The PPG (Photoplethysmography) data is provided by the E4 device. This is also known in the scientific literature as BVP (Blood Volume Pulse).
We suggest you to read the following articles before starting to analyze the BVP data:
- Pulse oximetry KK Tremper - CHEST Journal, 1989
- Sinex, James E. "Pulse oximetry: principles and limitations." The American journal of emergency medicine 17.1 (1999): 59-66.
The PPG data is also extremely related to the IBI (interbeat interval) data. If you want to read more of how they relate to each other read our What should I know to use the PPG/IBI data in my experiment? article. The PPG is the input signal to the algorithm that detects the heart beats and that provides the IBI signal as output.
The processing of the PPG in the E4 wristband happens as you can see in (eq 1)
(eq 1.) GREEN, RED -->[ Algorithm 1] --> PPG --> [Algorithm 2] --> IBI
The E4 wristband implements only a part of working principle of classic pulse oximetry and it relies in the reflective mode principles:
- E4 wristband uses green and red light instead of classic Red/IR coupling
- The way the green and red light are combined is optimized for maximizing the detection of the pulse wave, i.e. the estimation of the heart rate, instead of the oxygen saturation level (which is not part of the Empatica outputs).
The first part of the processing performed by the E4 is represented in (Fig 1)
The lights produced by the Green and Red LEDs are oriented toward the skin and are absorbed by the blood in different ways. Portion of the light is then reflected back and is then measured by the Light receiver.
- The measured light during green exposure contains most of the information on the pulse wave (i.e., the heart beats) and it is typically characterized by a sequences of valleys, whose time occurencies are used to estimete the heart beats. Note that the more the blood is oxygenated, the more the light is absorbed. Thus, during a heart beat, there is a high light absorption, which is observed as a valley in the light output signal.
- The measured light during the red exposure contains a reference light level which is used to cancel motion artefacts.
The PPG/BVP output
The PPG output is obtained by a proprietary algorithm which combines the light signals observed during both green and red exposure.
You might consider to use the PPG in you experiments to compute additional informations. In Fig 2 there is an example of PPG output that you can have on E4. Fig 2 is a typical condition but the output may vary depending on user and conditions.
- Diastolic points: local minima of the PPG are used to compute the IBI.
- Systolic points: local maxima of the PPG could be used in conjunction with the Diastolic point to estimate the vasoconstriction of the subject.
- Dicrotic notch under some conditions (i.e subject static) we observe the presence of this point which can be used to study different types of cardiac diseases.
- Dicrotic wave effect of Dicrotic notch
Note on the signal
- Zero mean value
- The measurement unit is fraction of nanoWatt (nW). This is basically a difference of light absorption observed by the light receiver.
- The signal is also inverted in sign thus basically the light observed at systolic point is less than the one observed at diastolic point.
- The difference PPG(Systolic point)-PPG(Diastolic point) is basically the difference in light between the most and least oxygenated blood condition. If the sensor is well positioned and there is no influence of motion on the data, this difference in light gives an information on the vasoconstriction of subject.