Assessment of Baroreflex Sensitivity Using Time-Frequency Analysis during Postural Change and Hypercapnia

Figure 1. Exemplary recordings of end-tidal CO2 level (EtCO2; upper plots) and arterial blood pressure (ABP; lower plots) signals from the performed experiment: (a) resting position in normocapnia and hypercapnia with hypoxia, (b) postural change in normocapnia, and (c) postural change in hypercapnia with hypoxia. Figures above the plots indicate the subject's body position and presence of rebreathing bag during each part of the measurement. Dashed vertical lines denote approximate times of change in EtCO2 level or body position.

Figure 2. Exemplary time (top plots), frequency (left plots), and time-frequency (centre graphs) representations of (a) pulse interval variability (PIV) and (b) systolic arterial pressure variability (SAPV) signals recorded during postural change in normocapnia stage of the experiment. The low (0.04–0.15 Hz) and high (0.15–0.4 Hz) frequency ranges used in baroreflex sensitivity estimation are shown together. Figures below the graphs indicate the subject's body position during each part of the measurement.

Figure 3. Exemplary representations of TF coherence (TFCoh), TF phase shift (TFPS), and TFCoh-TFPS binary mask for the same subject as in Figure 2 (postural change in normocapnia stage of the experiment). (a) Exemplary representation of TFCoh between pulse interval variability (PIV) and systolic arterial pressure variability (SAPV) signals. (b) Exemplary representation of TFPS between PIV and SAPV signals. White areas denote parts of the representation where TFCoh is lower than 0.9. (c) Binary mask M based on TFCoh and TFPS representations. Black areas denote parts of the representation which fulfil the following criteria: (1) TFCoh > 0.9 and (2) TFPS < 0. The low (0.04–0.15 Hz) and high (0.15–0.4 Hz) frequency ranges used in baroreflex sensitivity estimation are shown together.

Figure 4. Exemplary representations of modulus of TF cross spectrum and TFCoh-TFPS/band binary masks for the same subject as in Figure 2 (postural change in normocapnia stage of the experiment). (a) Exemplary modulus of TF cross spectrum of pulse interval variability (PIV) and systolic arterial pressure variability (SAPV) signals. (b) Binary mask MLF based on TFCoh-TFPS mask and instantaneous spectral peaks of TF cross spectrum in low-frequency band (0.04–0.15 Hz). (c) Binary mask MHF based on TFCoh-TFPS mask and instantaneous spectral peaks of TF cross spectrum in high-frequency band (0.15–0.4 Hz). In (b) and (c), black areas denote parts of the representations used in BRS estimation.

Figure 5. Correlation plots for BRS estimates obtained using the sequence and TF methods for data from the resting position in (a)normocapnia and (b) hypercapnia with hypoxia experiments.

Figure 6. Bland–Altman plots for BRS estimates obtained using the sequence and TF methods for data from the resting position experiment: (a) sequence vs. LF BRS in normocapnia, (b) sequence vs. LF BRS in hypercapnia with hypoxia, (c) sequence vs. HF BRS in normocapnia, and (d) sequence vs. HF BRS in hypercapnia with hypoxia. Dotted red lines indicate mean values of difference between BRS estimates obtained with the methods. Dotted black lines indicate ±1.96 ∗ SD values of difference between BRS estimates obtained with two methods.

Figure 7. Group-averaged time courses of respiratory and hemodynamic parameters and BRS estimates in (a) normocapnia and (b) hypercapnia with hypoxia. EtCO2: mean end-tidal carbon dioxide level; RR: mean respiration rate; MAP: mean arterial blood pressure; PP: pulse pressure; HR: heart rate; LF BRS: baroreflex sensitivity in low-frequency range; HF BRS: baroreflex sensitivity in high-frequency range. Black lines: median; grey lines: first/third quartile. Figures above the plots indicate the subject's body position and presence of rebreathing bag during each part of the measurement. Vertical lines denote approximate times of squat-stand transitions.

Figure 8. Comparison of hemodynamic parameters and BRS estimates in corresponding stages of postural change in normocapnia and hypercapnia with hypoxia. MAP: mean arterial blood pressure; HR: heart rate; LF BRS: baroreflex sensitivity in low-frequency range; HF BRS: baroreflex sensitivity in high-frequency range. Boxes indicate median (central line) and first/third quartile (edges), and whiskers extend to the most extreme data points excluding outliers marked by + sign. Forks indicate statistically significant differences by Wilcoxon-signed rank test: ∗p <0.05; ∗∗p<0.005, ∗∗∗p<0.0005. In each position, the left-hand side box represents normocapnia and the right-hand side box represents hypercapnia with hypoxia.