PPG Flashcards
What is a PPG
an optical measurement technique to record blood volume changes (light shines into skin and the amount of light that is reflected back or transmitted through is recorded.
What are the two parts of a PPG
- A light source
- A light sensor to measure the light
What are the two modes of a PPG
- Reflectance mode
- Transmittance mode
Describe reflective mode of PPG
light is sent in and it is sensed how much light has reflected back. The photodetector is at the same side as LED
Describe transmittance mode of PPG
light is sent in through the finger and it is sensed how much light is coming through. Photodetector is opposite side of LED
What is the beer lamber law
the absorption of light in a medium is proportional to the path length and the concentration of the absorbing species (A = (e)(b)(C))
what is the formula for wavelength and frequency
f = speed of light (c) / wavelength
what is the main factor affecting light absorption in a PPG
If we take a light and send it into fingertrip, the amount of light that is absorb depends on the concentration of blood in the finger
(A = molar absorption x distance x concentration of blood)
molar absorption of blood doesn’t change and the size of the finger doesn’t change either, so the concentration of the blood changes)
how does a fitbit track your heartrate?
it shines a green LED through your skin and measures how much light is absorbed. when your heart beats, more blood flows through your veins, which absorb more light
What is the difference between red and green LED?
Red is good for measuring blood oxygen level because it is more sensitive to oxygen changes in blood
Green light is good for heartbeat as it is very sensitive to volume changes
which LED colour is absorbed better by the skin?
green blood is absorbed better by our bodies, but the depth of penetration is limited.
red blood is not absorbed as well by our bodies, but the depth of penetration is deep, which tells us a lot more about the body
AC vs. DC Components of PPG Signal
AC : (Pulsatile), Varies with heartbeat. Measures heart rate, usually 1 Hz
DC: (Non-Pulsatile), Baseline blood volume in tissue, Affected by respiration, vasomotor activity, temperature
What is pulse oximetry and how does it work
measure the oxygen of the pulse
utilized PPQ measurements to obtain information about the arterial blood oxygen saturation
how does the two states of hemoglobin (oxygenated and deoxygenated) have different properties, which affect how they interact with light?
Oxyhemoglobin (HbO₂) → When oxygen is attached, hemoglobin folds in, changing its shape. absorbs more infrared light.
Deoxyhemoglobin (Hb) → When oxygen is released, hemoglobin opens up, exposing the iron atom in the heme group. absorbs more red light.
What is the isosbestic point of the light absorption curve
the point where the absorbent of oxygen-rich and oxygen-poor blood are equal
How does a pulse oximeter measure blood oxygen saturation (SpO₂)?
It shines red and infrared light through tissue, detecting how much is absorbed. Oxygenated blood absorbs more infrared light, while deoxygenated blood absorbs more red light. The ratio is used to calculate SpO₂.
