A healthy heart is not a metronome. Even though most of us can follow our heartbeat with ease, in between each heartbeat is a period of time that varies in duration from one beat to the next. This difference in time between any consecutive heartbeat is known as heart rate variability (HRV), and researchers are increasingly using it as a tool to understand mental health.
HRV reflects the delicate balance between two branches of our nervous system, the sympathetic (“fight-or-flight”) and parasympathetic (“rest-and-digest”) arms. These branches operate largely outside of conscious control and function to maintain balance within our body. The interaction between these systems is what keeps us from sweating when we are relaxed, for example, or causes us to sweat when our internal temperature gets too high. In both cases we aren’t consciously deciding whether to sweat or not. Instead, our nervous system is responding appropriately to our environments.
In the case of HRV, the sympathetic branch signals your heart to speed up and the parasympathetic branch signals your heart to slow down. This competition between “speed up” and “slow down” signals is what gives rise to HRV. Generally, higher HRV, meaning slightly less consistency between consecutive heartbeats, suggests that your sympathetic and parasympathetic arms are balanced and ready to adapt to quickly changing external demands. Lower HRV, meaning more consistency between consecutive heartbeats, suggests that one branch, typically the sympathetic “speed-up” signal, has more influence than the other. Higher HRV can be thought of as more flexibility within the nervous system. Your HRV is probably higher when you’re asleep or watching that favorite TV show than when you’re cramming for an exam or chasing kids around the house.
One of the major roles of the (para)sympathetic nervous system is to ensure that our bodies can adaptively respond to stress. HRV operates largely to this end. HRV changes during stress and the timing between consecutive heartbeats is more variable when resting than during some sort of stressor. This suggests that at rest, the body is prepared to flexibly respond to changing environmental demands. During a stressful time, however, the body is focused on resolving the heightened emotional and biological response and less concerned with being able to respond to future environmental changes. HRV re-elevates once the stressor has been resolved.
Owing to the importance of HRV in adaptively responding to stress, researchers have begun to use HRV to try and understand aspects of psychological disorders. There are two theoretical reasons HRV and psychological disorders may be linked. First, major stressful events often precede common psychological disorders such as anxiety or depression (e.g., loss of a loved one, divorce, starting college, or a new job). Second, many psychological disorders involve some amount of nervous system dysregulation. Anxiety, for example, is often associated with too much nervous system activity, whereas depression might be associated with too little. HRV might offer a window into understanding the role of (para)sympathetic functioning in psychopathology and ultimately inform treatment and prevention efforts.
One way to understand how HRV is impacted in clinical populations is to conduct meta-analyses on HRV within psychological disorders. A meta-analysis is a specialized type of data analysis, where data from many different individual studies on a topic are compiled and one metric that combines all the studies’ findings is created. Importantly, most meta-analyses on HRV in psychopathology examine HRV at rest when subjects are not performing any sort of task. This gives researchers an idea of how the system is functioning in general on a day-to-day basis.
In major depression, a psychological disorder characterized broadly by low mood and other behavioral, affective, and cognitive symptoms, a meta-analysis of over 4,500 individuals found support for reduced HRV. Similar findings exist for anxiety disorders, a class of psychological disorders characterized broadly by persistent and irrational fear or worry. A meta-analysis of over 4,300 people found lower HRV among patients with anxiety. Additional meta-analyses similarly suggest HRV is reduced in individuals with alcohol use disorder, schizophrenia, and autism spectrum disorders. In psychological disorders the heart sounds more like a metronome than it should.
These meta-analyses suggest that in several psychological disorders, the branch of the nervous system outside of conscious control looks like it is stressed when patients are actually resting. Reduced HRV reflects a physiological bias towards the sympathetic (“fight-or-flight”) branch of the nervous system. Overactivation of this system is a risk factor for many poor outcomes, including sleep loss, weight gain, and muscle tension, all of which are symptoms of both physical diseases and psychological disorders.
Characterizing HRV in psychological disorders is the first step in understanding its role in psychopathology. The next step is to examine how treatment, whether via medications or psychotherapy, impacts the outcome of interest – in this case, increasing HRV. Studies have begun to do this, albeit with mixed results. A second outstanding question is what came first, the psychological disorder or reduced HRV? Clarifying the developmental course of reduced HRV and psychological disorders will be especially important for informing future prevention and intervention efforts.
When you visit your doctor for your annual checkup, you likely have your blood pressure checked even if you have never had blood pressure issues to identify potential cardiovascular disease. Perhaps one day we will we reach a point where a brief test of HRV could be used similarly to predict or identify mental health symptoms early on.
Edited by Chloe Holden and Elizabeth Rosdeitcher