Consequently, the PPG is an attractive and convenient modality with which to potentially assess vascular age.įigure 1.Devices for measuring the photoplethysmogram (PPG) signal.
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1): PPG-based devices come in a range of form factors (e.g., from fitness bands to earbuds) measurements can be made either in contact with the skin or remotely (e.g., by finger probe or by webcam) PPG-based devices are used in clinical settings (e.g., pulse oximeters) and in daily life (e.g., smartwatches) devices can be used for continuous measurements (e.g., wearables) and intermittent measurements (e.g., placing a finger on a smartphone’s camera) and measurements can be taken at different body sites (e.g., finger, wrist, and ear), and even simultaneously at multiple sites.
The PPG is already measured by many devices with a range of designs and potential applications (see Fig. Indeed, some PPG-derived parameters have been found to correlate with age ( 14), providing insights into the effects of age on the vasculature. Second, the shape of the PPG pulse wave changes with chronological age ( 11) because it is influenced by both the speed of pulse wave propagation ( 12) and changes in the compliance of smaller, peripheral arteries that affect wave reflection ( 13). First, the time taken for the PPG pulse wave to arrive at peripheral sites reduces with chronological age, since arterial stiffness and therefore pulse wave velocity (PWV) increase with chronological age, particularly in the central arteries such as the aorta ( 10). The photoplethysmogram (PPG) signal is influenced in two ways by vascular aging. It has also been investigated as a modality with which to assess vascular age, although it is not widely used for this purpose. It is widely used in physiological monitoring, from its use in pulse oximeters for oxygen saturation assessment ( 7), to its use in toe blood pressure measurement for vascular assessment ( 8), and its use in smartwatches for heart rate monitoring ( 9). Photoplethysmography is an optical technique that captures the pulsatile change in vascular blood volume with each heartbeat. Photoplethysmography-based devices could provide a more convenient approach to assess vascular age. However, many current approaches to assess vascular age are not yet widely used, in part due to the need for a trained operator and standardized measurement conditions. Consequently, it is helpful to identify individuals with early vascular aging for clinical intervention ( 6): those whose vascular age (apparent age of the blood vessels) is greater than their chronological age (time since birth).
Other indicators are routinely used for diagnosis, such as the ankle-brachial index being used to diagnose peripheral arterial disease (PAD). Indicators of vascular aging have been found to be predictive of cardiovascular morbidity and all-cause mortality, such as the assessment of aortic stiffness by carotid-femoral pulse wave velocity (as assessed using applanation tonometry or vascular ultrasonography) ( 5). It can ultimately result in damage to the heart, kidney, and brain ( 1). This process, known as vascular aging, includes an increase in the stiffness and diameter of the larger arteries and lengthening of the proximal aorta ( 3, 4). The function and structure of blood vessels naturally degrade with age ( 2). Vascular age is an emerging indicator of cardiovascular health that is indicative of cardiovascular risk, and can prompt clinical intervention ( 1).
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