How the Breath Controls the Heart

The Biomechanics of HRV Biofeedback Training and Resonance Breathing

Heart rate variability (HRV) biofeedback training and resonance breathing are based on the fact that our breathing has a direct impact on the rest of our physiology, specifically our heart. In this short article I will outline the mechanisms behind this.

The process begins with the diaphragm.

On the inhale, the diaphragm moves downward, creating more space in the chest cavity and — like the plunger in a syringe — sucking in air.

But not only does this additional space pull air into your lungs, it also decreases the pressure on the heart and allows it to physically expand and take up more room. Increased heart volume in turn means that the blood flow in the heart slows down and the blood pressure decreases.

This drop in blood pressures is detected by a set of neurons called baroreceptors, which send information of the change via the vagus nerve to the brain, which in turn sends a message back to the heart to beat faster in order to stabilize the blood pressure — a process known as baroreflex.

And so, inhaling ultimately leads to an increased heart rate.

On the exhale, the reverse process takes place.

The diaphragm moves up. This reduces the size of the chest cavity, which pushes air out of your chest and makes the heart shrink. Like a garden hose being squeezed, the smaller heart volume speeds up the blood flow and increases blood pressure. Once again the baroreceptors communicate this change to the brain, which signals back to the heart to slow down.

In a nutshell, mechanical changes in the diaphragm and lungs cause mechanical changes in the heart. Then homeostasis — our body’s desire to stay in a stable state — kicks in and initiates a negative feedback process. To counteract the changes, the brain sends a signal that tells the heart to speed up on the inhale and slow down on the exhale.

This process leads to a natural fluctuation in heart rate, which can be quantified via various HRV metrics. And it is a sign of overall health and proper signaling of the autonomic nervous system.

As the authors of a recent review article in Frontiers in Neuroscience put it:

“Variability enables adaptability.”

Or, in other words: A healthy heart does not beat like a metronome but constantly adapts to the requirements of the present moment.

While this natural heart rate fluctuation happens to some extent on every in- and out-breath, the individual processes involved, like the baroreflex, don’t happen instantaneously. They all have specific times over which they unfold.

This is the key insight that enables resonance breathing and HRV biofeedback training: If we adjust our breathing rate to match the rate at which these processes take place, we can maximize the amplitude of our heart rate oscillations — just like pushing a child on a swing in exactly the right moment.

Heart rate oscillations during resonance breathing with the Yudemon HRV biofeedback app. While the average heart rate remains at just under 70 beats per minute, the instantaneous heart rate oscillates between less than 50 and more than 90 bpm. Yellow indicates inhale phases, blue exhale phases.

Practicing resonance breathing at this frequency trains our baroreflex, our heart, and more generally our autonomic nervous system (ANS), to be more adaptable not just during training but also during everyday life, particularly during challenging and stressful situations which tend to put a strain on the ANS and throw it out of balance.

The exact frequency which maximizes HRV — the resonance frequency — varies from person to person, and depends on many different and complexly interconnected factors (the more obvious being overall blood volume and body size). In general it is however significantly slower than normal breathing, usually somewhere around 6 breaths per minute (compared to normal rates which tend to fall between 12 and 16 breaths per minute).

My own resonance frequency is close to 5.4 breaths per minute, and I will write an in-depth article soon about how I determined this using the Yudemon HRV Biofeedback app.

For now, if you’d like to read more about HRV biofeedback training, I recommend a detailed article I wrote about my initial experience with a ten week HRV training program.

I can also highly recommend this recent Podcast episode by Dr. Andrew Huberman. In it he outlines a lot of control mechanisms between different parts of the body and the brain. The above article was directly inspired and partly informed by this episode.