Heart rate variability analysis has two roots. In the West it grew out of fetal monitoring and heart medicine. In the Soviet Union it grew out of space medicine, where Vasily Parin and Roman Baevsky used the changing gaps between heartbeats to judge how cosmonauts coped with stress (Baevsky and Chernikova, 2017). The two lines started separately. From the 1990s, Russian, American and German teams studied cosmonauts together, and Russian reviews cite the main Western papers.
Much of the Soviet story comes from Baevsky and his colleagues themselves, so this page names who says what.
Why space doctors watched the heartbeat
The first space flights raised a practical question: how is the person on board coping, right now? Baevsky and Chernikova (2017) write that HRV analysis “was first developed in space medicine”. The first crewed flights needed a fast way to judge a person’s reactions and their ability to keep working under stress.
Two people led the early work.
- Vasily Parin was a leading Soviet physiologist of blood circulation. A 2013 history by Grigoriev and Baevsky calls him the founder of space cardiology. It links his name with the use of HRV analysis and seismocardiography in space, and with the first medical equipment on board. His 1967 book with colleagues, Space cardiology, shaped the field for years.
- Roman Baevsky (1928 to 2020) began as a military flight surgeon on Sakhalin Island from 1952 to 1957. He was one of Parin’s last doctoral students. He built the first Russian ballistocardiography devices, which pick up the tiny forces of each heartbeat through sensors on the body (Tank and colleagues, 2020).
From the first dogs to Gagarin
According to his obituary, written by colleagues in Germany and Russia, Baevsky was directly involved in preparing the first animal and human space flights. It says he obtained heart rate recordings from the dog Laika and took similar measurements from Yuri Gagarin, the first person in space (Tank and colleagues, 2020). He also published an evaluation of seismocardiography recordings, which pick up the heart’s mechanical movements, from the Vostok 5 and Vostok 6 flights.
Baevsky and Chernikova (2017) write that animals in weightlessness showed large swings in heartbeat and breathing. Gagarin’s electrocardiogram ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG also showed marked variation in the gaps between heartbeats during his orbital flight. They read these changes as disturbances in the feedback of the body’s autonomic control system, “where the brain is commander”.
Reading the body’s regulation from the rhythm
Through the 1960s the Soviet group turned these observations into methods. Baevsky and Chernikova (2017) list the milestones:
- In 1966, at the first symposium on mathematical analysis of the heart rhythm, Parin gave a large report and said the method had come from space cardiology.
- In 1967, Parin, Baevsky and Nikulina presented “Heart rate as an indicator of neuroendocrine regulation” at the 18th congress of the International Astronautical Federation in Belgrade.
- In 1968, a two-loop model of heart rate control was proposed: a lower, automatic loop that follows breathing, and a higher, central loop that becomes more involved under stress.
- In the late 1960s, Nidekker and Baevsky proposed spectral analysis of heart rate, to find its repeating rhythms.
When the body is calm, the gaps between beats rise and fall with each breath. When stress pulls in the higher control centres, the rhythm becomes steadier and the gaps more alike. So a steady rhythm was read as a sign of strain. Baevsky’s stress index puts a number on this from a histogram of the gaps.
Baevsky stress index: A number built from the histogram of heartbeat intervals. It rises as the rhythm gets steadier and less variable, which Baevsky read as strain on the body's regulation. More about Baevsky stress index
In the Soviet Union, HRV research peaked in the 1970s and 1980s (Baevsky and Chernikova, 2017).
Between health and illness: prenosological diagnosis
Space medicine deals with healthy people in extreme conditions, so its question was how hard the body was working to stay well. Baevsky called the study of this in-between zone “pre-nosological diagnosis”, from the Greek nosos, disease. The aim was to catch the move from health towards illness before symptoms appear (Tank and colleagues, 2020). His colleagues write that his books on it influenced preventive, sports and rehabilitation medicine in Russia and the health policies of the Russian Ministry of Health.
