The effects of yoga on heart rate variability
There is growing evidence that distress alters the autonomic balance and can bring about adverse effects on our physical and mental health (Thayer 2009, 2012 and Lehrer). There exist numerous subjective questionnaires to measure distress, however, fewer non-invasive objective measures. Heart rate variability (HRV) has been suggested as a suitable proxy for distress, as it reflects the autonomic nervous system influence on the heart (Task Force of the European Society of Cardiology and the North Americal Society of Pacing Electrophysiology 1996).
Distress has become a growing public health concern in the past decades. In an achievement-oriented globalized world with inter-human connection through screen and social media, podcasts, worldwide news, and international standards of success within every field, we have seen a rise in lifestyle-related symptoms of anxiety and depression, commonly referred to as distress. The global cost of mental health is estimated at US$6 trillion by 2030 (Bloom et al., 2011).
Eustress vs. Distress
Stress is a normal part of everyday life (WHO). Hans Selye (1907–1982) is known as the father of stress research. He distinguished between acute and chronic stress, the latter has the potential to bring about ‘general adaptation syndrome’, which is also known in the literature as Selye’s Syndrome. The syndrome divides the total response from stress into three phases: the alarm reaction, the stage of resistance and the stage of exhaustion. Dr. Selye recognized that stress is simply a part of everyday life for each of us, but that we all respond to or experience it differently from one another. When the stress becomes negative and brings about symptoms of anxiety and depression, we call it distress. Note, there is a difference between eustress, which is a term for positive stress, and distress, which refers to negative stress.
The history of HRV and the discovery of RSA
The first accreditation to HRV is often given to Natural philosopher and inventor, Stephen Hales (1733), who noticed interbeat variances in heart beat cycles in a horse.
By the 19th century, it was well established that the heart beat tended to accelerate during inhale and decelerate during exhale, known as physiological respiratory sinus arrhythmia (RSA), and one could measure the interbeat time between the heart beats. Hering (1910) proposed a functional relationship between the vagus nerve and the RSA. Bainbridge (1920) sought to explain these cycles through changes in baroreceptors in the thoracic cavity. Eppinger and Hess (1915) first mentioned the link between autonomic dysfunction and pathophysiology. Hon (1958) was the first to draw the linkage between fetal distress and HRV, mediated by the brain-vagal-heart pathway.
HRV theory
We now know that RSA is a slight beat-to-beat variation due to changes in the vagal tone throughout the respiration cycle; the heart rate temporarily increases during inhalation and decreases during exhalation. The underlying cause is a short-term increase in intrathoracic pressure when the diaphragm relaxes during exhalation. This causes a short term increase in atrial pressure, causing a lower venous return to the heart as well as an increase in blood flow. Baroreceptors register these changes, and, in turn, through complex negative feedback loops signals the vagus nerve to slow the heart rate momentarily. This deceleration of our heart rate with every exhale makes up the foundation for RSA and HRV; In a a healthy human heart that is not much affected by distress, there should be RSA and relatively high HRV. If the body is affected by distress and is in a sympathetic state dominated by a rise in norephrine and cortisol, then the heart beat is overridden - the heart rate not only increases but the HRV falls. A non-stressed body has higher HRV - and, hence, HRV can serve as a proxy for distress, due to a positive correlation between HRV and parasympathetic activity.
The heart rhythm
In a healthy heart, the rhythm is determined by the SA-node. A sinus rhythm of about 60 beats per minute is considered average. The SA-node is modulated by innervation from both sympathetic and parasympathetic paths from the autonomic nervous system. Acetylcholine released from the vagus nerve will slow the heart rate. On the other hand, norephrine from the sympathetic branch will speed the heart rate. In addition to direct innervation, the heart rate can be altered through humoral pathways such as the hypothalamic–pituitary–adrenal axis and its release of adrenomedullary catecholamines and the renin-angiotensin system. These humoral processes have been seen even after denervation of the heart, for instance in cardiac transplantation patients in which there is no contact with the vagus nerve for some time after the surgery (Berntson et al 1997; Bernadi et al 1989).
HRV measurement and clinical significance
HRV is the variability between each successive heartbeat (interbeat intervals) and mirrors the interplay between parasympathetic and sympathetic influences on the heart rate as well as complex and not fully understood cardiac-brain interactions and autonomic responses. Clinicians measure these interbeat intervals in milliseconds (ms), using the R-R interval on an ECG. One can also use watches to measure pulse variation.
