NON-INVASIVE BLOOD PRESSURE MEASUREMENT APPROACH FOR CONTINUOUS AMBULATORY MONITORING

Ripoli A, Franchi D, Palagi G, Belardinelli A, Ghione S and Bedini R
Institute of Clinical Physiology, National Research Council,
Via P. Savi 8, 56100 Pisa, Italy.

The continuous non-invasive monitoring of blood pressure is an up to now unresolved problem. The available devices based on vexing occlusive cuffs are not suitable for long-time measurements, while the compressor noise causes an emotional artefact on the measurement itself. As a promising measurement method, avoiding the above mentioned problems, we investigated the relationship between the pulse wave velocity, namely the distance that the pulse wave travels in one second, and the systolic, diastolic and mean values of blood pressure. The greatest simplicity in the measurement is accomplished evaluating the pulse wave velocity by means of the delay between the R wave of the electrocardiogram and the onset of the pulse wave in a peripheral site. Although such a method has been already exploited, supposing pulse wave velocity to linearly vary with blood pressure, an explicit relationship between these two physiological parameters, being the velocity of the pressure wave a function of several parameters of the cardiovascular network (mainly the dimensions and the stiffness, the Young's modulus, of the vessels), has not yet been identified. In order to study the behaviour of the pulse wave velocity we built a mathematical model of the arterial network, so that an evaluation has been possible about the effect of blood pressure, age and body weight on it. The data for the model has been acquired, during cath-lab sessions, by means of an ad hoc developed computerised acquisition system of blood pressure waveform and pulse wave velocity. The pulse wave has been detected by means of transmission photo-pletysmographic device while the aortic pressure waveform is acquired by means of a fluid-filled pressure transducer, being our main interest on man systolic and diastolic values of blood pressure and not on its exact waveform The signals, acquired at a sampling frequency of 2000 Hz, to assure the necessary time resolution, and on-line displayed are: one lead ECG, pulse wave and aortic pressure. An algorithm has been developed for the delay evaluation between the QRS complex and the pulse wave onset; beat to beat systolic, diastolic and mean aortic pressure values have been carried out. The accomplished analysis showed an increasing in the pulse wave velocity with the pressure values, while the already exploited linear model seems not to fit the acquired data. A new estimation is here provided by means of a developed non-linear model calibrated, by means of alternative blood pressure measurements, on the single patient, while special care has been paid in the selection, for the photo-pletysmographic device, of a distal peripheral site, in order to minimize the error induced by the isovolumic contraction period of the heart.