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Citation
Kromenacker BW, Sanova AA, Marcus FI, Allen JJB, Lane RD. Vagal Mediation of Low-Frequency Heart Rate Variability During Slow Yogic Breathing. Psychosom Med. 2018 Jul/Aug;80(6):581-587. doi: 10.1097/PSY.0000000000000603. PMID: 29771730.
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Note: You can share anything you want from these 411s—the more sharing the better. But this is my attempt to condense the findings into a bite-sized wisdom nugget:
By separately blocking the parasympathetic and sympathetic nervous systems during slow breathing, a 2018 study in Psychosomatic Medicine conclusively found that observed increases in HRV during slow breathing are primarily due to increased vagal activity.
4 Fundamentals
1. Essential Background Material
Slow yogic breathing (which I’ll just call “slow breathing”) has consistently been shown to reduce stress, anxiety, and depression and lower inflammation and blood pressure.
These benefits are thought to be—at least partially—the result of better vagal tone (more activity of the parasympathetic rest-and-digest nervous system [PNS]).
However, the way vagal tone is measured (via heart rate variability [HRV]) introduces uncertainty to this interpretation for two reasons:
Low-frequency HRV (LF-HRV) usually increases significantly during slow breathing. But the interpretation of LF-HRV is not concrete: some studies suggest that increases in LF-HRV could indicate increased sympathetic (fight-or-flight) activity [SNS].
A problem with LF-HRV measurements is that the breathing rate itself interferes. When LF-HRV increases, it’s unclear how much is due to increases in vagal activity and how much is an artifact of the breathing rate itself.
Vagal nerve stimulation is used for many health conditions (and slow breathing is often even called “vagal stimulation”). Thus, from a treatment perspective, it’s vital to know if these slow breathing rates are, in fact, stimulating the vagus nerve.
This study used a neat approach to assess this directly.
Nick’s Sidenote: Even though this study was focused on LF-HRV, which may be of no interest to you, the methods they used were awesome and the results are super important to anyone interested in how slow breathing works.
2. What Did this Research Do?
This study had 6 participants (4 male, 2 female). The procedure was pretty intensive and required a smaller sample size. Still, this is a limitation, but the intervention produced substantial effects, making this a significant study despite the small sample.
Procedure
The participants rested for 10-20 minutes. Then, they were injected with one of three drugs:
Glycopyrrolate: Blocks activity of the parasympathetic nervous system
Esmolol: Blocks activity of the sympathetic nervous system
Saline: Placebo
Next, they performed 11 different slow breathing rates, ranging from 4 to 9 breaths a minute, using 0.5-step increments. Although the breathing rates varied, they used the same ratio of 4-4-6-2 (inhale 4, hold 4, exhale 6, hold 2), which is commonly used in Sudarshan Kriya Yoga.
They performed each breathing rate for 1 minute, followed by 4 minutes of normal breathing.
Example: They took glycopyrrolate and slowly breathed at five breaths a minute. HRV was measured to see if it went up despite the blocked PNS.
3. What Were the Major Findings?
Blocking the Sympathetic Nervous System
When they blocked the SNS, HRV was virtually unchanged compared to the placebo saline solution. This means the SNS plays a minimal role in HRV measurements during slow breathing.
Blocking the Parasympathetic Nervous System
HRV reduced significantly (basically to zero) when they blocked the PNS regardless of which breathing rate was used. This means the PNS (and thus vagal activity) is the primary reason HRV increases during slow breathing.
One Exception
At the slowest rate of 4 breaths a minute, there was a slight (but significant) increase in HRV even when the PNS was blocked. This indicated that the sympathetic nervous system contributes a small amount to HRV readings when breathing is less than 5 breaths a minute.
HRV plots during slow breathing at different rates and under different nervous system blockades. THIS IS NOT REAL DATA. It’s a loose adaptation of Figure 2 of this paper, Vagal Mediation of Low-Frequency Heart Rate Variability During Slow Yogic Breathing. Psychosom Med. 2018.
4. Why Do These Results Matter?
Together, these findings conclusively show that increases in HRV during slow breathing result from increased vagal activity, meaning slow breathing is indeed “vagal nerve stimulation.”
To reiterate the results, when the PNS was blocked, HRV basically went to zero. But when the SNS was blocked, HRV was basically unchanged. This means that when we see HRV increase during slow breathing, it’s almost entirely due to increased vagal activity. That’s super neat.
1 Big Takeaway
By separately blocking the parasympathetic and sympathetic nervous systems during slow breathing, this study conclusively found that observed increases in HRV during slow breathing are primarily due to increased vagal activity.
1 Practical Application
Practice slow breathing to stimulate your vagus nerve and increase the health of your cardiovascular and autonomic nervous systems.
More specifically, consider trying this 4-4-6-2 ratio to see how it makes you feel. I’ve been practicing it for 10 minutes before bed and find it very helpful for falling asleep.
You can easily program the 4-4-6-2 rate using the Breathe app: