Body position does not affect respiratory sinus arrhythmia magnitude (1103.4)

Abstract

Body position does not affect respiratory sinus arrhythmia magnitude Respiratory sinus arrhythmia (RSA) is the normal fluctuation in heart rate in phase with respiration. Instantaneous heart rate (IHR) increases with inspiration and decreases with expiration. The mechanisms underlying RSA are unclear, but some evidence suggests that central brainstem mechanisms drive RSA, independent of afferent feedback. Cardiac loading associated with inspiration or gravity may also trigger the Bainbridge reflex, although the contribution of this reflex is unclear in humans. We investigated the effects of incremental increases in inspired volume on RSA magnitude during both 45° head‐up and head‐down tilt (HUT, HDT). Six healthy subjects were positioned on a tilt table and instrumented with ECG to measure IHR (60/period) and a pnuemotachometer to measure respiratory volumes. Following a forced vital capacity (FVC) measurement, subjects were coached to breathe 30 consecutive breaths at each inspiratory volume of 10, 15, 20, 25 and 30% of FVC using a computer screen for visual feedback. The average RSA magnitude of the middle 10 breaths of each trial was plotted against the inspired volume, and linear regression was performed. The increase in RSA magnitude with inspired volume was highly linear in HUT and HDT (R2>0.9), but the mean slopes were not different between positions. We have determined a range of inspired volumes that generate a linear RSA magnitude response. Further, our data demonstrate that gravity‐dependent cardiac loading does not influence RSA magnitude associated with increases in inspired volume.