Comparison between Reflection - mode 3 Photoplethysmography ( PPG ) and Arterial Diameter 4 Change Detected by Ultrasound at the Region of Radial 5 Artery

1 Continuous monitoring of arterial blood pressure (ABP) can provide vital information on the 2 cardiovascular system in clinical practice and research. To achieve this, methods have been 3 developed to determine ABP continuously and non-invasively, including the wrist volume 4 clamp method, which uses reflection-mode photoplethysmography (PPG) to indicate the 5 volume change of the radial artery. However, this kind of indication is reportedly not accurate 6 enough, and an in vivo test for its accuracy is still lacking. As ultrasound can detect the 7 arterial diameter accurately, we have developed a sensor comprised of a PPG sensor and an 8 ultrasound transducer, which can collect reflection-mode PPG and A-mode ultrasound signals 9 simultaneously. Tests on fifteen volunteers were conducted under the similar conditions of 10 using wrist volume clamp method for a comparison between the PPG signal and the radial 11 arterial diameter. It was observed that there were obvious differences between the shapes of 12 these two signals, mainly between their drifting trend parts, with the pulsatile components of 13 the signals matching very well. The mean RMS value of the differences of the trend parts 14 (0.287 ± 0.072) was found to be much larger than that of the pulsatile parts (0.107 ± 0.028). 15 The results indicated that using PPG signal as a reference for ABP measurement at the wrist 16 region might be not accurate enough, and ultrasound has the potential for replacing it in the 17 wrist volume clamp method for more accurate ABP determination. 18 19