A multi-element carotid tonometry sensor for non-invasive measurement of pulse wave velocity

A new pulse wave velocity (PWV) measurement system has been developed using a novel multi-element tonometry carotid sensor combined with a heart sound sensor. In this system, PWV is derived from the time lag between the second heart sound (S2) obtained from the heart sound sensor and the dicrotic notch in the carotid pulse waveform, and the physical distance between the heart and the neck. We assessed the accuracy of the system in an animal model. The study was divided into two groups: in Group I the tonometric sensor was directly applied to the exposed artery, while in Group II the sensor was applied over the skin and subcutaneous tissues covering the artery. To examine the fidelity of the dicrotic notch, the ejection time with the tonometry sensor was compared with that obtained from the intra-arterial catheter measurement. The correlation coefficients between them were 0.99 in both groups. The bias error (defined as the mean of the differences between the tonometry and the catheter measurements) +/-2SD was 0.13+/-1.45 ms in Group I and 0.16+/-1.64 ms in Group II. These results confirmed that the arterial wall, subcutaneous tissue and skin did not affect the accuracy of the dicrotic notch fidelity. The reproducibility of the system was assessed in 18 human subjects. The 2SD of intraobserver and interobserver reproducibility of the S2-carotid PWV measurement were 0.54 and 0.38 m/s, respectively, demonstrating high reproducibility of the measurement. From a clinical point of view, the S2-carotid PWV was compared with the aortic PWV. The bias error +/-2SD between the two measurements was -0.14+/-3.24 m/s with the correlation coefficient being 0.73. Although the S2-carotid PWV may not replace the aortic PWV directly, we believe that the S2-carotid PWV with the new system may become a new clinical parameter for early detection of cardiovascular disorders such as cerebrovascular diseases.