The ability of photoplethysmography (PPG) to track various sleep-related data by measuring blood volume changes in the skin has fueled the increasing demand for better sleep monitoring solutions. This shift has led to a growing interest in long-range PPG-based wearables for home sleep testing (HST) due to their convenience and cost-effectiveness. Companies like Viatom are at the forefront of this innovation, refining new strategies for PPG-based FDA-approved continuous oximeters that enhance the accessibility and effectiveness of sleep apnea tests. Explore this article to discover how PPG is transforming sleep apnea monitoring and improving overall sleep health!
Quick Guide to Understanding Photoplethysmography (PPG)
1.1 What is a Photoplethysmogram?
Photoplethysmography (PPG) is an optical method for detecting volumetric changes in blood within peripheral circulation. This non-invasive and low-cost technique measures blood flow at the skin's surface using low-intensity infrared (IR) light emitted from a light-emitting diode (LED)[1]. As the light penetrates biological tissues, it is absorbed by bones, skin pigments, and blood, allowing detection of changes in blood volume by measuring transmitted or reflected light at a photodiode (PD). Since blood absorbs light more effectively than surrounding tissue, PPG can identify even minor blood volume fluctuations through light intensity changes [1].
PPG can be either transmission or reflection mode, depending on the position of the light source and the photodetector relative to the tissue being measured. In reflection PPG, both the light source and the photodetector are situated on the same side of the tissue, such as positioned above the radial artery[2].
![Transmissive & Reflective Mode of PPG Sensors[3]](https://static.wixstatic.com/media/6391ef_49928bb81f5d4e3980b263f371e8181c~mv2.jpg/v1/fill/w_147,h_41,al_c,q_80,usm_0.66_1.00_0.01,blur_2,enc_auto/6391ef_49928bb81f5d4e3980b263f371e8181c~mv2.jpg)
Several key factors, such as the measurement site and the contact force between the sensor and the skin, can influence PPG readings. For accurate results, the PPG sensor should be positioned on specific anatomical locations where the arrangement and composition of the tissues ensure effective transmission from the LED to the photodiode. The fingertip and earlobe are commonly preferred sites for measurement.
1.2 How to interpret a PPG signal?
The time-series PPG data is converted into 2D spatial images through the extraction of the interbeat interval (IBI), and the blood volume pulse (BVP). A PPG signal comprises several elements, including volumetric changes in arterial blood associated with heart activity, variations in venous blood volume that modulate the PPG signal, a direct current (DC) component reflecting the optical properties of the tissues, and subtle energy changes within the body[4]. In a typical PPG signal waveform, the Diastolic Point represents the lowest pressure during diastole, indicating minimal blood flow, while the Systolic Point marks the beginning of systole when the heart pumps blood to the lungs, resulting in maximum blood pressure[5].
Role of PPG in Sleep Monitoring Through Wearable Oximeters
2.1 Clinical validation of PPG in sleep technology
Recent studies have validated the clinical effectiveness of PPG in continuous oximeters for monitoring sleep patterns, diagnosing sleep disorders, improving the accessibility and reducing the technical challenges of sleep apnea tests. The following four points illustrate some of the clinical validation:
01 PTT & PWA Data Collection
A groundbreaking advancement in PPG’s application for sleep monitoring is its integration with vital parameters such as Pulse Transit Time (PTT) and Pulse Wave Amplitude (PWA). PTT effectively indicates the autonomic imbalances, respiratory effort, and arousal in sleep. It measures the duration required for the pulse pressure waveform to travel from the aortic valve to the peripheral regions. A decrease in intrathoracic pressure during obstructed breathing is associated with a drop in blood pressure, which may be followed by a rise in PTT[6]. Therefore, PTT is an indirect marker of sleep fragmentation and can be used to monitor arousal from sleep due to respiratory events.
