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The autonomic nervous system changes with age, too.
Normally, the parasympathetic side sets the basic resting heart
rate, while the sympathetic side affects the heart in anticipation
of and in response to physical activity, stimulating timely
and proportional increases in blood circulation necessary to
support ongoing activity. Continuous adjustment between the
sympathetic and parasympathetic systems result in a heart-
rate variability evident on a beat-to-beat basis, which is a sign
of a healthy control system. With age, the contribution of the
parasympathetic side wanes, the sympathetic activity increases
(even at rest), heart-rate variability disappears and heart rhythm
becomes more prone to derailments. Low heart-rate variability
and increased resting heart rate (due to loss of parasympathetic
tone) independently increase the risk of SCD more than two
and a half times.
EffEcts of diving on thE cardiovascular systEm
Diving exposes divers’ bodies to various stressors that
independently affect cardiovascular function. The major
stressors are immersion, exposure to cold, increased partial
pressure of oxygen and increased work of breathing.
The combined effect of these factors is that the volume
of blood in the vessels of the chest and heart increases
significantly, stretching the walls of heart and large vessels.
Pressure in the right atrium and blood pressure slightly increase
— more so in cold water. The heart has to work harder to
maintain circulation. These conditions contribute to various
arrhythmias, from bradycardia (slow heart rate) caused by cold
to tachyarrhythmia (racing heart rate) caused by cardiac and
neuroendocrine responses to stress. Older people, especially
those with structural cardiovascular changes and weaker
function, are at greater risk of adverse reactions to these stresses.
The autonomic nervous system is affected by diving, too.
In healthy individuals, diving increases parasympathetic
effects on heart rate, and heart-rate variability is preserved.
Diving that is perceived as stressful pushes the balance of the
autonomic nervous system in the other direction: Sympathetic
effects prevail and increase the heart rate, diminish heart-rate
variability and increase the risk of arrhythmia.
Although diving is often perceived as a leisurely activity, the
level of exertion may sometimes exceed the physical capacity
of divers with functional limitations as well as healthy but
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