Piezoelectric probe for pressure waveform estimation in flexible tubes and its application to the cardiovascular system

The problem of using a piezoelectric (PZ) probe to non-invasively measuring the pressure wave propagation through a fluid contained in an elastic tube is considered in this paper. In particular, we describe a probe system designed to non-invasively reproduce the morphology of the pulsatile arterial pressure waveform (APW). The study is focused in three main issues: the mechanical interface that transmits the forces associated to the distension of the wall of the tube to the sensor, the electronic conditioning circuit and the methods to assess the global accuracy of the system. The circuit, incorporates a, new to our knowledge, baseline restorer (BLR) that contributes to maintaining a stable (non-floating) baseline of the cardiac pressure pulses, making real-time observations more effective. Identification and correction of the systematic errors, responsible for deviations of the correct output morphology, are also discussed and tested for different waveforms. To assess the performance of the probe a special purpose test bench was developed that can originate an arbitrarily shaped pressure wave and launch it through a silicone-rubber tube. Finally, preliminary results, taken at the carotid site of a set of human volunteers, are shown. The probe can be incorporated in a collar, and its pulse waveforms exhibit high intra-patient repeatability. It has the potential of being used as an alternative to costly techniques such as ultrasound or applanation tonometry. The root mean square error (RMSE) of the probe when reproducing cardiac-like pressure waveforms yielded a value of 1.8 ± 0.22%.