Naturopathic Medical Student Adam Sadowski back for more with this insightful post on anxiety, the autonomic nervous system and heart rate variability!
With the recent passing of World Mental Day on October 10th, (which we can all agree isn’t a one day deal) and Dr. Abbott’s passion for all things meditation and mental well being, it makes sense to have a little (or lengthy) discussion about a nearly ubiquitous human experience: Anxiety, and more specifically it’s association with heart rate variability, or HRV for short.
General anxiety disorder and panic disorders are some of the most common mental conditions in the United States (1,9). The difference between the two is that anxiety is mainly characterized as excessive worrying that is out of control, and panic disorder is characterized by unexpected panic attacks that can come on at any time (1,10). Unfortunately, like many mental health conditions, the medical community as a whole is not entirely sure as to what is causing this, and understandably so. The causal factor may not be so black or white, as anxiety is paired with multiple conditions creating a chicken or the egg scenario.
Anxiety has been associated with but not limited to fibromyalgia3, chronic obstructive pulmonary disease (4), PTSD (5), psychosis (6), depression (7,8), borderline personality disorder (19), and cardiovascular disease (8,21). (This list could take the whole post, so we will stop there).
A few interesting theories have been developed to explain these relationships such as the polyvagal theory (9), neurovisceral integration model (9), and a compromise to the central autonomic network (5).
Before we discuss these theories/models, it is important to understand the role of the nervous system, and yes, it’s important to understand the intricate relationships between both anxiety and HRV to our fascinating nervous system, so buckle up…
From an overly simplistic standpoint the nervous system is divided into two main branches: The Central Nervous System (commonly referred to as the CNS), and the Peripheral Nervous System (commonly referred to as the PNS). The CNS consists of the brain, brainstem, and spinal cord while the PNS consists of everything that isn’t the brain, brainstem or spinal cord.
From there, the PNS can be divided into the Somatic Nervous System and the Autonomic Nervous System. The somatic nervous system is mainly involved with controlling skeletal muscles and you have more conscious regulation over it, while we have less conscious control of the Autonomic Nervous System as it is primarily involved with organ functions11 (allowing us to not think about breathing or contracting your heart 24/7).
** Note: This “automatic” regulation of breathing is not an EXCUSE to avoid becoming aware of one’s breathing and the realization that through mindful attention of one’s breathing AND conscious manipulation of such breathing, one may likely benefit in ways we will discuss later on in this post.
The autonomic nervous system is further subdivided to involve 3 functions: sympathetic, parasympathetic, and enteric functions with the latter often being grouped into parasympathetic. With that being said, the sympathetic aspect can be thought of as a “fight-or-flight” response, and the parasympathetic aspect can be thought of as the “rest-and-digest” response. If the sympathetic component of the autonomic nervous system is overly activated, we get things like hyperventilation, palpitations, dizziness, tremors, chest discomfort, sweating, hot flashes, and GI distress (10). When the parasympathetic component is more active, our heart rate decreases/normalizes, we are much more relaxed, we salivate, and we are able to digest our food properly along with other functions that are not necessarily noticeable.
It’s important to realize, that because the autonomic nervous system has two functions it’s trying to maintain a state of homeostasis where neither system is not excessively overriding the other (12). Again, this is not a black or white situation, and the interaction between the two systems is much more complicated than this post allows for. However, as we will see, when there is a disruption to this homeostasis, problems such as anxiety might arise. The wonderful Dr. Mike T Nelson uses the analogy of a car break and a gas pedal where “stepping on the gas” can be synonymous with sympathetic tone, and pressing on the brake is synonymous with parasympathetic tone. The harder you press on either pedals, the greater the tone respectively. Although parasympathetic sounds great, you don’t want to strictly press on the brakes because you won’t move, much like if you only press on the gas you’ll end up joy riding until you eventually crash. So as you can see, there is a sweet spot that we want to (ideally) stay in when it comes to a sympathetic or parasympathetic state. If you’re too sympathetic you’ll constantly feel stressed out, and if you’re too parasympathetic you might just want to spend all day in bed and never move.
