Sinus arrhythmia in man at rest.

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Abstract
The separate effects of inspiration and expiration on the heart rate were investigated in 10 healthy resting subjects. Inspiration was shown to produce a biphasic fluctuation in heart rate of 15.9 beats/min. mean amplitude and 13 sec. duration. The underswing was 1.3 beats/min. mean, representing 9.4% of the total response. Expiration had little or no effect on heart rate under normal resting conditions. Super-position of these inspiratory transients takes place to produce various patterns of arrhythmia depending upon a given subject''s rate and depth of breathing. Only at very low respiration rates is clear rise and fall of heart rate to be seen. The underlying mechanisms of sinus arrhythmia were discussed. It was suggested that though a stretch receptor might be involved the greater part of the effect was probably due to afferent impulses arising from vasoreceptors on the left side of the heart as a result of blood flow changes which accompany respiratory movements.

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