V.B. Parashin, M.N. Simonenko

Non-invasive methods of blood pressure measurement show in the majority the limit values of blood pressure, such as systolic and diastolic blood pressure. The method of unloaded artery suggested by Penaz in 1969, is not widely used because of the complexity of implementation and interpretation of ambiguous results, including the ratio of pressure in the arteries of the fingers with the pressure in the aorta. The method of continuous pressure signal recovery proposed in the article, can be implemented on existing devices for blood pressure measuring. The resulting signal reflects changes in blood pressure in the brachial artery, which correlates better with the pressure in the aorta. To implement the method, the signal pulse oscillation of cuff pressure (signal waveform) is used in the measurement of blood pressure by occlusive-oscillometric method at the stage of decompression. The mechanism of the signal waveform can be represented as the interaction of the three functions: f1 function of pressure pulsation of blood in arteries, f2 - compliance of the arteria and f3 - function of changing the external pressure. Then, the true waveform (change in cross-section of the vessel during the measurement of blood pressure) can be represented by the following formula: f2(f1(t)+f3(t)), and the signal from the pressure sensor as K∙f2(f1(t)+f3(t))+f3(t). Using this approach it is possible to get a BP signal during several cardiac cycles. To select the f1 function from the real signal it is necessary to know functions f2 and f3. Finding of the function f2 (volume compliance curve (VCC)) is possible after the subtraction of the function f3 from the oscillogram signal. For the construction of the VCC it is possible to use the middle of the amplitude of each oscillation, or notch in the «normal» form of the pulse pressure signal. When finding the function f3, it is necessary to consider the features of the technical realization of devices, namely the flow characteristics of the bleed valve (orifice). The algorithm to obtain the reconstructed signal is shown for the case of the function f3, which is close to exponential in the area of negative transmural pressure, and consists of the following stages: 1) record of the signal by the pressure sensor; 2) selection of informative section of the signal and then taking the logarithm; 3) constructing a linear approximation function and subtracting it from the signal obtained in step 2; 4) constructing the VCC (finding of f2) for each selected level of the amplitude of the oscillations; 5) finding the inverse function f2; 6) application of the inverse function and subtracting a linear relationship to obtain the reconstructed pressure signal. These results include the variation of the amplitude and shape of the oscillation from cycle to cycle, but the signal has a steady level of systolic and diastolic blood pressure.