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reason this is considered the gold standard of h r v measurement is because it has been found have contributions from both the sympathetic nervous system and para sympathetic nervous system. So we can get full insight into our autonomic response with this measurement. Like i mentioned before, stin is most accurate when it is assessed over 24 hours than during short periods, because the longer periods can provide data about cardiac reactions to a great range of environmental stimulation, or lack thereof in the case of sleep. It is also the 24 hour recordings that allow us to assess the sympathetic nervous system contribution to s t n n. For assessing cardiac risk, s t n n is almost always used and can be a valuable predictor of both morbidity and mortality. In looking at 24 hour recordings, an estiin in less than 50 millo seconds has been demonstrated in those classified as unhealthy, those with 50 millo seconds to 100 millo seconds is a classification for compromised health, and 100 millo second sent above is considered a healthy individual from a cardiaca cardiac standpoint. When we look at hartattack survivors, those who have had a higher classification had a greater risk of living, are, sorry, i should say, a greater probability of living during a 31 month followup period. One study demonstrated that individuals with an s t n n greater than 100 millo seconds had a five point three times lower risk of mortality at follow up than those with values less than 50 millal seconds. We can actually correlate the st n n metric with other frequency domains that i will discuss later. S d n n is correlated with the ultra low frequency ban, the very low frequency band and the low frequency ban. Now let's turn to another time domain index for h r v. And for short term time domain evaluation is probably my most preferred measure, which is the r m s s d metric. So r m s s d is the root main square of successive differences between normal heart beats. And now that i have sufficed in spinning your heads, let me explain it. In order to calculate this value, we must calculate each successive time difference between adjacent inter beat intervals in terms of milla seconds. Then each of these values is squared, and the result is then averaged, before the square root of the total is is obtained. Oh. Now, that is a lot of what we do in research, so i wanted to give you that equation, but for the sake of practicality, it may not be necessarily that important. The measurement is mediated or provides us insight into the workings of our pera sympathetic nervous system, and is highly correlated with the high frequency ban. We actually know that r m s s d is more influenced by the pera sympathetic branch than s d n n. While a five minute gold stand while five minute measurement is the gold standard for r m s s d, you can get usable data in ultra short term domains that i mentioned at the beginning of this podcast, this makes it really good as an on the go measurement. One thing that i like about r m s s d is that it is not mediated by respiratory sinus arhythmia like s d n n. Is what i mean by this is that with s d n n, you can effect change in this number by taking those low and slow diaphragmatic breaths. Whereas r m s s d does not appear to be mediated by respiration or slow breathing. And may provide more sight into recovery, as it is primarily assessing tonic or relaxed or static h r v. So if you want to gain insight into your recovery and remove the effects of respiration to assess non manipulated h r vr, m s s d is the way to go. So that's the one i use for recovery. This is one of the best measures of vagal tone due to the lack of manipulation of respiration. This is why i actually like the use of the ora ring to assess h r v at night while i sleep. But more about that later. Another one that is used quite frequently and is very easy to assessin th h r v time domains, is something called h r v amplitude. This is a very simple measurement that is an average of the difference between the highest and lowest heart rate during each respiratory cycle. So remember, r s a, a respiratory sinus a rhythmia, is the fluctuation of heart rate from the start to the finish of a breath cycle. Normal r s a occurs when you inhale and your heart rate increases, and when you exhale, your heart rate decreases.