What is HRV?

Heart rate variability (HRV) is an express method to assess the functional state of cardiovascular system (CVS) and autonomic nervous system (ANS) by analysis of syncs rhythm.


Method of mathematic analysis of HRV has been widely used to assess cosmonauts’ functional state since 1961, when Yuriy Gagarin made the first cosmic orbital flight. The HRV was the only way to real-time monitor rapidly changing state of cosmonaut’s CVS and ANS, reflecting general state of the organism.

Principle of the method

CVS transports oxygen and nutrients to every cell of the body. That is why it is the main energy supplying system in organism. Thus, fluctuations of CVS activity reflect the need in oxygen and nutrients in organs and tissues.

ANS regulates the metabolism’s focus towards the prevalence of trophic or energetic processes in organs and tissues. As a matter of fact, the functioning of each organ and each cell of the body is dependent on phylogenetically formed and genetically defined biorhythms. Every day, each system of organs, each organ, each cell cyclically passes through phases of minimum and maximum of their functional activity. The rule and the necessary condition of normal functioning of organ (or the whole human body) is alternation of energy-consuming period of maximum activity with a recovery period. CVS and ANS play a great role in keeping tissue metabolism at required level according to the demands of organs and their systems as well as during adaptation of organism to changing conditions, such as fulfilment of different occupational tasks or everyday activities.

The hierarchical structure of heart rhythm control

Sinoatrial node is the normal pacemaker of the heart. It has multilevel autonomic and humoral regulation. There are three levels of regulation.

The first level (autonomic level) includes nucleus of the vagus nerve and the respiratory center of medulla oblongata. It works rapidly with low latency (reaction goes very fast) – about 2.5 – 6 sec (High Frequency domain – HF). So, HF domain characterizes basically parasympathetic regulation.

The second level (baroreflex level) includes vasomotor center of the medulla oblongata. It has mixed nature – predominantly sympathetic and to a lesser extent parasympathetic. Its reaction time is about 6 – 25 sec (Low Frequency domain – LF). So, LF domain reflects adaptive vasomotor reactions.

The third level (sympathetic ergotropic and humoral level) consists of subcortical sympathetic ergotropic nerve ganglia. It reflects slow humoral and temperature regulation as well as transitory processes associated with changes in body position (vertical – horizontal). Its reaction time is about 25 – 300 sec (Very Low Frequency domain – VLF).

Our body works according to the following rules:

  • Normally, when the workload or everyday activity demands do not exceed the capacity of CVS (e.g. rest or light intensity work), the heart is regulated predominantly by autonomic level.
  • When the workload becomes higher, the organism starts to involve baroreflex level of regulation.
  • When the workload becomes too high or external conditions become too severe, the organism starts to involve sympathetic ergotropic and humoral level.
  • So, the higher intensity of work or adaptation, the higher level of regulation is involved.
  • Each higher level of regulation inhibits activity of lower levels.

The state of stress is characterized by increased heart rate (HR), increased sympathetic tone of ANS, increased blood pressure. State of relaxation is characterized by decrease of HR, increase of parasympathetic tone (synonym – vagal tone) of ANS.

Heart rate variability analysis methods will be described in our future posts.