In my last post about this study (1), I described how four months of pranayama practice affected respiratory function in elderly yoga practitioners. Today, we take a look at how the practice of bhastrika impacted heart rate variability and the functioning of the autonomic nervous system.
Before delving into heart rate variability, a review of the autonomic nervous system is in order. The autonomic nervous system regulates numerous systems of the body, controlling circulatory, respiratory, digestive, endocrine, immune and metabolic processes. In general, it manages internal functions that occur without conscious control. That’s where the name comes from: early researchers envisioned it as acting autonomously, or independently, of conscious volition. In fact, however, all parts of the nervous system are integrated and function cohesively. It’s true that most people can’t voluntarily alter their heartbeat as you might vary the rhythm of a tapping finger. Nevertheless, input from the conscious part of your brain as well as from emotional centers affects the autonomic nervous system, and vice versa. Think of how your heart beats as you wait to go on stage to make a major presentation, and how you could choose to slow it down by taking several long, conscious breaths. That indirect linkage between volitional action and autonomic function explains how practices such as yoga can influence internal physiology.
The autonomic nervous system is generally divided into two major divisions, the parasympathetic and the sympathetic. You’ll often hear the parasympathetic division referred to as the “rest and digest” system, and the sympathetic division as the “fight or flight” system. As the catch phrase suggests, the sympathetic division prepares your body for action, priming you to react to dangerous or stressful conditions. Your liver releases glucose (blood sugar), your breathing speeds up, air passages in your lungs widen, your heart pounds, and systolic blood pressure rises. Those responses are vital for survival when you’re faced with an immediate threat. Because exercise is also a stressor, albeit a controlled one, the same mechanisms kick in when you’re moving through a series of vigorous sun salutations or working out in the weight room.
Problems start when stress becomes overwhelming or overly prolonged. Stressors like car alarms, stock market crashes and tax deadlines aren’t physically threatening, yet the sympathetic nervous system reacts as if they were. And although short-lived spikes in glucose, breathing rate or blood pressure are healthy and necessary when dealing with a real threat, if they become chronic they cause serious health problems.
The parasympathetic nervous system brings us back to normal when danger has passed. While the sympathetic nervous system responds to external events, marshaling resources to deal with threats, the parasympathetic system maintains the body’s normal internal environment, what French physiologist Claude Bernard called the “milieu interieur.” Many aspects of that environment must be maintained within narrow limits for us to function and stay healthy, a process termed homeostasis (2). For example the parasympathetic system stimulates digestion, keeps heart rate and blood pressure within normal levels, and promotes healthy immune function.
How does heart rate variability fit in?
The primary pacemaker of the heart is the sino-atrial node, a region of specialized cardiac muscle cells in the right atrium of the heart. The sino-atrial node rhythmically fires at a rate of about 60 to 100 beats per minutes, creating electrical signals that propagate throughout the heart, stimulating contraction.
Inputs from the autonomic nervous system modify that basic sinus rhythm, particularly inputs from the vagus nerve, the major nerve of the parasympathetic system. When vagal (parasympathetic) tone increases, heart rate slows down. As that input is withdrawn, the sino-atrial node returns to its baseline firing rate. When the sympathetic system kicks in, heart rate can speed up above 100 beats per minute
Left to its own devices, the sino-atrial node would beat out a regular rhythm like a metronome. But, largely because of vagal stimulation, a healthy person’s heart rate actually varies considerably under normal circumstances. It isn’t regular, and shouldn’t be (3). In fact, too regular a heartbeat is a sign of an unhealthy autonomic nervous system and a risk factor for death from heart disease (4).
You may be able to feel the normal variation in your heartbeat that accompanies breathing—termed respiratory sinus arrhythmia—when your heart is beating strongly after a few rounds of vigorous sun salutations. Pause in tadasana and place your hand over your heart (under the left nipple is the best place to find the heart beat). Take a few long, slow breaths and notice how your heart rate accelerates as you inhale and decreases as you exhale.
Slow breathing increases respiratory sinus arrhythmia, making it easier to sense, but even when you’re sitting quietly and breathing normally, your heart rate varies with the breathing cycle. As you exhale, parasympathetic tone increases and your heart rate slows. When you inhale, there is a decrease in vagal input, and your heart rate speeds up. It’s not clear why that is, but one theory suggests that soaking the lungs with extra blood during an inhalation uses the heart’s output more efficiently.
Whatever the reason, the fact that heart rate variability is tied to autonomic tone makes it a useful marker for measuring the balance between the sympathetic and parasympathetic divisions. The Rev. Stephen Hales first described heart rate variability in 1733, but it was only with the development of modern digital processors that it became a useful measure. Today researchers use complex mathematical models to identify several frequency components of heart rate variability (5). The low-frequency range is generally a marker of sympathetic tone while the high-frequency band is linked to parasympathetic activation. The ratio between the two describes the relative balance between the two autonomic divisions (3).
In this study, Santaella and his colleagues found that the group who practiced pranayama experienced a reduction in the low-frequency band as well as in the low-to-high-frequency ratio, suggesting a shift from a sympathetic to a more parasympathetic state. In other words, the bhastrika practitioners were less stressed than their counterparts in the control group.
What can we take from these results?
One result that I found curious was that only the pranayama group showed improvements, since both groups participated in regular yoga classes. The authors don’t say much about those classes other than that they included asanas and stretching exercises. However, other studiessuggest that asana practice can also improve cardiovascular function and induce more parasympathetic dominance. So why the control group didn’t show some significant improvements isn’t clear.
Like just about every scientific research study you’ll ever read, this one ends with the comment that further research is necessary. That’s not just because scientists are a bunch of killjoys. In fact, the conclusions we can draw from any individual scientific study are pretty narrow. We can conclude that this particular practice (rounds of kapalabhati alternating with inhalations through the right nostril followed by breath retentions) for this particular amount of time (four months) has these particular effects (improved respiratory pressures and heart rate variability) in this particular population (healthy senior citizens). Would other pranayama practices in other populations have the same effects? We don’t really know from this study. Scientific knowledge grows as the result of many, many studies, some of which confirm and some of which contradict each other.
On the other hand, we don’t need certain knowledge to make educated guesses for ourselves. The results from this study agree with those from other studies suggesting that pranayama improves respiratory function and autonomic balance. You may not have scientific certainty that it will have the same effects for you, but it seems like a pretty good bet.
Other posts in this series
1. Santaella DF, Devesa CRS, Rojo MR, et al. Yoga respiratory training improves respiratory function and cardiac sympathovagal balance in elderly subjects: a randomised controlled trial. BMJ Open 2011;1:e000085. doi:10.1136/ bmjopen-2011-000085
2. Porges SW. Cardiac vagal tone: A physiological index of stress. Neurosci Biobehav Rev. 1995 Summer;19(2):225-33
3. Parati G, et al. Point: cardiovascular variability is/is not an index of autonomic control of circulation. J Appl Physiol. 2006 Aug;101(2):676-8
4. Tsuji H, et al. Reduced heart rate variability and mortality risk in an elderly cohort. The Framingham Heart Study. Circulation. 1994 Aug;90(2):878-83
5. Billman GE. Heart rate variability: A historical perspective. Front Physio. 2:86. doi: 10.3389/fphys.2011.0008
Copyright Joseph Miller