Non-invasive Techniques for Assessing the Endothelial Dysfunction: Ultrasound Versus Photoplethysmography

Endothelial dysfunction, which can be non-invasively assessed by flow mediated vasodilation (FMD), predicts an increased rate of adverse cardiovascular events. The endothelial dysfunction is considered as a leading cause of development and progression of the atherosclerotic process. The main aim of this study is to review different non-invasive methods to assess the endothelial dysfunction and to propose enhancements to a new method based on photoplethysmography (PPG). First, non-invasive techniques developed for the evaluation of peripheral endothelial function (including the Doppler ultrasound (US)) will be reviewed. Although non-invasive, US-based techniques present a few disadvantages. To remedy to the above disadvantages, another technique based on pulse wave analysis is introduced. Although amplitude-based features from the photoplethysmogram has produced encouraging results, there are cases where the compete equivalence with US-FMD based on ultrasound measurement cannot be established. Therefore more elaborated features combined with data processing techniques are proposed which seem promising enough for the PPG-based technique to be used as a replacement method for US-FMD measurement. The ultimate aim is to assess the endothelial function and the presence of significant atherosclerotic events leading to peripheral vascular disease using a practical, simple, low-cost, non-operator dependent and alternative non-invasive technique in a clinical setting.