HRV values are individual, and good reference values are lacking. The best approach is therefore to follow your own values over time.
- Age: HRV is in general blunted with increasing age. Baroreflex sensitivity also decreases, but seems to stabilise after middle age.
- Sex: on average, women have somewhat higher activity in the brake but lower total variability than men.
- Fitness: endurance training increases resting HRV over time. In one of my studies, three participants had strikingly higher power in all frequency bands than the rest. All three were athletes who exercised more than four times a week.
- Sleep, alcohol and illness: poor sleep, alcohol the evening before and infections lower HRV, often clearly.
- Hard training: HRV drops the day after a tough session and rises again once the body has recovered.
- Stress and breathing: mental strain lowers HRV and increases low-frequency (LF) power, while calm, deep breathing increases variability in the moment.
- Chronic pain: in the Tromsø Study, more than 1,100 people with chronic pain had lower HRV and baroreflex sensitivity than pain-free controls, which may contribute to their higher risk of high blood pressure.
- Altitude: HRV falls in all frequency bands during the first days at altitude (see below).
- Medication: many drugs affect heart rate. Beta blockers increase all components of HRV.
- Heart rhythm: ectopic beats and atrial fibrillation produce numbers that look high but say nothing about the nervous system. See Pitfalls.
High blood pressure
In my doctoral work we compared 41 people with high blood pressure with 34 people with normal blood pressure, using 24-hour ECG. Those with hypertension had lower total power, lower RMSSD (29 vs 36 ms) and lower baroreflex sensitivity (7.6 vs 10.4 ms/mmHg). The differences were largest among women: the difference in baroreflex sensitivity within the female group was twice that within the male group.
Altitude and low air pressure
In a hypobaric chamber we followed healthy volunteers for a week while the air pressure was lowered stepwise to the equivalent of 4,500 metres. Hypobaric hypoxia is regarded as an activator of the sympathetic nervous system, but during the first days we found the opposite: HRV, baroreflex sensitivity, plasma noradrenaline and the responses to stress tests were all blunted. A possible explanation is that oxygen is needed to make the stress hormones. Even at an air pressure equivalent to 2,400 metres – similar to the cabin of an airliner at cruising altitude – baroreflex sensitivity was reduced.
A higher heart rate is not always more stress
A raised resting heart rate is traditionally interpreted as a shift towards stronger sympathetic and weaker parasympathetic influence. Yet both at high altitude and in people with hypertension, we found reduced baroreflex sensitivity together with reduced total, low- and high-frequency power. This implies reduced parasympathetic and sympathetic activity. The higher heart rate may therefore partly reflect generally reduced autonomic control, which brings the heart rate closer to the heart’s intrinsic rate of 110–120 beats per minute.
Body Battery and similar scores
Garmin’s Body Battery and similar scores from other manufacturers combine HRV, stress and activity into a single number – for example from 1 to 100 – meant to show how much «energy» you have. At rest, when the parasympathetic system dominates, the heart rate is low and variability high, the battery charges. When the sympathetic system dominates, it drains. The numbers are useful for seeing patterns in your own life, but the actual calculation is usually proprietary and not publicly known.
HRV in research
HRV has a long history in research. In the 1980s and 1990s, several large studies showed that patients with very low HRV after a heart attack had higher mortality. Measures of total variability, such as SDNN and total power, have proven to be the strongest predictors, while results for HF and LF power have been less convincing. Low HRV has also been found in people with high blood pressure and in panic disorder. Such findings apply to groups, and a single HRV value cannot be used to assess an individual’s health risk.
References
- Sevre K. Autonomic cardiovascular regulation in healthy subjects and patients with cardiovascular disease. Doctoral thesis. University of Oslo, 2001.
- Sevre K, Lefrandt JD, Nordby G, et al. Autonomic function in hypertensive and normotensive subjects: the importance of gender. Hypertension 2001; 37: 1351–6.
- Sevre K, Bendz B, Nakstad AR, et al. Reduced autonomic activity during stepwise exposure to high altitude. Acta Physiol Scand 2001; 173: 409–17.
- Sevre K, Bendz B, Rostrup M. Reduced baroreceptor reflex sensitivity and increased blood pressure variability at 2400 m simulated cabin altitude. Aviat Space Environ Med 2002 [VOLUME AND PAGES].
- Bruehl S, Olsen RB, Tronstad C, Sevre K, et al. Chronic pain-related changes in cardiovascular regulation and impact on comorbid hypertension in a general population: the Tromsø study. Pain 2018; 159: 119–27.
- Kleiger RE, Miller JP, Bigger JT, Moss AJ. Decreased heart rate variability and its association with increased mortality after acute myocardial infarction. Am J Cardiol 1987; 59: 256–62.
- Bigger JT, Fleiss JL, Steinman RC, et al. Frequency domain measures of heart period variability and mortality after myocardial infarction. Circulation 1992; 85: 164–71.