Effect of Posture on Respiratory Sinus Arrhythmia in Young Females

Abstract

Effect of posture on the amplitude of respiratory sinus arrhythmia (RSA) at some breathing periods between 4 and 12 sec was studied in standing and supine female students of a physical education college.
Tidal volume increased in accordance with the respiratory period. RSA amplitude, measured as heart period (i. e, pulse interval) change, was greater in supine position than in standing posture when the respiration period was rather short, that is at 4, 6 and 8 sec. When the interval change was expressed as percent of existing mean heart period, the difference was significant only at breathing period of 4 sec. RSA amplitude expressed as heart rate change (beats/min) was greater in supine than in upright position at respiration period of 4 sec, but at 10 and 12 sec period the relation was reversed. The greater heart rate amplitude in standing posture at slower (and deeper) respiration seems to be partly apparent, because at shorter basal heart period (i. e, in standing), a similar change in pulse interval is expressed as larger heart rate change than at longer basal heart period. The regression coefficient of RSA amplitude on tidal volume was slightly greater in standing than in supine posture.
This difference in the slope of regression line would explain, also partly, the absence of RSA amplitude difference between the two postures at slower respiration rate.
The difference of heart rate level by posture was negatively correlated with mean RSA amplitude in standing. This seems to come from the fact that the difference is determined mainly by the heart rate level in standing, and RSA amplitude changes as a function of the mean heart period.
Although these results are almost in agreement with the relation of RSA with vagal activity shown by Katona et al. (1975), other factors, like the sympathetic influence on baroreflex sensitivity suggested by Eckberg et al. (1976), should be considered.