One breathing technique that can produce a beneficent effect on the autonomic nervous system is 2:1 breathing—taking twice as long to exhale as to inhale. For those who are in good condition, 6-second exhalations and 3-second inhalations are about right, and if you can regulate this without stress, the practice will slow your heart down and you will have a subjective experience of relaxation. As with almost all breathing exercises in yoga, both inhalation and exhalation should be through the nose.
This connection between heart rate and breathing, known as respiratory sinus arrhythmia, involves reflex activity from the circulatory system to the brain stem that causes the heart to beat more slowly during exhalation than it does in inhalation. It is a natural arrhythmia, called "respiratory" because it is induced by respiration, and called "sinus" because the receptors that stimulate the shifts in heart rate are located in the aortic and carotid sinuses, which are bulbous enlargements in those great vessels. If you take longer to exhale than to inhale, especially when you are relaxing, the slowing-down effect of exhalation will predominate. This is an excellent example of how we can willfully intervene to produce effects that are usually regulated by the autonomic nervous system.
There are limits on both ends to the effects of 2:1 breathing. If you are walking briskly, exhaling for two seconds and inhaling one second, you will not get this reaction, and if you take it too far in the other direction, which for most people means trying to breathe fewer than five breaths per minute (8-second exhalations and 4-second inhalations), the exercise may bccome stressful and cause the heart rate to increase rather than slow down. The golden mean—that which is entirely comfortable—is best.
There is one well-known practical consequence of respiratory sinus arrhythmia. For decades doctors have known empirically that pursed-lip breathing against moderate resistance is helpful for those with obstructive lung disease. What is not generally realized is that the practice is helpful mainly because it lengthens exhalations, slows the heart rate, decreases the amount of air remaining in the lungs after exhalation, and reduces fear and anxiety. Knowledgeable yoga teachers realize that the same end can be accomplished through a different approach—lengthening exhalations by pressing in gently with the abdominal muscles while at the same time breathing through the nose.
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