Schematic diagram showing Hilbert-Huang decomposition of the original blood pressure (BP) signal into the empirical modes corresponding to amplitude-frequency modulation for different time scales. Panel A shows the profile of the BP waveform over the course of the VM: I- indicates the beginning of the maneuver, II- the duration of straining, III-the end of straining and IV- the BP overshoot above baseline. (Note that the transient BP decrease in phase III is due to inspiration.) Panel B shows the empirical modes for each component frequencies and their corresponding amplitudes were detected from the signal (mode 1–10). Empirical modes corresponding to the faster frequencies (modes 1–5) were removed from the original BP and BFV signals. The empirical mode corresponding to the characteristic BP profile induced by the VM (BPVM – mode 6 in this example) was used to obtain phase information. Modes 7–10 reflect BP modulations at slow frequencies. Similarly, the empirical mode corresponding to the characteristic BFV profile was extracted from the raw BFV waveform (not shown).