Interplay in human nervous system
To better understand what heart rate variability (HRV) is, let’s briefly delve into the anatomical mysteries of the human nervous system.
The nervous system is made up of two major components. First there is the central nervous system (CNS), formed by brain and spinal cord. It is where diverse information from the entire organism is received, processed and responded to. Its operation is accompanied by the peripheral nervous system (PNS) comprised of nerves and ganglia spread across the body. Within the peripheral system, sensory nerves detect physical or chemical cues and transmit the signals towards the brain. Motor nerves carry the signals in the opposite direction to stimulate effectors, such as muscles or internal organs.
Motor functions can be performed as a voluntary or involuntary response. When the response to the stimulus is controlled and performed consciously, it is supervised by a part of PNS known as the somatic nervous system. Most notably, this includes muscle contraction that underlie locomotor activity. The autonomic nervous system, on the other hand, is responsible for processes related to uncontrollable responses, such as cardiovascular adaptation to exercise, body temperature regulation, digestion or breathing.
Finally, the autonomic nervous system is sub-divided into two components, whose roles are contradictory to one another. The sympathetic nervous system is activated in the face of threat or stress. Among its numerous functions, the sympathetic system increases the heart rate in response to various stimuli. The time intervals between the successive heart beats become then shorter, more blood is pumped per unit time and blood pressure spikes. Once the stressful situation is gone, the parasympathetic nervous system is activated to restore the impaired balance and to eventually induce the state of relaxation.
Heart rate variability (HRV)
The sympathetic and parasympathetic nervous systems work in a carefully harmonized manner. The energy-consuming ‘fight or flight’ state governed by the sympathetic nervous system should be naturally followed by a shift towards the ‘rest and digest’ parasympathetic response. A prolonged deviation from the equilibrium, however, may pose a threat to human health.
To monitor the balance between the two extreme states of autonomic nervous activity, a notion of heart rate variability was developed. HRV is a measure of changes in the duration of time intervals between the successive heart beats. These intervals can be best observed on an ECG reading as distances between the so-called R peaks of the QRS complexes indicating ventricular contractions. Although difficult to spot with the naked eye, there are always some irregularities between individual intervals. They are, however, not a reason for concern.
The variation in beat-to-beat intervals yields information on how the autonomic nervous system modulates heart muscle activity. It is a valid non-invasive indicator of the general health of the patient.
Normal or increased HRV means the organism is currently capable of adapting to a possible sudden change of activity or environmental conditions. Patient’s heart is thus able to readily switch between different modes of heart rate intensities, if required.
Lack of balance between the sympathetic and parasympathetic nervous systems, often caused by stress or anxiety, is typically reflected in low HRV. The link between decreased HRV and certain types of cardiovascular or autonomic pathologies is so strong, that HRV can serve as a diagnostic tool for evaluating the prognosis of patients with many chronic cardiovascular conditions.
HRV in hypertension
A decreased HRV is commonly reported in patients with elevated blood pressure. What many people do not realize, however, is that it is possible to somewhat influence your HRV level with the use of certain conscious practices, including meditation , contemplative training , or breathing exercises . As a consequence, one should observe an increase in the fitness level, stress adaptation and emotional stability possibly disturbed by hypertension.
Heart rate variability is affected by a number of factors, which include diet and respiration pattern. The variation of the latter is often employed by designers of smart wearables. Electronic watches or wristbands occasionally ask their users to slow down and breathe calmly for a minute. There is more to it that meets the eye. Although this method will not cure hypertension per se, it can largely improve the quality of everyday life of patients with elevated blood pressure .
Slow breathing exercises repeated periodically throughout the day act as triggers activating the parasympathetic nervous system in a way similar to how it works on its own. As a result, the heart rate slows down (in this case voluntarily and consciously), which entails a decrease of blood pressure. Repeated daily, such a training to balance the distorted HRV can help live a better life with hypertension.
Recommended for listening:
1. Changes in Heart Rate Variability During Heartfulness Meditation: A Power Spectral Analysis Including the Residual Spectrum. Anne Léonard, Serge Clément, Cheng-Deng Kuo, Mario Manto. Frontiers in Cardiovascular Medicine 2019, 6:62.
5. Effects of slow breathing rate on heart rate variability and arterial baroreflex sensitivity in essential hypertension. Changjun Li, Qinghua Chang, Jia Zhang, Wenshu Chai. Medicine (Baltimore) 2018, 97(18): e0639.