“Coherence”, a measure of the consistency of wave phenomena, is often used in the context of the heart beat. Here, it can pertain to the beat itself, i.e. the physical consistency of consecutive beats where each beat is a wave, or it can pertain to the longer term cycle of variation in the heart beat. Note that the latter is not itself a wave but a mathematical abstraction of the heart beat rate. Yet, when breathing slowly, deeply, and rhythmically, the abstraction certainly resembles a wave – why?
Many years ago, the breathing induced change in heart rate was given the name “Respiratory Sinus Arrhythmia” or RSA, referring to the sine wave-like variation in heart rate as a consequence of breathing. (Figure 1)
While the groundwork for this understanding goes back thousands of years, in the first half of the 1900s researchers made a series of measurements with the newly invented catheter, which was inserted into the thoracic cavity, major arteries, and chambers of the heart in an effort to understand the subtle relationships between breathing, blood flow, and the heart rate.
Paraphrasing their conclusions, the lungs perform the little known function of storing and ejecting a large volume of blood with each inhalation and exhalation, this having been recognized before them. During inhalation, heart rate speeds up to shuttle blood through the veins and right heart into the lungs. During exhalation, heart rate slows down as the large volume of blood (this author estimates that it’s on the order of 500ml) exits the lungs via the left heart, making its way into the arterial tree. Its interesting to note that while heart rate slows down during exhalation, the quantity of blood moved with each beat increases dramatically.
At first this seems counter-intuitive. Most of us accept that the heart is the primary determinant of blood flow. Secondly, we’d expect that as heart rate increases blood volume should increase, and as heart rate decreases blood volume should decrease (which is the way it works under some circumstances).
As of the last couple of years, we can observe this blood wave (which we refer to as the Valsalva Wave) non-invasively, simply using plethysmography, which involves shining a light into the finger, earlobe, or other point where capillary circulation is accessible and measuring the light passing through. As blood increases during exhalation, transmitted light diminishes; as blood decreases during inhalation, light increases.
As breathing depth increases (to a point) the amplitude of the Valsalva Wave issuing from and returning to the lungs increases, heart rate changing in opposition in order to facilitate arterial and venous flows as well as maintain viable flow and pressure throughout the body.
The autonomic nervous system senses these changes via baroreceptors, specialized neurons distributed throughout major blood vessels, and stretch receptors of the heart and lungs. These receptors are part of the feedback loop that allows the autonomic nervous system to gauge cardio-plumonary-circulatory status, allowing corrections to occur in real-time.
Returning to matter of why the heart rate variability cycle looks like a wave – because breathing induced HRV is the autonomic response to a real wave, the Valsalva Wave entering and exiting the lungs – and it is a very accurate indicator thereof. This understanding invites a revised definition of heart rate variability coherence:
Breathing induced HRV coherence is a measure of the degree to which the heart rate mirrors blood flow.
When we synchronize respiration with the heart rate, exhaling at peaks and inhaling at valleys, it yields maximal alignment and “coherence” of the HRV cycle – wave-to-wave consistency of amplitude, phase, and frequency. Phase correlation between the Valsalva Wave and the heart rate approaches -1. (Figures 2 & 3)
This is because at near resonance, the HRV cycle tracks the Valslava Wave with about 1 second delay. The heart rate changing about 1 second behind an opposing change in the blood wave. Therefore if we change our breathing when the heart rate changes we are aligned with the autonomic perspective.
This method is universally appropriate because HRV is the real time autonomic response to the breathing induced blood wave and the myriad other factors of concern to the ANS, i.e. time of day, digestive status, etc. In this regard, the autonomic nervous system is always right, as it is the keeper of the body.
Because it is necessary to change the breathing pattern when the autonomic perspective changes, it is important to have near real-time detection of both heart rate and Valsalva Wave. For this reason, monitoring plethysmographically at the earlobe or via EKG is preferrable.
Stephen Elliott is President and life scientist for COHERENCE – The New Science Of Breath. He is the principal author of The New Science Of Breath (2004) and Coherent Breathing – The Definitive Method (2007). COHERENCE is the developer of BreatheHeart (2010) and Valsalva Wave Pro (2009) which allows monitoring and training of the “Valsalva Wave”.
Stephen’s research colleague is Dee Edmonson, R.N., BCIAC-EEG (www.neurologics.us)