Baevsky’s team later described this as a “traffic light of health”. The yellow zone lies between health and disease: the body is strained but not ill, and prevention can still help (Baevsky and colleagues, 2015). HRV measures fed into scores of “adaptation risk” for each person (Baevsky and Chernikova, 2016), and into an index of regulatory system activity that runs from 1 to 10 (Baevsky and Chernikova, 2017).
These scores describe risk and strain. They are not diagnoses, and they were built for monitoring people at work, especially in aviation and space medicine (Baevsky and Chernikova, 2016).
Long flights, Mir and the ISS
The work continued as flights got longer.
- Mir. In 1994, a team from Harvard Medical School, NASA’s Johnson Space Center and Baevsky’s Moscow institute reported on heart rate dynamics in cosmonauts on long flights aboard the Mir station (Goldberger and colleagues, 1994).
- A 14-month flight. Baevsky’s group followed physician-cosmonaut V. V. Polyakov through a flight of 438 days, the longest at the time of the report. Using 24-hour heart recordings and other methods, they concluded that adapting to weightlessness happens in stages, with higher levels of control coming into play over time (Baevsky and colleagues, 1998).
- The ISS. Russian, German and American researchers recorded eight male cosmonauts on flights of 162 to 196 days. Heart rate and HRV stayed stable in flight, while blood pressure and breathing rate fell. The authors read this as healthy adaptation. HRV during slow, paced breathing fell in six of the eight. They took this as a smaller reserve of vagal (“ rest and digest Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system ”) control (Baevsky and colleagues, 2007).
- Pneumocard. This ISS experiment involved 25 Russian crew members and 226 recording sessions, 130 of them aboard the station, using heart, breathing and pulse sensors. The authors report that vagal influence typically rose at the start of a mission, and argue that HRV analysis plus prenosological assessment could support medical care on long missions (Baevsky and colleagues, 2014).
How the Western line of HRV research grew
Billman’s 2011 review of HRV history does not mention the space programme. It dates the first clinical use to 1965. Hon and Lee saw that distress in unborn babies was preceded by changes in the gaps between heartbeats, before the average heart rate changed. In the 1970s, HRV was used to detect nerve damage in people with diabetes and linked with survival after a heart attack (Billman, 2011).
In 1981, Akselrod and colleagues used spectral analysis to show that the sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system and parasympathetic nervous systems each leave their mark at different frequencies of heart rate change. Billman counts it among the pioneering studies after which the field grew quickly. In 1996, a European and North American Task Force published shared standards for measuring HRV, interpreting it and using it in the clinic (Task Force, 1996). Russian methodological recommendations followed in 2001 (Baevsky and Chernikova, 2017).
Where the two lines differ and where they meet
| Soviet and Russian space medicine | Western cardiology and physiology | |
|---|---|---|
| Starting point | Healthy cosmonauts under stress, early 1960s | Fetal distress (1965), then heart attack and diabetes patients |
| Main question | How strained is the body’s regulation, and how well is it adapting? | Who is at higher risk of illness or death? |
| Typical measures | Histogram measures such as the stress index Baevsky stress index A number built from the histogram of heartbeat intervals. It rises as the rhythm gets steadier and less variable, which Baevsky read as strain on the body's regulation. More about Baevsky stress index , plus spectral analysis | Time-domain measures such as SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN and RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD , and spectral power |
| Key reference | Baevsky’s methods and the 2001 Russian recommendations | The 1996 Task Force standards |
The two lines did draw on each other. Baevsky and Chernikova (2017) cite Hon and Lee, Akselrod and the Task Force, and compare their histogram with the Western “triangular index”, which is also built from a histogram of beat intervals. The Mir and ISS studies were joint work between Russian, German and American teams.
You can see both traditions on this site. Most wearables report RMSSD, a measure from the Western standards (read how HRV is measured). The chest-strap test also shows Baevsky’s stress index when the recording is long enough.