highly individualistic, it is also important to closely characterize how such conditions affect autonomic control to see if common patterns can be observed.  

References

Berntson, G., & Cacioppo, J. (2004). Heart Rate Variability: Stress and Psychiatric Conditions. In Dynamic Electrocardiography (pp. 56-63). Elmsford: Futura.