HRV appears to be a powerful predictor of disease and death, as HRV is correlated with indicators of both physical and mental health. Beauchine 2015 and Kemp 2013 found that patients with distress disorders display reduced resting-state HRV. Lower HRV has also been reported in patients with schizophrenia (Berger et al 2010), attention deficit hyperactivity disorder (Buchhorn et al 2012), conduct disorder (Beauchaine et al 2007, and bipolar disorder (Henry et al 2010, Lee et al 2012), and depression (Kemp et al 2010). HRV has also been found at lower levels among a wide range of clinical populations such as patients with epilepsy (Lotufo 2012), Type 2 diabetes (Benichou et al 2018), Parkinson disease (Alonso 2015), rheumatoid arthritis [22], atherosclerosis [23] and Crohn ’s disease [24]. In a study involving 1882 subjects with ages ranging 21-76 years, a decreased HRV was identified as a significant predictor for the development of diabetes, cardiovascular disease and early mortality [25]. Overall, it appears that a low HRV is associated with a wide range of health conditions and is even associated with an increased risk of early mortality.
Determinants of HRV
There have been attempts to standardize normal range values of HRV, but it has been difficult as HRV is dependent on so many factors and also varies greatly between individuals (Tsuji et al 1996). One study found that age and heart rate were the two main determinants of HRV (Tsuji et al 1996). Other key determinants include smoking, exercise, and use of medication such as beta-blockers (Tsuji et al 1996). A better way to use HRV as a tool in monitoring health status could be for each individual to use her or his own measurements to compare over time. Below is the table for suggested normal range values.
Ways to manipulate your HRV
Exercise
Cardio exercise has been shown to increase HRV. Grässler et al (2021) found in their meta-analysis an effect of endurance, coordinative, and multimodal training on HRV in an elderly healthy population. They found no effect of resistance training on HRV. This is supported by the 2005 meta analysis from Sandercock et al, who found significant increases in HRV due to exercise. A recent meta analysis showed that exercise training increased HRV in patients with diabetes mellitus type 2 (Picard et al 2021).
Yoga (breathing, meditation, physical postures, and philosophy combined)
There only exists one meta-analysis on the effects of yoga on HRV, which reports no effect of yoga on HRV (Pozadski et al 2015). The meta analysis highlights the poor quality and methodological drawbacks in most of the included RCTs. No newer meta analysis on yoga and HRV exists.
Meditation
Two meta analyses on meditation fail to identify a link between meditation and HRV (Brown et al 2022; Radmark et al), claiming that there is no evidence of an association between meditation participation and changes in HRV. Many single RCTs report on significant effects of yoga on HRV, however, just as many report no effect. In their meta-analysis from 2022, Waleed et al, reports that meditation caused short-term changes in HRV, but there is limited evidence for significant long-term effects. There exist no study that explores the effects of various forms of meditation on HRV.
Slow breathing exercises
A recent well performed RCT by Kippusary et et al 2020 found a small but significant increase in HRV among healthy adolescents that did slow yoga breathing against a passive control group. This is supported by the systematic review of Zaccaro et al 2018 who not only reports a raise in HRV from slow breathing, but also increased alpha power in EEG and higher cortical and sub-cortical activity as shown by MRI-scans. Another RCT reports slow breathing pace (respiratory rate 6 breaths/min) for 5 mins decreased heart rate and blood pressure, however, no effects found of the same practice when administered Buscopan (anticholinergic), suggesting that pranayama may cause a parasympathetic boost (Pramanik et al 2009). A recent meta analysis suggests an effect of voluntary slow breathing on HRV (Laborde et al 2022), they paced the breathing at around 6 breaths/minute.
Chanting
'OM' chanting indicates limbic deactivation, possibly mediated through the auricular branches of the vagal nerves (Kalyani 2011), but no study has specifically been done on the effects of HRV. A recent RCT by Inbaraj et al (2022) suggests that Om chanting performed by experienced yoga practitioners increases HRV and vagal tone.
Conclusion
Meditation, breathing, and chanting may have a positive effect on HRV. Clinical relevance remains to be fully understood. Moreover, comparison between studies is hard due to the various statistical calculation variations and lack of data set standardization. More studies are needed. And this author would argue that subjective wellbeing and health status is key; in a society where we all benefit from taking a step out of the mind and into the body.
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