Calculation of an Improved Stiffness Index Using Decomposed Radial Pulse and Digital Volume Pulse Signals

Figure 1. Measurement of digital volume pulse (DVP) by transmission of infrared rays through the finger. Generally, DVP comprises a systolic peak (the first peak) and a diastolic peak (the second peak). ΔT = the time difference between the first and second peak; stiffness index (SI) = body height/ΔT.

Figure 2. We chose IMF5 and IMF6 from Equation (3) to calculate SI indexes after decomposition of the radial pulse and DVP using EEMD. We automatically computed the time difference (ΔTnew) between the first IMF5 peak (P1) and IMF6 trough (T) within the same period. We attributed IMF7 to respiratory signals, with a frequency of around 0.2 Hz. Since this signal did not affect the accuracy of the IMF5 signals, we deemed its measurement unnecessary.

Figure 3. Variation in the radial pulse and DVP curve with age and diabetes. Non-stationary waveform signals were decomposed using ensemble empirical mode decomposition (EEMD) into nine intrinsic mode functions (IMFs), including IMF5 and IMF6. (A) Radial pulse from subject A (healthy older individual; 1.73 m, age 50) with ΔTnew = 0.43 sec and SInew-RP = 4.02 m/s; (B) DVP from subject A, with ΔTnew = 0.48 sec and SInew-DVP = 3.60 m/s; (C) radial pulse from subject B (type 2 diabetic patient; 1.63m, age 52) with ΔTnew = 0.28 sec and SInew-RP = 5.82 m/s; (D) DVP from subject B, with ΔTnew = 0.38 sec and SInew-DVP = 4.29 m/s. The graphs show lower ΔTnew values for the type 2 diabetic patient regardless of the signal type (radial pulse or DVP). For ease of viewing, the radial pulse and DVP signals of only 1600 samples are shown.

Figure 4. Significant correlation between the SI measured by six-channel ECG-PWV system [7] and SI calculated using the new method based on decomposed radial pulse signals (measured by a refined air pressure sensing system) [15]. SInew-RP—SI calculated using the new method based on decomposed radial pulse signals proposed in Equation (4); SInew-DVP—SI calculated using the new method based on decomposed DVP signals proposed in Equation (4).