A pilot study to assess the origin of the spectral power increases of heart rate variability associated with transient changes in the R‐R interval

FIGURE 1. Example of evaluation of changes in powers of very‐low‐frequency (VLF), low‐frequency (LF), and high‐frequency (HF) components associated with a transient RRI increase during standing. In this case, the length of the data segment was set to 30 s. (top) RRI data sequence during the first 80 s of a 600‐s RRI time series data plotted at 1‐s intervals. From this sequence, 51 30‐s RRI data segments were made consecutively with a 1‐s shift. (bottom) Plots of the magnitudes of powers of three frequency components calculated in 51 30‐s RRI data segments. The magnitudes of the powers in each segment are plotted at the end of the segment, indicated by red down arrows.

FIGURE 2. Two typical types of changes in the R‐R interval (RRI) and LF and HF powers that occurred upon standing from the supine positions. The RRI time series in the supine position and during standing each for 600 s, and the time course of changes in the LF and HF powers obtained by spectral analysis of 30‐s RRI data segments consecutively with a 1‐s shift are shown for type 1 (a1) and type 2 (b1) cases. The power spectral densities (PSDs) for these 600‐s RRI time series in the supine position and during standing are shown in (a2) and (a3) for the type 1 case and (b2) and (b3) for the type 2 case, respectively. (a1)–(a3): A typical type 1 response to the postural change from supine to standing. Both HF and LF powers were markedly reduced upon standing. (b1)–(b3): A typical type 2 response to postural change. Although the HF power was markedly reduced upon standing as in (a), the LF power increased. The PSDs (b2) in the supine position and (b3) during standing indicate that although the HF power generated by respiratory sinus arrhythmia (RSA) was almost completely suppressed during standing, there remained small but significant HF power due to the overspill of the tail of the LF…

FIGURE 3. Simultaneous increases in VLF and LF powers associated with transient RRI increases in the 600‐s RRI time series data during standing. The time courses of the VLF power increase and its temporal relationship with the LF power increase linked to the transient RRI increase in type 1 (a) and type 2 (b) cases were tracked continuously by connecting the magnitudes of the powers calculated by spectral analyses of 30‐s data segments, each conducted consecutively with a 1‐s shift. The time course of the HF power change is also shown.

FIGURE 4. Effects of changing the length of the data segment on the detection of power increases associated with transient RRI increase in five different frequency ranges: range 1 (0.003–0.04 Hz), range 2 (0.04–0.08 Hz), range 3 (0.08–0.12 Hz), range 4 (0.12–0.15 Hz), and range 5 (0.15–0.20 Hz). Type 1 (a) and type 2 (b) 600‐s RRI time series during standing (the first panels in (a) and (b)) shown in Figure 2 were analyzed. The second to fifth panels in (a) and (b) show the time courses of the changes in the powers in the five frequency ranges, which were obtained by connecting the magnitudes of the powers calculated by spectral analyses of 5‐s, 30‐s, 100‐s, and 200‐s data segments, each conducted consecutively with a 1‐s shift.

FIGURE 5. (a1) and (a2) Graphs to summarize the effects of changing the data segment length in the 600‐s RRI time series during standing on the estimation of power increases in five different frequency ranges in type 1 (a1) and type 2 (a2) cases. The average values of six type 1 and six type 2 cases are plotted as the mean ± standard deviation (SD). Each value of the power magnitude was normalized by the total power obtained in the 600‐s segment. (b) Effects of changing the data segment length on the ratio of VLF power relative to LF power. The LF power was obtained as the sum of powers in ranges 2–4. This ratio in the type 1 case markedly increased when the segment length was increased to 100 s, and it reached a level similar to that of the 600‐s segment at the 200‐s segment. The ratio was significantly larger in the type 1 case than in the type 2 case for all segment lengths (p < 2.1×10−4).

FIGURE 6. Simultaneous recordings of arterial blood pressure, interbeat interval (IBI), RRI, and changes in VLF, LF, and HF (0.15–0.20 Hz range) powers in type 2 (a) and type 1 (b) cases. In both (a) and (b), the top (first) panel shows the time series of arterial mean blood pressure (MBP) changes, the second panel shows that of IBI estimated by the time point of the systolic blood pressure, and the third panel shows the RRI time series. The time courses of IBI and RRI were similar. The fourth panel shows the time course of changes in VLF, LF, and HF power obtained by the 30‐s segment analysis of the RRI time series shown in the third panel. The bottom (fifth) panel shows the time course of change in HF power obtained by the 5‐s segment analysis. Three black arrows in the top panels in both (a) and (b) show the occurrence of higher blood pressure increases with a faster rate of rise than other blood pressure increases. These larger pressure increases initially caused a decrease in IBI (RRI), followed by a sudden reversal to the sharp increase. Three pairs of red and black vertical lines are drawn from the top to the bottom panel at the black arrows. The red line indicates the onset of the…