Modulation of respiratory sinus arrhythmia by respiration rate and volume: stability across posture and volume variations.

We tested the assumption that the slope of the within-individual regression equation of respiratory sinus arrhythmia (RSA) normalized by tidal volume (V(T)) upon respiratory cycle length (T(TOT)) can characterize ventilatory RSA modulation independent of cardiac vagal tone, whereas the intercept is varying as a function of vagal tone. We also explored whether a variation of V(T) is necessary to estimate slopes reliably. Four 3-min sequences paced at 8-18 cycles/min were performed supine and standing. Participants also breathed the same sequences in supine posture with voluntarily varying V(T). The mean slope of RSA/V(T) upon T(TOT) was identical, and the mean intercept was lower for standing than supine (spontaneous V(T)) conditions. Stability of slopes was low between body postures, and was higher between spontaneous V(T) versus varying V(T) at the same body posture. The regression of RSA/V(T) upon T(TOT) allows for a valid estimation of ventilatory influences on RSA.