Photoplethysmography(PPG) Introduction, Sowa Rigpa and philosophy
Introduction
To understand the human body we like to measure and quantify various units related to the body or the functioning of the body. We get information from the body through various means. A few examples are
One of the simplest and cheapest ways today to measure is using light and photodetectors. This is called photoplethysmography (PPG) and it is the basis for most of the wearables out in the market. These wearables measures your pulse or pulse rate (many call this for heart rate but experts only talk about heart rate in combination with ECG). Because this measurement is so common there is no end to what people have tried to achieve using this signal. PPG is mainly used to measure
Physical principles
PPG measures volume change when the heart pumps. Light is scattered in two ways in tissue
The difference between them has to do with molecule vs wave length of light. In addition we have the absorption curve of blood. Depending on the absorption curve we select wavelengths we like to study. There are two types of hemoglobin, either saturated with oxygen (arterial blood)) or deoxyhemoglobin (venous). From the extinction coefficient we see where to measure what:
Here is color vs wavelength. The interesting wavelengths to measure could be e.g. 440 nm, 550 nm, 660 nm and 940 nm depending on application.
A particular technology to measure in multispectral sensing. In this measurement you have a white light source and can capture many frequencies in one shot saving energy and time?
This is the basic PPG signal consisting of systole and diastole. The cardiovascular system is not a terminated system but pulses are reflected back. In other words there are echoes. The echo is the dicrotic notch and when you get old then you lose the echo.
Pulse and traditional Tibetan medicine (Sowa Rigpa)
The PPG is a pulse measurement and understanding pulse is ancient understanding. Apple did not invent pulse measurement. The pulse is a key measurement in Sowa Rigpa or traditional Tibetan medicine, a holistic system of medicine, body constitution, rhythms, food and mind that developed over hundreds of years through experience. The pulse reading is performed by a trained doctor who senses the pulse using 3 fingers for an extended period of time. Out of the measurement the doctor can extract information about the general balance of the patient e.g, in west we would call this for ANS.
Heart rate and poor signals
The PPG signal has a terrible signal to noise ratio and in particular, PPG is sensitive to movement. The remedy is that most watches has some signal compensation involving an 3-axis accelerometer. It is a multi year project to develop a reliable heart rate extraction algorithm. The quality of Apple took a decade of systematic research to reach so buy the algorithms if you ever need one. A good source is SATheart, CSEM or Valencell.
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SpO2
SpO2 is an even more difficult measurement. You must capture many wavelengths that are extremely noisy simultaneously at certain times. Long wavelengths travel deeper into tissues but seems to bring more noise.
In the cold, measurement place, black skin
Low ambient temperature will cause vasoconstriction causing even poorer signals. Some use longer wavelengths to penetrate deeper to compensate. Unfortunately, penetrating deeper seem to bring more noise.
The wrist is poorly capillarized and a bad place to measure(Apple, Fitbit, Garmin). Oura has a good place at the finger and high quality PPG signals.
Black skin causes many issue due to melanin that overlaps the spectrum. This makes the SpO2 measurement far more complex.
These are only a few sources of disturbances.
Blood pressure
Unfortunately in many applications you need not the signal but the the second derivative of the signal. The reason is that the second derivative is the motion of the heart and pressure is force / area, force is proportional to acceleration p= F / A= m a / A.
Whenever you form differences between points in a noisy signal you get more noise. In the picture (i) down one we have two areas related to blood pressure. It is the systole and diastole. Again, blood pressure is really hard to measure using the pulse so speak with Aktiia in Switzerland. Aktiia spend more than a decade to perfect the measurement. Valencell is another example investing into bloodpressure. Apple has not managed this.
Autonomous nervous system (ANS)
The activity of the ANS can be extracted from the pulse wave. Sympathetic activity is related to physical activity and stress. Parasympathetic activity is related to relaxation, sleep and recovery. This can all be extracted by looking at changes peak to peak, and, possibly, some other parts of the signal.
Information in the PPG
What kind of information could we expect to find in the PPG? The PPG is the result of the heart beating and a pressure pulse travelling down a complex tree of branches, What we would expect to find is
Summary
PPG is a versatile measurement that can capture a wealth of information. The interesting information is fairly well understood and what is left is probably details, The development of reliable algorithms take years of experience to do and this is the hard part.
What is missing today is life style guidance that would take a complete sets of information to improve life in a holistic manner. The western world is unfortunately even lacking words and concepts to talk about many aspects of health so we have a way to go. This is one of the goals of SATheart.
Here are a few quotes of Wittgenstein