Continuous relation between high frequency component of HRV and respiratory frequency during postural change

Our aims were to use a chirp-type breathing maneuver to obtain a continuous relation between the high frequency component of HRV (HF
_RR
) and respiratory frequency (RF), and to assess if it indicates the vagal withdrawal induced by a postural change. ECG and lung volume were registered from 30 subjects who performed, in sitting and standing, a 70-s continuous linear RF increase from 0.05 to 0.8 Hz at constant tidal volume. From the smoothed pseudo-Wigner-Ville distribution of the RR intervals and tidal volume series, HFRR and RF were computed to obtain the relation. After log-transformation, correlations were -0.88±0.03 in sitting and -0.89±0.04 in standing. With the postural change the intercept decreased (p<0.001) and the slope increased (p<0.009), indicating a vagal withdrawal. The continuous HF
_RR
-RF relation we obtained can assess the vagal effect of the postural change independently from the RF influence, improving the interpretability of HFRR as vagal index.