What are the low medium and high coherence ratios on the emWave program? How is it related to heart rate variability? Is there any mathematical equation to derive the scores? Also how do I interpret the results of the ratios?
The coherence scores are based on how symmetrical the wave-like HRV pattern is. The low coherence score is the percentage of time there is no wave-like activity in the HRV wave. Medium coherence is percentage of time that there is some wave-like activity and high coherence is the percentage of time there is very wave-like activity.
If you look at the power spectrum screen, you see three colored regions; blue bars in the dark blue, (in emWave PC 1.0, brown) region on the left are an indication of sympathetic activity. Blue bars in the medium blue, (purple region in 1.0) on the right are an indication of parasympathetic activity. Blue bars in the light blue, (the yellow region in 1.0) indicate that the sympathetic and parasympathetic are synchronizing.
In low coherence the blue bars tend to be scattered across the graph with more blue typically in the dark blue/brown sympathetic region. As you move towards high coherence the blue bars focus in the light blue/yellow region and can get very large in amplitude. We won't publish the mathematical algorithm that is used to produce the coherence ratio scores as it is proprietary.
Note that the coherence scores do not have much of a relationship to traditional HRV parameters like SDNN Index which primarily measure HRV amplitude not how wave-like the HRV pattern is. If you are interested in generating traditional HRV measures you'll need to use another piece of equipment to record HRV data as the emWave program stores interpolated data; the inter-beat interval in milliseconds is stored every half second.