Now that we have a basic understanding of the nervous system, the first theory, the polyvagal theory, is an evolutionary model that suggests the autonomic nervous system evolved in mammals to suit their social environment via the use of the vagal nerve which is the head honcho of the autonomic nervous system. When parasympathetic function is working adequately, vagal stimulation primarily sends parasympathetic signaling allowing for slower heart rates, and better digestion. On the flip side, when vagal stimulation is overridden with sympathetic signaling, you start to feel nervous, get butterflies in your stomach, sweaty palms, and feel like your heart is going to fly out of your chest when giving a presentation to a large audience. The importance of the vagus nerve in the polyvagal theory according to Alvares et al, is in “promoting engagement, or disengagement with an individual’s social environment”9 and if someone thinks they are in a safe environment, defensive circuits of the sympathetic “fight-or-flight” are overridden by parasympathetic signals (13). Similarly, if someone perceives their environment as unsafe for whatever reason (ex. a large audience), the ability to “express positive social cues is compromised” (13). Note this can ALSO BE an entirely unconscious process of “perception”, think: air and sound pollution and even EMF; live in the city, near airports or sleep next to your Wifi?). It might be that impairment of these neural processes causes people to not know if their environment or the people they are around are safe, causing the development of anxiety (5), heighten arousal, or at least anxiety like symptoms.
The neurovisceral integration model, which is similar to the polyvagal theory, suggests that the inability for the vagus nerve to inhibit sympathetic function prevents regulation of inflammatory processes, glucose regulation, and HPA axis function (13). Remember, the autonomic nervous system has nerves going to all organs and glands throughout the body, and whether they are more sympathetic or parasympathetic dominant, may determine if these organs or glands will up-regulate or down regulate the production of various substances. If there is too much sympathetic tone, this can lead to excess production/secretion of inflammatory cytokines, impaired fasting blood glucose by causing the liver to pump out glucose into the bloodstream, and a variety of other hormonal signals, which may be one explanation as to why stress is associated or possibly the cause to many chronic diseases.
Lastly, the central autonomic network, (which isn’t really a theory) is a network within the brain which regulates adaptive responses to various stimuli (14). If we have a more robust nervous system that allows us to adapt to a multitude of situations, perhaps we can express a more parasympathetic state during times of non-threatening stress and thus have an experience of lessened anxiety. If you grew up in a larger family with constant arguing at the dinner table, perhaps over time your central autonomic network has been trained to create sympathetic responses to the surrounding stimuli such as multiple people in one room, the food that was being served, the yelling, etc. Now you get a little (or very) anxious when you go to parties because there are lots of people in one room, loud noises, an aversion to a specific food/food group or smell. Maybe it’s possible to train our nervous system to eventually express more parasympathetic tone during these situations over time such that we reduce our levels of anxiety in these environments.
This robustness of the nervous system is what brings me to the topic of HRV. Most people know of HRV as a tool used to monitor recovery for athletes and exercise enthusiasts in response to various stress loads, however, this same tool may be beneficial for monitoring relative states of hyperarousal or anxiety.
Simply put, HRV measures the R waves between heart beats (16). What do I mean by R waves?
This is what I mean by R waves.
In cardiology when we try to visualize the heart beating, we rely on ECG strips which can reveal to us a great deal of information about the heart in regards to how it is beating (are they normal beats? Are there arrhythmias present? Is someone suffering a heart attack? Etc.). There are 5 wave-forms that an ECG will show: “P-wave”, “Q-wave”, “R-wave”, “S-wave”, and lastly a “T-wave”. In a normal heart, this pattern almost always occurs where the first little bump is the “p-wave”, followed by a subtle downward “q-wave”, followed by the large upward rising “R-wave”. Following the R wave is the “s-wave” and we end with the last little hump of the “t-wave” before starting all over again. One of the reasons HRV is looking at the R-R intervals is because of how easily shows up on the ECG.
