Department of Electronic Systems, Norwegian University of Science and Technology, 7034 Trondheim, Norway.
Department of Clinical and Biomedical Engineering, Oslo University Hospital, 0372 Oslo, Norway.
Sensors (Basel). 2024 Oct 10;24(20):6505. doi: 10.3390/s24206505.
Inversion of the photoplethysmographic (PPG) signal is a rarely reported case. This signal anomaly can have implications for PPG-based cardiovascular assessments. The conditions for PPG signal inversion in the vicinity of the dorsalis pedis (DPA) artery of the foot were investigated. Wireless multi-wavelength PPG sensing with skin-probe contact pressure and local skin temperature were studied at different sensor positions, and the occurrence of inversion (OOI) was investigated. Twelve healthy adult volunteers were studied over four LED wavelengths at three levels of contact pressure for 11 probe positions. A novel algorithm quantified the proportion of inverted samples with respect to the abovementioned variables. Our algorithm classifying inverted vs. non-inverted pulses achieved 98.3% accuracy. Ten of the participants had at least one inverted signal identified. The impact of interindividual variation on inversion prevalence was large, but different LEDs, relative position to the DPA and sensor contact pressure also affected OOI. Skin surface and room temperatures showed no impact on OOI. Lateral measurements showed 39.6% more inversion at maximum compared to minimum contact pressure. Mechanical capillary bed variations and arterial reflections during venous engorgement are considered viable explanations for our observations. These findings motivate an expanded study of the occurrence of PPG signal inversion.
光体积描记图（PPG）信号的反转是一种罕见的报告案例。这种信号异常可能会对基于 PPG 的心血管评估产生影响。本研究调查了足部背动脉（DPA）附近 PPG 信号反转的条件。研究了具有皮肤探头接触压力和局部皮肤温度的无线多波长 PPG 传感，在不同的传感器位置，研究了反转（OOI）的发生。12 名健康成年志愿者在 3 个接触压力水平和 11 个探头位置的 4 个 LED 波长下进行了研究。一种新算法根据上述变量量化了反转样本的比例。我们的算法对反转和非反转脉冲进行分类，准确率达到 98.3%。其中 10 名参与者至少有一个反转信号。个体间变异对反转发生率的影响很大，但不同的 LED、相对于 DPA 的相对位置和传感器接触压力也会影响 OOI。皮肤表面温度和室温对 OOI 没有影响。与最小接触压力相比，最大接触压力下的横向测量显示出 39.6%的反转更多。机械毛细血管床的变化和静脉充血期间的动脉反射被认为是对我们观察结果的合理解释。这些发现促使我们对 PPG 信号反转的发生进行了更广泛的研究。
