This part is devoted to the heart rate variability (H.R.V).
We will successively present the following points:
· The branches of the autonomic nervous system that
innervate the heart and influence its rhythm and contractions.
· Study of the cardiac variability on the two analysis
plans (temporal and frequency) and the parameters measured.
· Physiological interpretation of different parameters of
heart rate variability II.1. Autonomic nervous system
The autonomic nervous system (ANS), also called vegetative
nervous system (VNS) or neurovegetative intervenes in the regulation of many
functions of the body. It can be considered as a common final pathway,
stretched between the neural axis and effectors organs, and subject to the
double influence of peripheral afference and supra segmental centres of the
central nervous system. Its involvement does not lead to paralysis but
dysfunction of the organ that innervates, which organ most often has a specific
functional autonomy that the vegetative system adapts incessantly to the
conditions of the environment (Mathias &Bannister,
2002).
The ANS thus has a role of modulator and regulator of the
unconscious vegetative life while fine-tuning the activities of the organs,
with respect to the environment and respecting their independence
(Appenzeller & Oribe, 1997, Mathias, 2000). It acts on
metabolism and electrolyte balances, blood pressure, body temperature, blood
composition and is involved in the functioning of the cardiovascular,
respiratory and digestive systems (Guyton, 2006).
ANS effectors are the tissues and organs responsible for
maintaining homeostasis, mainly the myocardium, the smooth muscles of the
vessels and hollow viscera, such as the bronchi, the digestive tract and the
bladder, as well as the glands and secretory cells. Its functioning is reflex,
unconscious and autonomous (Spalding, 1969) but is under
control of other parts of the nervous system.
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ANS reactions are fast, of the order of a second, and are
distributed in the body, whereas the somatic nervous system has reactions of
the order of a millisecond and are local. Moreover, the ANS can also solicit
the somatic nervous system to feel sensations, such as thirst, hunger, urge to
urinate, or pain. The involvement of the ANS means a dysfunction of the organ
and not a stop. The organs have functional autonomy that the SNV only adapts.
If it is no longer active, the organs continue to function but their activities
are no longer maintained in homeostasis and in the reaction to aggression
(Langley, 1921, Cannon, 1929). While an attack of the somatic
nervous system will cause a loss of function, identical to anaesthesia or
paralysis.
The ANS is composed of two subsystems: sympathetic and
parasympathetic. At the level of an effectors, there is a double innervations
by the two sub-systems whose effects are conjugated, opposed or succeed one
another. However, sweat glands, piloerector muscles, and some subcutaneous
vessels do not exhibit parasympathetic innervations. These two subsystems are
composed of afferents, specific centres located in the central nervous system
and an efferent pathway, formed by two neurons within the SNA. There are also
relays in the ANS outside the central nervous system, in cell clusters called
ganglia, between centres and effectors. We then distinguish Pre-ganglion
neurons, which have cell bodies located in the central nervous system (spinal
cord), and postganglionic neurons, so-called effectors, located in the ganglia
(Pruvost, 2007).
Many organs, such as the heart, have a double innervations;
sympathetic and parasympathetic. Now, the effects of the two branches of the
autonomic nervous system are antagonistic. Their actions interact constantly:
the parasympathetic influence is restricted by sympathetic influence and vice
versa. Nerve modulation on the heart causes a change in heart rate, called a
chronotropic effect. It should also be noted that the heart rate is also
influenced by hormonal control mediated through the bloodstream, but hormonal
control is less rapid and less powerful than direct nerve control
(Pocock & Richards, 2004). It has been suggested that
abnormal regulation of the autonomic nervous system is a biological process
leading to arrhythmias and cardiovascular events during stress
(Bhattacharyya & Steptoe, 2007). For example, an increase
in cardiovascular events has already been reported following earthquakes and
major sports competitions (Wilbert-Lampen et al., 2008).
It is therefore clear that the autonomic nervous system can
be divided into two major parts: the sympathetic nervous system and the
parasympathetic nervous system. Their origins are
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found at different levels of the spinal cord and at the base
of the brain. The effects of these two systems are often antagonistic, but they
always work together, although for more methodological reasons we have to study
them separately.
