The two numbers that form the blood pressure measurement are the systolic and the diastolic pressure. When the heart beats, the pressure reaches its highest level. That's the systolic pressure. Between the beats, the pressure reaches its lowest level. That's the diastolic pressure. The classic way of measuring the blood pressure is by pressurizing a cuff, which strangles the blood vessels in the arm. Slowly releasing the pressure in the cuff will eventually let the blood pass during the beat phase. Releasing further the pressure will allow the blood flow between the beats, too. A pressure sensor is needed to determin the exact pressure at which the blood starts to flow and when it flows continuously. An old school device consists of a mercury manometer and a stetoscope. Besides the inflatable cuff. A column of mercury will reveal the pressure. Modern devices use electronics to detect the blood flow as well as to sense the pressure. But even modern devices use the old school unit of mm Hg, that is millimeter of mercury. A systolic pressure of 120 means literally a pressure that can raise a 120 mm high column of mercury.Electronic devices
Electronic devices still use the inflatable cuff. The cuff is automatically inflated and then deflated, while a pressure sensor reads the pressure continuously. A microphone may listen to the blood flow. Alternatively the variations in the pressure in the cuff can reveal the moment when the decreased pressure releases the blood flow.About this project
The text you are reading describes a work in progress, which will hopefully end in a fully working device, where the blood pressure is measured from my fingertip. What I'm going to do is to create a device, which will read
- the amount of infrared light passing through the fingertip
- the amount of red, green and blue light passing through the fingertip
- the temperature of the fingertip
- the electrical resistance of the fingertip.
The main focus is on the infrared light. Blood absorbs infrared light. When the heart beats, the blood vessels dilate and more infrared light is absorbed. This variation can be detected by the IR sensor. If we put it simple, the more pressure, the less infrared light gets through. So this would be the main parameter affecting the values of the systolic and the diastolic pressure.
But a lot of other things can be affected by the blood pressure. Therefore I add as much sensors as possible to the system. My hope is that machine learning will reveal the correlations between all thes values. The amount of visible light passing through the fingertip might tell if the tissues in the hand are swollen. We know how difficult it is to remove a ring from the finger in the evening, while it's easy in the morning. This will most surely also affect the readings from the infrared sensor. The temperature and the electric resistance of the fingertip might as well vary a lot and affect the readings. As the project advances, I might add more sensors.