Figure 6: Extracting the respiratory component of heart rate variability.
From: In vivo cardiac phase response curve elucidates human respiratory heart rate variability

(a) The variances of the RSA-HRV and non-RSA-HRV signals, as functions of the variance of the original HRV. Green diamonds show the sums Var(RSA-HRV)+Var(non-RSA-HRV), these are nearly equal to Var(HRV) (black dashed line), which means that RSA-HRV and non-RSA-HRV are almost uncorrelated. (b,c) Power spectra of original HRV (green), the RSA-HRV component (red) and of the non-RSA-HRV (blue), in the cases of maximal (b) and minimal (c) relative RSA-HRV component; in (b) ≈67% of Var(HRV) is contained in RSA-HRV, indicating vagal predominance, whereas in (c) only 10% Var(HRV) is in RSA-HRV, indicating sympathetic predominance. In (b) the letters A, B, C mark three respiratory-related peaks, corresponding to average respiratory frequency 0.23 Hz and to side-bands of the heart rate (average value 0.92 Hz), that is, to 0.92±0.23 Hz. Notice that these peaks (at which the green and red curves practically coincide) are definitely higher than the surrounding, which indicates a strong contribution of RSA-HRV in these frequency bands. Correspondingly, the residual non-RSA-HRV (blue line) is much weaker than HRV and RSA-HRV in these bands. Contrary, in (c) the peaks of RSA-HRV, marked by D (corresponds to average respiratory frequency 0.15 Hz) and E (side-band of the heart rate peak at ≈0.82 Hz) are low and the corresponding peaks in HRV only slightly exceed the surrounding; hence, the residual non-RSA-HRV is relatively high.