Parasympathetic saturation
Also called vagal saturation, HRV saturation
When the vagal brake is so strong, usually in very fit people with very slow resting heart rates, that the beat-to-beat swing shrinks. HRV then falls while vagal activity is rising.
Normally, more vagal activity Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone means a slower heart and a bigger beat-to-beat swing, so RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD rises as resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate falls. At the far end that stops working. The vagal brake is pressed so hard, so steadily, that there is little room left for it to ease off with each breath. The heart is slow and very even, and RMSSD drops.
Buchheit (2014) explains that the drop in vagal HRV at very low heart rates comes from more vagal activity, not less, because HRV measures the size of the swings in vagal outflow rather than its level. He places the tipping point at gaps between beats longer than about 1000 ms, a heart rate under 60. Plews and colleagues (2013) describe the same pattern in elite endurance athletes: HRV falling while resting heart rate keeps dropping. Quigley and colleagues (2024) note that the breathing swing is “nearly abolished” at very high vagal activity and advise checking for this ceiling.
Why it matters for HRV. A falling HRV in a very fit person is not automatically a warning. Look at resting heart rate at the same time. Buchheit recommends tracking the ratio of lnRMSSD lnRMSSD The natural logarithm of RMSSD, with no unit. Taking the log evens out the spread of RMSSD values, which makes day-to-day changes easier to compare and analyse. More about lnRMSSD to the mean RR Mean RR The average time between heartbeats in a recording, in milliseconds. Heart rate is the same number upside down: 60,000 divided by the mean RR. More about Mean RR interval: in saturation the gaps get longer while the ratio drops.
Turn on saturation below and slide the heart rate under 60.
The same nudge, a slower heart
A teaching model, not a measurement
- Mean gap
- 1000 ms
- One beat a minute
- 17 ms
- RMSSD
- 42 ms
- lnRMSSD
- 3.73
- lnRMSSD:RR ×1000
- 3.7
At 60 bpm the average gap is 1000 ms and one beat a minute is worth 17 ms. The same 6 beats a minute swing gives an RMSSD of 42 ms; lnRMSSD is 3.73 and the lnRMSSD:RR ratio 3.7.
The slider works when JavaScript is on.
A chart of RMSSD against heart rate from 40 to 100 beats a minute. Without saturation the curve falls steadily: about 60 ms at 50, 30 ms at 70 and 15 ms at 100. With saturation on, a dashed second curve bends down below 60 beats a minute. A slider sets the heart rate, a checkbox turns saturation on, and readouts give the mean gap in milliseconds, the milliseconds per beat a minute at that rate, RMSSD, lnRMSSD and the lnRMSSD:RR ratio.
Why does my HRV fall when my resting heart rate falls? puts this in context.
Sources
- Monitoring training status with HR measures: do all roads lead to Rome?
- Training adaptation and heart rate variability in elite endurance athletes: opening the door to effective monitoring
- Publication guidelines for human heart rate and heart rate variability studies in psychophysiology—Part 1: Physiological underpinnings and foundations of measurement