Concurrently, PWA drops are a reflection of peripheral vasoconstriction caused by sympathetic activation. They often occur during sleep in conjunction with cortical arousals, such as those caused by sleep apnea, leg movements, or other nocturnal events[7].
02 Assessment of Respiratory Patterns
The gold standard method for measuring respiratory effort, esophageal pressure (Pes) measurement, is invasive and uncomfortable. A catheter with an air-filled balloon is inserted into the esophagus, which can be done either orally or nasally[8]. A research by Khandoker el at was conducted in 2013 to explore the relationship between the features extracted from PPG and variations in Pes. Result showed that significant correlations were found between reductions in Pes and that in PPG based surrogate respiratory signals PPI, PWA and Wv, proving that PPG-based relative respiratory effort signal can be considered as an alternative to Pes as a means of measuring changes in inspiratory effort of OSA events[9].
03 Heart Rate Variability Analysis
The previous research[10][11] has demonstrated the feasibility of deriving heart rate variability (HRV) signals from PPG data for the detection of sleep-related issues, and the capability for continuous PPG recording offers an innovative, non-invasive method for assessing sleep apnea. While HRV is a well-established method for measuring the sympathovagal balance, PPG provides complementary insights by monitoring peripheral circulation. Both HRV and PPG exhibit changes during sleep deprivation, indicating alterations in autonomic regulation, but PPG may enhance the understanding of OSA severity due to its non-invasive nature and ability to reflect fluctuations in the sympathetic nervous system[12]. The increasing availability of continuous oximeters that accurately collect both HRV and PPG data supports the feasibility of using PPG as a reliable alternative for sleep problem detection.
04 Determination of Sleep Stages
The sleep cycle encompasses five stages: four non-rapid eye movement (NREM) stages and one rapid eye movement (REM) stage. Traditionally, an electroencephalogram (EEG) is the gold standard for identifying these stages[13]. However, a study in 2023 explored the feasibility of using a singular PPG sensor approach to detect sleep stages through continuous oximeters. This research analyzed 16 PPG recordings to derive surrogates for respiratory and cardiovascular parameters, alongside a series of nonlinear features pertinent to sleep stage classification. The findings demonstrated that automated multi-stage sleep classification utilizing solely PPG is not only viable but also achieved impressive accuracy rates of 84.66%, 79.62%, and 72.23% for two-, three-, and four-stage classifications, respectively, in individuals with sleep disorders[14].
2.2 Practical advantage of PPG-based continuous oximeters
The high rates of undiagnosed OSA remain a significant public health concern, given its potential link to complications such as hypertension, stroke, heart failure, obesity, metabolic syndrome, erectile dysfunction, Alzheimer's disease (AD)[16] and more. While polysomnography (PSG) serves as the gold standard for diagnosing OSA, its complexity and high cost often discourage patients from seeking evaluation. The national average cost for a sleep study comes as high as $2,925, and the range is from $1,250 to $6,700, according to New Choice Health[17]. After weighing factors like cost, accessibility, and diagnostic accuracy, the focus in the sleep apnea test is shifting toward home sleep tests (HST). PPG-based continuous oximeters have emerged as a more accessible alternative for monitoring sleep health compared with other costly HST options, such as EEG, actigraphy, and ECG. So what are the practical advantages that position continuous oximeters as an optimal solution for expanding business in the sleep health domain?
01 Accessibility of PPG-based continuous oximeters
On a practical level, PPG embedded in wearable oximeters presents significant advantages in the realm of sleep monitoring. When one has symptoms of sleep apnea and is prescribed to take a traditional PSG test, he/she has to visit specialized sleep laboratories, with the common waiting time to conduct an in-lab sleep study being 4-8 weeks, with extra waiting time to get test results[18]. In contrast, PPG-based continuous oximeters such as Viatom Checkme O2 Series can be used freely in a familiar environment, without troubling the normal sleep of the individuals while the data compare well with simultaneous in-lab PSG in the diagnosis of suspected OSA[19]. This ease of access not only enhances convenience but also encourages individuals from various demographics, including those in rural areas, to take charge of their sleep health without facing significant barriers.
