Reliability of Finger Photoplethysmogram Assessment: Effect of Mental Stress and Day-to-Day Variability

The frequent assessment of vascular health may contribute to increased awareness and prevention of cardiovascular events in general populations. It has been well recognized that arterial stiffness and wave reflection measured by the aortic pulse wave velocity and Augmentation Index (AIx) are the strong predictors of future Cardiovascular events and all-cause mortality [1,2]. However, these measures require technical expertise and knowledge. In this context, the Second Derivative waveform of fingertip Photoplethysmography (SDPTG) can be measured easily from a fingertip placed on the photoplethysmography (PTG) sensor and has the potential for selfmonitoring of vascular health during daily living [3]. Previously, several studies have demonstrated clinical and pathophysiological importance of SDPTG indices such as the SDPTG Aging Index (SDPTGAI) [4–6]. Since SDPTGAI is strongly correlated with age, it can be rescaled to years of age which is easily comprehended by general populations [3]. For these reasons, SDPTG indices including the vascular aging index have recently been implemented for ambulatory monitoring devices, but its usability and reliability are not well documented. Mental stress is an external stimulus that can affect physiological parameters such as arterial tone [5,7], blood viscosity [8], and heart rate [9]. It is reported that pulse wave velocity and AIx are increased with acute mental stress [10,11]. Moreover, skin blood flow shows transient reflex changes in response to various external stimuli (e.g., arousing or painful stimuli, emotional stress) [12]. Since photoplethysmogram optically evaluates blood volume changes in the microvascular bed of cutaneous tissue, SDPTG may be influenced by mental stressor independent of changes in vascular tone and stiffness of the relatively proximal vessels.