Heart rate variability (HRV) is defined as the variation in the interval between consecutive heartbeats. It is under the influence of the autonomic nervous system (ANS). The latter is composed of two branches: the parasympathetic and the sympathetic nervous system. HRV is regulated by the ANS via a region in the base of the brain called the hypothalamus which either relaxes or stimulates the different body functions depending on external stimuli such as exercise or sleep.
Imbalances in the ANS can be identified by assessing HRV. For instance, if the body is in the “fight or flight” or sympathetic mode, HRV is reduced. Conversely, if the predominating mode is the “relaxed” or parasympathetic mode, HRV is increased.
HRV is a crucial measurement as it not only predicts the risk of cardiovascular disease but also allows for the objective assessment of emotional health. In general terms, a high HRV reflects good cardiovascular health and a lower risk of developing mental illness due to improved stress tolerance.
The relationship and synchrony between breathing and HRV is known as respiratory sinus arrthymia (RSA). RSA is the product of interaction between baroreceptors (pressure sensors) and the medulla oblongata of the brainstem, where cardiac and breathing rhythms are generated. In fact, the ANS plays a vital role, via the vagus nerve, in the interplay between breathing and the cardiovascular system.
In simple terms, the two branches of the ANS are under the control of the breathing centres and exert opposite effects on the heart. During the inspiratory phase of the cycle, HRV is reduced due to the inhibition of the vagus nerve and the consequent promotion of the sympathetic system increasing the heart rate. On the contrary, during exhalation, the vagus nerve is reactivated slowing down the heart rate increasing HRV.
These variations are normal and occur in the absence of respiratory disease. In fact, they are in place to help achieve homeostasis by promoting efficient gas exchange.
Obstructive Sleep Apnea (OSA) is a breathing disorder characterized by periods of night time breathing interruption due to total or partial upper airway collapse. The distruption in ventilation leads to hypoxemia (low levels of oxygen (02)) and hypercapnia (high levels of carbon dioxide(C02)). This is detected by chemoreceptors (sensors that detect changes in C02 and 02) that feedback to the central nervous system resulting in the upregulation of the sympathetic branch of the ANS changing breathing patterns and consequently decreasing HRV.
Therefore, chronic OSA can lead to a chronically upregulated sympathetic response and a low HRV as elucidated by a systematic review on 513 participants conducted in 2019. Hence, the resultant ANS dysregulation is the main culprit for the cardiovascular implications of OSA. That being said, this can potentially be reversed by the various available treatment modalities for OSA.
HRV can be trained and increased by breathing exercises- simple non-invasive strategies that impact the ANS.
For instance, resonance breathing is a form of slow diaphragmatic breathing (at a rate of 5-7 breaths/min) that maximizes HRV by stimulating the vagus nerve calming the body’s sympathetic system. All humans have their individualized resonance time. Therefore, it is prudent to identify the duration of your own breathing cycle, by using special instruments that monitor HRV while performing a breathing exercise. That being said, there is currently no consensus on whether breathing at an individualized rate versus a specific set rate results in a greater improvement in HRV- more studies are needed to clarify this.
A popular form of slow deep diaphragmatic breathing exercise is Yoga training. A meta-analysis, conducted in 2018, evaluated the effects of yoga training on HRV. In conclusion, the analysis indicated a significant improvement in HRV and a decrease in stress levels in participants that performed atleast 60 mins of yoga/week.
There is a clear evidence that HRV is influenced by breathing exercises, however, debate remains regarding the best breathing exercise for improving it. A recent randomized control trial conducted in 2021 evaluated the best breathing technique to improve HRV. Participants that attempted breathing at 6 breaths/min along with those in the soothing rhythm breathing achieved a significant increase in HRV compared to the group that watched relaxing nature videos. That being said, there is a need for a systematic review comparing different breathing techniques to confidently ascertain the best breathing exercise for you.
Heart rate variability in adults with obstructive sleep apnea: a systematic review
Obstructive sleep apnea syndrome and autonomic dysfunction
The physiological effects of slow breathing in the healthy human
A Practical Guide to Resonance Frequency Assessment for Heart Rate Variability Biofeedback
Acute effects of resonance frequency breathing on cardiovascular regulation
Effects of Mind–Body Exercises (Tai Chi/Yoga) on Heart Rate Variability Parameters and Perceived Stress: A Systematic Review with Meta-Analysis of Randomized Controlled Trials
Integrating Breathing Techniques Into Psychotherapy to Improve HRV: Which Approach Is Best?https://www.frontiersin.org/articles/10.3389/fpsyg.2021.624254/full