How does the Stress Eraser or the emWave measure the function of the Sympathetic and Parasympathetic Nervous Systems? Short Answer: It doesn’t. (Not directly.)
The Stress Eraser and the emWave are the Heart Rate Variability (HRV) Feedback devices currently available on the consumer level. I’m generally a big fan of the user reviews on Amazon, however with these devices there is a lot of confusion. Consider the following excerpt from a retired physician:
“based on the “feedback” it gives the user, I really don’t understand what it is measuring. It simply can’t be sensitive enough to detect what it purports to detect without being very much more complex than it is.”
The Stress Easer and emWave measure one thing, and one thing only- the inter-beat interval. That is, the time between heart beats. With this one measure it is possible to calculate the heart rate which is constantly changing. For example if the inter-beat interval is 10 seconds the heart rate is 6 beats per minute. If the next interval is 15 seconds the rate has dropped to 4 beats per minute. (Unrealistic numbers chosen to simplify the math.) So how does it measure the output of the Autonomic Nervous System (ANS)?
The devices don’t measure directly the output of the ANS. Using the inter-beat interval data, and the mathematical technique of Fast Fourier Transform, the rate of change of the heart rate is calculated, on the fly, in real time. This is the “magic” of the technology.
Sometimes the heart rate is changing quickly, and sometimes slowly.
Fast changes in rate are caused by shifts in parasympathetic tone (by opening an ion channel which is virtually instantaneous).
Slow changes in rate are caused by shifts in sympathetic tone (linked by a second messenger system which is relatively slow taking roughly 4 seconds to change the rate).
So the rate of change of the heart rate, fast or slow, is a fairly accurate reflection of the activity of the ANS (particularly the parasympathetic system), even though it is not measured directly.
Next time we’ll look at “coherence” and see how that is measured (or not).