Parasympathetic saturation

Blog post by Marco Altini

​In this blog I briefly discuss parasympathetic saturation and introduce a new feature we have released in HRV4Training Pro to allow you to determine the likelihood of parasympathetic saturation in your data or your athlete's data.

What's parasympathetic saturation?

Parasympathetic saturation refers to a situation in which parasympathetic activity is particularly high, but this is not reflected accurately in HRV data. As Kiviniemi et al. explain in [1], "possible physiological mechanisms underlying saturation could be due to the dose response of the heart to the acetylcholine secreted by vagal nerve ending. The dose response to acetylcholine has been considered to be linear until its concentration reaches the level at which a further increase in acetylcholine concentration does not produce a change in the response", or in Daniel Plews' words [3]: "a heightened vagal tone may give rise to sustained parasympathetic control of the sinus node, which may eliminate respiratory heart modulation and reduce HRV".

It follows that as reported in another of Daniel's papers [2], "in some circumstances, such as vagal saturation, decreases in cardiac parasympathetic indices of HRV during this particular training phase can be related to positive performance outcomes and consequently reductions in HRV, so should not be viewed negatively". 

How can we identify parasympathetic saturation?

Looking at the relationship between HRV and RR interval length you can identify possible parasympathetic saturation. Parasympathetic saturation is a rare event which can happen in elite endurance athletes during high load training blocks.

In particular, parasympathetic saturation typically requires:

1. average low heart rate (in the 40s or lower)
2. high volume training
3. elite endurance athletes

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Under these circumstances, you can look at the relationship between HRV and the average RR interval length (basically the inverse of heart rate), to determine the likelihood of saturation.

Normally, higher HRV is associated to lower HR (see data below for an example), and therefore we expect a linear relationship. However, if you are in a period of high training load and HRV is low, together with low HR, and therefore the correlation between HRV and the average RR interval length is small or negative (we lose the linear relationship that we were just discussing), parasympathetic saturation is plausible (below I'll show how you can look at this in our platform). 

What can you do about it?

Depending on how you measure your HRV, you could be pro-active and collect data that is less likely to be affected by the issue of parasympathetic saturation.

In particular:

• if you measure your HRV during the night, there isn't anything you can do. Hence you should use the procedure below to determine if parasympathetic saturation is likely in your own case. In this case it might be preferable to use a morning measurement.
• If you measure in the morning, you can measure while sitting (or standing), so that you add a little stress on your body and potentially prevent the issue of parasympathetic saturation, as recommended by Andrew Flatt [4]. Some practitioners mention lower compliance when asking athletes to take their measurement in a way other than while still lying down in bed, and as usual, no data won't help, hence first make sure that this is not affecting compliance. Make also sure to always measure in the same body position, so if you decide to sit or stand for your measurement, first always allow for 30-60" of rest to allow your body to re-adjust to the change in posture, then take your measurement always in the same body position every day.

New feature in HRV4Training Pro to determine likelihood of parasympathetic saturation for you and your athletes

As mentioned earlier, if you are in a period of high training load and HRV is low, together with low HR, and therefore the correlation between HRV and the average RR interval length is small or negative, parasympathetic saturation is plausible.

We have developed a new feature in HRV4Training Pro to help you analyze this relationship. In the plot below, you would see the darker dots in the lower right corner (low HRV, low HR or high RR interval length). In this case, the suppression in HRV should not be interpreted negatively, as reported by ​Plews et al.: "the lack of correlation between the R-R interval and Ln rMSSD indicate that athletes are more likely to undergo parasympathetic saturation".

​You can find the plot below under Insights / Resting Physiology in HRV4Training Pro, we also report the correlation between HRV and the RR interval length for the past 2 and 6 weeks, so that you can more easily spot any recent changes:

Finally, we have added Possible parasympathetic saturation as an automatically detected trend, hence the system will try to do the math for you:

Is this something that should concern you?

In general, parasympathetic saturation is a rare event. Over the years, I have received a few emails from users reading online that they should be sitting instead of lying down when measuring, and I just want to make something clear here: this is typically a non-issue.

If you are an elite endurance athlete, and you have experienced periods of suppressed HRV, low HR, and performed well in training or racing under high loads while your HRV was low, then you are a good candidate and it would be beneficial to measure while sitting or standing.

Otherwise, please do not obsess over something that most likely will never happen, and keep taking your measurements in a consistent manner, so that you can benefit the most from analyzing the data in the long term, as shown in many case studies here. 

If you have any doubts, you can login in Pro at HRV4T.com and check your own data.

I hope you will enjoy the new feature, take care!

References

[1] Kiviniemi AM, Hautala AJ, Seppanen T, Makikallio TH, Huikuri HV, Tulppo MP. Saturation of high-frequency oscillations of RR intervals in healthy subjects and patients after acute myocardial infarction during ambulatory conditions. American Journal of Physiology-Heart and Circulatory Physiology. 2004 Nov;287(5):H1921-7

[2] Plews DJ, Laursen PB, Buchheit M. Day-to-day heart-rate variability recordings in world-champion rowers: appreciating unique athlete characteristics. International journal of sports physiology and performance. 2017 May 1;12(5):697-703

[3] Plews DJ, Laursen PB, Stanley J, Kilding AE, Buchheit M. Training adaptation and heart rate variability in elite endurance athletes: opening the door to effective monitoring. Sports medicine. 2013 Sep 1;43(9):773-81

[4] Andrew Flatt's blog: https://hrvtraining.com/