Abstract
Changes in heart rate with respiration ii.e.respiratory sinus
arrhythmia "RSA") have been known since the last century.More
recently,it has been shown that there are other rhythms in heart
rate not associated with respiration. Collectively, the "RSA" and
the other rhythms are called heart rate variability "HRV". One of
these slower rhythms occurs at the rate of around 6 cycles /
min.C'iO second "rhythm) . The factors influencing this rhythm and
its autonomic control were investigated in this thesis.
An initial survey of the effects of posture on "HRV" was carried
out on 79 subjects. Maintained standing (chapter III) was found to
be a strong stimulus to potentiate the "10 second" rhythm in about
half the subjects who showed predominantly "RSA" when supine.
In chapter IV two different age groups were compared and the
results showed that fewer older subjects exhibited a prominent "10
second" rhythm during maintained standing.
The mean heart rate (MHR) during maintained standing was
significantly higher than supine MHR. So the difference in heart
rate pattern between the two postures was possibly due to the
difference in MHR. Therefore, in some oscillators (chapter V) the
heart rate was increased to a level similar to that during
maintained standing by exercising in the supine position. However,
this increase in heart rate failed to induce the "10 second"
rhythm.
The effect of tilting the subjects to 70° angle (chapter VI) caused
a similar change in heart rate pattern to that of maintained
standing .
Since both active and passive change of posture caused a
potentiation of the "10 second" rhythm, investigations were made
(chapter VII) to elucidate the contributing factors. The heart
rate pattern changed from a predominantly "10 second" rhythm
during standing in air to a predominantly "RSA" rhythm during
standing immersed in water up to the level of the diaphragm. This
change in heart rate pattern with the prevention of venous pooling
suggests that the decrease in venous return during maintained
standing and the subsequent change in the pattern of discharge from
the receptors on the low pressure side of the circulation may be
important for the increased prominence of the "10 second" rhythm.
The efferent pathways involved in controlling the "10 second"
rhythm were investigated (chapter VIII) by using propranolol to
block the sympathetic system and atropine to block the
parasympathetic system. Propranolol caused a marked decrease in the
amplitude of the "10 second" rhythm during maintained standing
suggesting that the sympathetic system is important in its control.
Atropine also decreased the "10 second" rhythm, however, the
pathway mediating this effect is not clear.
In conclusion, the change in posture from supine to maintained
standing caused a remarkable change in the heart rate pattern in
about 507. of subjects. As the "RSA" is predominantly
parasympathetic and in this study it was shown that the sympathetic
system was important in controlling the "10 second" rhythm,
therefore the change in heart rate pattern with posture reflects
the change in the balance between the sympathetic and
parasympathetic supply to the heart. This effect of posture on
heart rate pattern can be developed as a test for the autonomic
nervous control of the heart.