02 Improved Patient Compliance Rates
The simplicity and comfort of a wrist or fingertip pulse oximeter make it easier for individuals to incorporate the device into their nightly routines. Unlike PSG, where intricate electrodes should be placed precisely with uncomfortable caps and alter normal sleep behaviors, PPG-based continuous oximeters require minimal set-up and provide a comfortable, familiar option that allows for more natural sleep patterns[19]. Furthermore, continuous oximeters have the benefit of gathering longitudinal data, with the ability to evaluate sleep daily over days/weeks in contrast to one night in the sleep laboratory. Additionally, many wearable devices come equipped with user-friendly applications that promote engagement and facilitate regular tracking of sleep data. This increased usability and comfort led to enhanced patient adherence, improving the quality and reliability of the collected sleep data.
03 Cost-Effectiveness of PPG Solutions
PPG-based continuous oximeters offer substantial financial savings for both healthcare systems and patients. For healthcare providers, it involves not only the expenses related to specialized equipment and facilities but also the costs associated with the time and expertise of healthcare professionals. In contrast, PPG devices provide quicker results, reduce the need for extensive specialist involvement. This approach not only minimizes immediate healthcare costs but also fosters long-term savings by allowing for continuous management of sleep disorders.
Viatom’s Continuous Oximeters with PPG Meeting Commercial needs
3.1 How to choose reliable sleep oximeters
As awareness of sleep apnea and sleep tracking continues to grow, a plethora of PPG-based sleep oximeters has emerged in the market. Clinicians and sleep specialists now face an overwhelming array of options, including smartwatches, rings, and wristbands from numerous brands. For clinical and professional applications, it is suggested that healthcare providers select FDA-cleared pulse oximeters to ensure diagnostic accuracy and reliability.
Understanding the complexities of FDA regulations for PPG technology and its application in specialized devices is essential. The FDA's guidance on the premarket evaluation of pulse oximeters, issued on March 4, 2013, delineates the standards for Class II devices regulated under specific codes. Key requirements include conducting desaturation studies with a minimum of 10 diverse subjects and accumulating over 200 data points, ensuring that at least 15% of participants represent dark-skinned demographics, thereby reflecting U.S. population diversity. Additionally, the guidance mandates rigorous testing for accuracy under conditions of motion and low perfusion, aimed at minimizing errors in oxygen saturation readings[20].
3.2 Viatom Checkme O2 Series widely used as continuous sleep monitors
Viatom's Checkme O2 Series continuous oximeters are an excellent choice for healthcare providers needing reliable and accurate measurements of blood oxygen saturation. Cleared by the FDA under K191088 in 2019, they meet stringent regulatory standards and utilize PPG technology to perform effectively under various conditions, including motion and low perfusion. Its clearance confirms the device's performance, reliability, and safety, making it a significant advancement in patient monitoring technology and a valuable tool for clinicians dedicated to quality patient care.
Viatom’s continuous oximeters have rapidly gained prominence in the sleep health market, significantly enhanced by their partnership with Company E. Since their collaboration commenced in 2020, Company E strategically acquired a substantial number of Viatom Checkme O2 Series continuous oximeters to conduct various sleep apnea tests. With a sleek design and impressive performance, Viatom continuous oximeters accurately monitor vital sign data such as SpO2 level, HR, body motion, etc., instrumental in developing Company E’s sleep diagnostic software.
The precise data from Viatom Checkme O2 Series enables the software to detect sleep-disordered breathing events and identify sleep stages. Now FDA-cleared, Company E’s software is widely adopted, speeding up the identification, diagnosis, and treatment of sleep-disordered breathing[21]. Furthermore, Viatom continuous oximeters continue to play a vital role in Company E’s development of an AI-based system for sleep staging, helping establish a strong presence in the U.S. sleep diagnostics sector.
