A Survey on the Affordances of “Hearables” <s> Photoplethsmography (PPG) <s> ### Summary points ::: ::: Pulse oximetry is arguably the greatest advance in patient monitoring since electrocardiography. It enables oxygenation, an important physiological variable that is poorly detected by clinical means, to be monitored continuously, simply, and non-invasively. Hypoxaemia is commonly found in all aspects of medical practice and is a major cause of organ dysfunction and death. Pulse oximetry should be widely available and used routinely in clinical practice both in primary care and in hospital. ::: ::: An understanding of the principles of operation is essential for the proper use and interpretation of the values displayed by an oximeter. The absorption spectra of oxygenated and reduced haemoglobin differ so that arterial blood appears red while venous blood appears “blue.” When two compounds with differing absorption spectra are together in solution, the ratio of their concentrations can be determined from the ratio of the light absorbed at two different wavelengths. The aim of oximetry is to measure the ratio of oxygenated haemoglobin to total haemoglobin in arterial blood, the oxyhaemoglobin saturation. In living tissue, however, light is also absorbed by the tissues and by the haemoglobin in venous and capillary blood. To overcome this, early instruments heated the tissues to increase flow and to “arterialise” the blood in the capillaries and veins and some used up to eight wavelengths to allow for absorption of light by tissues. These instruments were useful research tools but were too cumbersome for clinical use. ::: ::: ### Sources of error ::: ::: varnish, and pigmentation ::: ::: The breakthrough was the realisation that the light absorbed varied with each pulse and that if the absorption was measured at one point of … <s> BIB001 </s> A Survey on the Affordances of “Hearables” <s> Photoplethsmography (PPG) <s> This paper describes the development and evaluation of wearable sensor earphones coupled with a mobile platform for comfortable assessment of cardiovascular function. Photoplethysmographic sensors were integrated into regular earphones to provide discreet and non-stigmatizing heart rate measurements. Preliminary results indicate that the proposed system has a high degree of accuracy at rest with an average error of 0.63%. <s> BIB002 </s> A Survey on the Affordances of “Hearables” <s> Photoplethsmography (PPG) <s> Photoplethysmography (PPG) technology has been used to develop small, wearable, pulse rate sensors. These devices, consisting of infrared light-emitting diodes (LEDs) and photodetectors, offer a simple, reliable, low-cost means of monitoring the pulse rate noninvasively. Recent advances in optical technology have facilitated the use of high-intensity green LEDs for PPG, increasing the adoption of this measurement technique. In this review, we briefly present the history of PPG and recent developments in wearable pulse rate sensors with green LEDs. The application of wearable pulse rate monitors is discussed. <s> BIB003 </s> A Survey on the Affordances of “Hearables” <s> Photoplethsmography (PPG) <s> Because it is more convenient to measure photoplethysmography (PPG) than ECG, PPG is supposed as a surrogate of ECG for heart rate variability (HRV) analysis. In this study, we measured the spectrum of pulse rate variability (PRV) from PPG and its square coherence spectrum with HRV from ECG before and after exercise. The results showed that the spectrum of PRV corresponds almost well with HRV, especially for the healthy subjects at rest. However, stimuli conditions such as exercise will decrease the correlation, especially for the high frequency (HF) component. It indicates that whether PRV can be used as an alternative to HRV depends on the applications and conditions. <s> BIB004
Reflective photoplethsmography (PPG) sensors have appeared in a number of different wearable devices, including various health trackers, smartwatches, and hearables  BIB002 BIB003 , allowing a relatively inexpensive and non-invasive method for obtaining data pertaining to the user's heart rate and heart rate variability. The underlying technology for obtaining this type of heart rate and other blood related activity is remarkably simple, and therefore some wearable devices on the market even contain multiple PPG sensors for redundancy. Using a paired light source and photosensitive measuring component, PPG makes it possible to measure changes in blood flow, as oxygenated and deoxygenated blood reflect light differently BIB003 . The ear has a number of affordances compared to other PPG measurement locations, as the sensor can be naturally protected and secured within the ear canal, thus making measurements less susceptible to motion and external light artifacts. With specific regards to affective affordances, large-scale heart rate data may be useful for identifying general levels of user activity/restfulness. Further, smaller scale heart rate variability measurements can be useful for measuring changing mental states, stress responses, and potentially monitoring a number of clinical diagnosis markers across many different medical specialties  . Optical measurements of heart-related data are quite different from electrical measurements, and therefore, the use of PPG for certain clinical applications (such as monitoring abnormal heart rhythms) may not be possible BIB004 . Beyond heart-specific information, PPG is also used in applications that monitor blood-oxygen levels BIB001 . Within the specific context of in-ear measurements, real-time blood oxygen monitoring and alerting could be useful in a number of high-altitude applications (aviation, mountain-related sporting, etc.), especially considering in-ear measurements could be obtained hands-free.