Features - Notes

Features

1) Pulse transit time

Difference between the time at which the PPG signal rises and Peak in ECG signal.

2)Photoplethysmograph intensity ratio

= Highest intensity of PPG / Lowest intensity of PPG

= IH / IL

3) Womersley number

=alpha = il*sqrt(1060*Heartrate/meu);

1060= density of blood

meu = AC component max Amplitude

4) Heart Rate

5) IH

6) IL

7) Meu (AC component max Amplitude)

Features extracted using PPG signal:

1. SUT (Systolic Upstroke Time) : It is the time interval between the start of the pulse and the peak value of the pulse. It is calculated by finding the time interval between the max peak value location and min peak value location.

2. DT (Diastolic Time) : It is the time interval between the peak and end of a pulse. The time interval between adjacent max and min peak gives this value.

3. ST (10,25,33,50,66,75) : It is the time interval in the systolic range where the amplitude of PPG is 10%( or 25,33,50,66,75%) of peak value. The calculation is done as follows:

• First the peak value of signal is calculated.
• Then, all PPG values lying in between the min and max peak (systolic range) is copied onto a temporary array.
• From peak, 10% of peak is found out, and the temporary array containing PPG values is checked for exact presence of 10% value or a value which is closest and slightly lower than the required value.

4. DT(10,25,33,50,66,75) :

It is the time interval in the diastolic range where the amplitude of PPG is 10%( or 25,33,50,66,75%) of peak value. The calculation is done as follows:

• First the peak value of signal is calculated.
• Then, all PPG values lying in between the max and min peak (diastolic range) is copied onto a temporary array.
• From peak, 10% of peak is found out, and the temporary array containing PPG values is checked for exact presence of 10% value or a value which is closest and slightly lower than the required value.

From the above values the below features are calculated:

• • SUT, DT
• • 10%: DT10, ST10+DT10, DT10/ST10
• • 25%: DT25, ST25+DT25, DT25/ST25
• • 33%: DT33, ST33+DT33, DT33/ST33
• • 50%: DT50, ST50+DT50, DT50/ST50
• • 66%: DT66, ST66+DT66, DT66/ST66
• • 75%: DT75, ST75+DT75, DT75/ST75

Note:

• Some features may have NaN values because findpeak(matlab fn. does not work in some cases).
• Features like ST or DT 75,66 may have many zero values because the time interval between peak and time of occurance of the 75% peak value may be very small that it becomes zero.

Features extracted using SDPPG signal:

1. A_a : This feature is calculated by finding all the peaks, and taking the peak with max value.

2. A_b : This feature is calculated by finding all the peaks in negative direction, and taking the peak with max value.

3. A_e: This feature is calculated in following way:

• All the SDPPG values lying in between two peaks in positive direction is stored in a temporary array.
• The peaks lying between those are found and the max of those peaks(lying between 2 adjacent positive peaks) gives the value A_e.

4. A_d : The values of SDPPG in between A_b and A_e are stored in temporary array, and all peaks in negative direction is found. The peak with max value in negative direction gives A_d.

5.A_c : The values of SDPPG in between A_b and A_d are stored, and the peak (only one peak lies between b&d) is found out which gives A_c.

By using the value and location of above, the folowing features are found out:

 A_b/A_a

 A_c/A_a

 A_d/A_a

 A_e/A_a

 T_a

 T_c

 T_e

 T_b

 T_d

 AI = (A_b - A_c - A_d - A_e) / A_a

Once the location of a,b,c,d,e are known, the AP_a, AP_b..... can be calculated from the array containing PPG values.

The value of PPG signal at the location of feature a in SDPPG gives the value of AP_a, (i.e) if feature 'a' occurs at location loc_a in SDPPG, the value of PPG array at index loc_a gives AP_a.

Similarly AP_b,AP_c... etc are calculated and below features are obtained:

 AP_b/AP_a

 AP_c/AP_a

 AP_d/AP_a

 AP_e/AP_a

Physiological Features:

Taken from Link