Why do we care about these intervals? It goes back to the point I made about not wanting to be too sympathetic or too parasympathetic. If you are too dominant in one or the other, your heart rate won’t have any variability to it: For example…
That flat line (indicative of no contractions) is not variable. In fact, it’s the complete opposite, it’s entirely predictable and going at a constant rate (0 beats per minute). Since there are so many interactions between neural, hormonal, and mechanical control systems both in the peripheral and central nervous system, a normal resting rhythm is not exactly monotonous (15).
What is preferred to be seen is something like this:
Granted, we don’t want to see so much variability that it looks entirely erratic and emergency medical interventions need to be used, it is very small millisecond differences between intervals that are sought after (i.e. 1.68 milliseconds at one interval and 1.66 milliseconds at the second interval followed by 1.7 milliseconds at the third, etc.)
HRV can further be measured in 3 different ways, and they don’t require an ECG machine and a host of electrodes to hook yourself up to. There are smart-phone apps such as heart math and ithlete that can hook up to a variety of devices such as a polar monitor strap depending on the program.
The first way HRV can be measured is through a time domain, the second way is through a frequency domain, and lastly, it can be measured through what is referred to as a non-linear domain.
Time Domain looks at multiple R-to-R intervals over time (so how much time takes place between each wave). If there is more variability, the time between waves will be slightly longer or shorter. This also means your heart rate is slower and more indicative of a parasympathetic state. When vagal parasympathetic tone is in a “sweet-spot” the variability from one R wave to the next increases which can be used as a marker for health and recovery of the autonomic nervous system. It’s a good way to get an overall representation of HRV status (17). In some studies, those with anxiety have shorter time domain HRV compared to “healthy” controls, however, this findings may also be skewed by relative publication bias (5).
Frequency Domain is useful when looking at how different one wav form is from another (so quite literally, the beat to beat difference). The main components that seem to be represented in the literature from frequency domain are the high frequency (largely parasympathetic influence), low frequency (largely sympathetic influence), very low frequency (a mix of the two, although there seems to be disagreement with researchers in it’s use ), and the low frequency to high frequency ratio.
Non-linear Domain is more complicated to explain and is more important with regards to collecting data, and from what I have read in the literature thus far, is not reported as much as frequency or time domain. Regardless of the method used to obtain data, HRV is done for anywhere from 2 minutes to 24 hours (5).
So what does all this HRV stuff mean when it comes to anxiety?
It seems to be, that across the board those with anxiety have a decreased heart rate variability (8,16,18,20,22), indicating a more stressful state (which also makes sense as anxiety can often be a comorbidity of stress and vice-versa). Those with general anxiety disorder and social anxiety disorder have a decrease in high frequency HRV (5), and interestingly people with anxiety do not have much of a difference in low frequency HRV compared to “normal” people although there seems to be quite a bit of heterogeneity across studies which might explain this5.
I do want to make a disclaimer in that, almost all of these studies I have referenced had pretty low numbers of participants and much of this information is a correlation not necessarily a causation. We do not know EXACTLY why people who have anxiety have lower HRV compared to people who do not, however, there are still some interesting reasons that were discussed above that may cause us to think about sympathetic and parasympathetic regulation as a factor.
If we follow the model that those with anxiety tend to have depressed HRV and higher levels of stress, reducing stress and increasing HRV theoretically could be used to improve, or dare I say, cure one’s anxiety. The benefit of using HRV is that it will give you a quantitative measure to help monitor your stress each day. If the HRV is going up, your body may be coping better with stressors and can help to see if what you are doing is helping to alleviate your anxiety. By having that quantitative data, it may help to encourage you to keep doing whatever it is you are doing or what may need to be improved.
Perhaps increasing HRV can be possible through pharmaceutical agents, practices such as yoga or meditation, exercising properly, play (whatever that means to you), adequate sleep, nutrition and/or supplementation. These are things that need to be investigated further and will be addressed in a future post. Regardless of the cause or treatment, it needs to be dealt with as it is currently costing the world one TRILLION dollars annually, (2) and it is projected to increase to six trillion by 2030 (9).
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