PPG (Photoplethysmography)

<What is PPG?>

- PPG

> Optical technique used to detect volumetric changes in blood in peripheral circulation

→ Low cost & non-invasive method (Measurements at the surface of the skin)

→ Measurable list: Oxygen saturation, Blood pressure, Cardiac Output, etc.

 

- Principle

> When heart beats, the blood vessels are slightly inflated and arterial blood flow changes by pressure. Accordingly,  light transmittance is measured → PPG signal

> Shine the LED light on human tissue which includes microvascular and detect the difference of light by using photo detector

 

 

<Charateristic of PPG>

- (Quasi) DC + AC

> PPG waveform comprises a pulsatile physiological waveform attributed to cardiac synchronous changes in the blood volume with each heart beat, and is superimposed on a slowly varing baseline with lower frequency components attributed to respiration, sympathetic nervous system activity and thermoregulation

> Average respiration rate of adult: 20 per minute → About 0.3~0.4 [Hz] (Breathing Rate)

> Average heart rato of adult: 60 ~ 120 per minute → About 1~2 [Hz] (Heart Rate)

 

- PPG signal characteristic by light wavelength

> Green: The light wavelength where hemoglobin absorb most compared to other lights (Great absorptivity) -> Clearly distinguished PPG signal is detected and commonly used for measuring heart rate

> IR (Infrared Ray): Less effected by (skin) color -> used in wear detection. But, very vulnerable to motion effect. Also, by measuring red and IR simultaneously and calculating light transmittance difference of oxy, de-oxy hemoglobin, we can detect oxygen saturation.

 

 

<Circuit of PPG>

- Main device: LED (Light Emitting Diode), PD (Photo Detector), TIA (Transimpedance Amplifier)

> TIA: For amplifying PD current because PD current is small

 

<Drawback of PPG>

- Vulnerable to Motion: MA (Motion Artifact)

1. Motion result in the leakage of lights → Directly affect to PPG

2. Deform the microvascular system 

→ But, the frequency range of PPG (0.5~4.0 [Hz]) is considerably overlap to the frequency range of MA (0.01~10 [Hz]) ∴ Normal filtering method for eradicating MA noise couldn't be applied 

→ Many algorithm research for removing MA noise are in underway