Heart rate variability in man

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.