My thesis is about the design of a blood pressure monitor and the algorithms used to determine the blood pressure. The device is using the most common blood pressure measurement strategy, the oscillometric method. It is a non-invasive method, employs an occluding cuff and operates by sensing the magnitude of oscillations caused by the blood as it increases the volume of the occluded artery. The method measures only the mean artery pressure, and calculates the systolic and diastolic pressure using constants determined by statistical basis. The best grades of the standards (AAMI, BHS) allow more than 5mmHg deviation from the reference in the 40% of the measurements. It can be seen that the applied algorithms offer opportunity for improvements.
At the beginning of my thesis I show the physiological background of blood pressure and ECG signals. Then I present the historical turning points of blood pressure monitoring, and the recently used blood pressure measurement strategies. At the second chapter I describe the hardware modules.
The blood pressure is not a permanent quantity, it changes with every heartbeat, depending on the time of the day or the stress level. Even so it is usually described with two numbers, the systolic and the diastolic pressure. For that reason it’s very important to measure the blood pressure every time under the same circumstances and at the time which can represent a longer interval. I show an algorithm which can determine the stress level, the tool can decide if it was a representative result or it was influenced by the stress level.
As I mentioned before the oscillometric method determines the systolic and diastolic pressure by calculation. The calculation uses constants determined by statistical basis. In my thesis I present algorithms based on the magnitude of oscillometric pulses or the gradient of oscillometric pulses. Most of the blood pressure monitors measure during the deflation. The measurement during the inflation has some technical problems, however it has a huge advantage. In the inflation period the method of measurement influences the circulation less than in the deflation period, by occluding the artery. That is why I present an algorithm which measure during the inflation period.
There are some common mistakes in home blood pressure monitoring: the cuff can be wrapped around the arm e.g. upside down, loosely or even above a shirt sleeve. In my thesis I show how to check if the cuff is applied properly, using the time function of the cuff-pressure. The arrhythmic heartbeats also can influence the result of measurement. I present how to find these arrhythmic pulses and how to eliminate the effect.
One of the main questions of my thesis was the personalization. It plays a part at the determination of the stress level. And I suggested the calibration of the tools to specify the constants used to calculate the systolic and diastolic pressure.