Investigation of Photoplethysmography Behind the Ear for Pulse Oximetry in Hypoxic Conditions with a Novel Device (SPYDR)

Abstract

Photoplethysmography (PPG) is a valuable technique for noninvasively evaluating physiological parameters. However, traditional PPG devices have significant limitations in high-motion and low-perfusion environments. To overcome these limitations, we investigated the accuracy of a clinically novel PPG site using SPYDR®, a new PPG sensor suite, against arterial blood gas (ABG) measurements as well as other commercial PPG sensors at the finger and forehead in hypoxic environments. SPYDR utilizes a reflectance PPG sensor applied behind the ear, between the pinna and the hairline, on the mastoid process, above the sternocleidomastoid muscle, near the posterior auricular artery in a self-contained ear cup system. ABG revealed accuracy of SPYDR with a root mean square error of 2.61% at a 70–100% range, meeting FDA requirements for PPG sensor accuracy. Subjects were also instrumented with SPYDR, as well as finger and forehead PPG sensors, and pulse rate (PR) and oxygen saturation (SpO2) were measured and compared at various reduced oxygen profiles with a reduced oxygen breathing device (ROBD). SPYDR was shown to be as accurate as other sensors in reduced oxygen environments with a Pearson’s correlation >93% for PR and SpO2. In addition, SPYDR responded to changes in SpO2 up to 50 s faster than PPG measurements at the finger and forehead.

The Use of Pulse Oximetry in the Assessment of Acclimatization to High Altitude

“…This complicates to identify devices that are appropriate for their usage at high altitudes without restrictions. However, certain parameters may require increased attention when the decision for a specific pulse oximeter has to be made: (1) accuracy, precision, and bias of the device [ 12 ]; (2) environmental conditions such as maximum operating altitude respectively minimum air pressure or the minimum operating temperature; (3) the availability of advanced algorithms to reduce motion artifacts or the detection of low perfusion; (4) the selection of the sensor location, typically using the finger [ 12 , 15 , 27 , 28 ]—however, other common positions such as the forehead may be considered, in particular if the measurement is conducted during motion [ 28 , 29 , 30 ]; (5) if required: the opportunity to sense carboxyhemoglobin and methemoglobin. In addition to the technical aspects, however, the measurement process itself becomes more error-prone with increasing altitude [ 6 , 12 ].…”

In-Ear SpO2: A Tool for Wearable, Unobtrusive Monitoring of Core Blood Oxygen Saturation

“…Other devices such as wearing a helmet for behind the ear [ 12 ] or a headband for the forehead [ 13 ], whilst providing good quality signal, giving a proxy for core blood oxygen rather than peripheral and being less obstructive to daily life, are less suitable for 24/7 use as they can be perceived as stigmatising which decreases adherence to wearing the device in the patient and consumer populations.…”

“…The PPG from the ear canal has also been shown to be far more sensitive than earlobe and finger PPG to amplitude variations that arise from respiration, thus allowing for a better measurement of respiration rate [ 18 ]. Furthermore, a significant delay has been observed between earlobe/behind-the-ear pulse oximetry, and pulse oximetry on the hand or the foot for detection of hypoxemia (low levels of blood oxygen) [ 12 , 19 ], which is primarily caused by the distance from the core blood supply. Although never previously shown, a similar fast response time was hypothesised from the ear canal given that its vasculature is supplied by the carotid artery, as is the brain.…”

Photoplethysmography behind the Ear Outperforms Electrocardiogram for Cardiovascular Monitoring in Dynamic Environments

“…Based on the need in many high-risk industries, especially tactical military flight, and the current state of the field for routine monitoring of operators, this study seeks to validate the accuracy of a human centrifuge, of a novel multiwavelength reflectance PPG sensor platform, called SPYDR ® (Standalone Performance Yielding Deliberate Risk), specifically designed to accurately measure and withstand the most rigorous physiological monitoring environments. SPYDR was previously shown to be as accurate as commonly approved finger and forehead PPG sensors through the gold-standard Arterial Blood Gas (ABG) testing used in accuracy approval of novel PPG sensors [ 26 ].…”

“…Signal processing within SPYDR was enhanced and optimized with novel, proprietary built-in algorithms specifically designed to detect and counter motion-artifact. In addition, several features of SPYDR’s design and anatomical measurement site are important contributing factors to its accuracy [ 26 ]. SPYDR’s custom-fabricated ear-cup design is an innovation to PPG sensor platforms, in that it provides adequate pressure to hold the sensor in tight yet comfortable contact with the skin while additionally blocking out ambient light that could throw off readings.…”