3.3 Reasons to choose Viatom Checkme O2 Series continuous oximeter
01 Exceptional Sampling Frequency
Viatom Checkme O2 Series offers an impressive sampling rate of up to 150 Hz, which enhances measurement accuracy high above the standard. Studies show that lower sampling frequencies, even as low as 50 Hz, can yield reliable results[22]. The reduction in sampling frequency is primarily aimed at minimizing power consumption. However, Viatom Checkme O2 Series effectively integrates its high sampling rate with essential features such as extended battery life. Furthermore, it is equipped with a clear display, ample storage capacity, abnormal vibration alerts, and more features to better meet clinical testing and long-term disease monitoring needs.
02 Adjustable Average Window
Viatom Checkme O2 Series offers an adjustable average signal time ranging from 4 seconds to 2 seconds. According to Dr. Nancy Collop, the lead author of the AASM’s PM guidelines, The maximum acceptable signal averaging time for an oximeter is 3 seconds (at a heart rate of > 80 bpm). This standard was also incorporated into the PM guideline paper[23].
Customers focused on minimizing arterial oxygen desaturation events during sleep—particularly in high-altitude environments—have opt for Checkme O2 Series with longer average times. This choice effectively reduces unwanted noise and enhances the reliability of readings[24]. Moreover, for sleep lab scenarios, clients also prefer Viatom continuous oximeters with longer averaging times, which can help mitigate alarm fatigue among clinicians, preventing desensitization to non-critical alarms[25].
In contrast, clients who require precise diagnostics for interventions like CPAP therapy favor Checkme O2 Series with shorter average times, such as two seconds, to improve the detection of brief desaturation events[26]. In sleep medicine, where therapeutic decisions hinge on the frequency of desaturations over specified timeframes, an incorrect choice of average time could lead to delayed or inappropriate interventions.
In summary, the adjustable average time feature of Viatom Checkme O2 Series allows different healthcare providers to customize their monitoring strategies to better suit their specific requirements. To further address the sleep apnea test market, Viatom is cementing its place as a leading supplier by actively developing high-resolution pulse oximeters that feature 1-second signal averaging times, hoping to revolutionize blood oxygen monitoring and further enhance diagnostic accuracy.
03 Prolonged Battery Life and Abundant Device Storage
The battery life of the Checkme O2 Series can be as long as 72 consecutive hours on a single charge, able to monitor patients over a full week, providing seven complete nights of data. Research has demonstrated that a single-night sleep study can result in misdiagnosis. A 2017 study indicated that such studies might overlook moderate OSA in 60% of cases and mild OSA in 84% of instances[28].
In contrast, multi-night at-home sleep testing with Viatom's Checkme O2 Series has delivered profound insights into sleep patterns and simplified reliable access to sleep apnea diagnosis for thousands of people.
For patients, Viatom's Checkme O2 Series provides a comfortable, cost-effective, convenient alternative with enhanced patient engagement, eliminating “first-night” effect linked to the changes in environment. For sleep specialists, the access to multi-night data for providers heightened diagnostic accuracy, instrumental in diagnosing mild to moderate sleep apnea cases. Unveiling night-to-night variability contributes to a more holistic understanding of the patient’s sleep health and personalized treatment plans.
04 Future Trends of PPG Applications
PPG Data Analytics enhanced with machine learning
The study conducted by Filipa et al. in 2021 successfully developed a model demonstrated to be effective in detecting and delineating PPG signals, even in the presence of noise-corrupted data[29]. The primary objective was to achieve robust detection of PPG waves, which includes accurately identifying the temporal limits of each heartbeat to facilitate peak detection. Additionally, the findings suggested that time-frequency transforms show promise as a tool for generating deep learning features, indicating that further exploration of various model architectures and hyperparameters could yield improved results.
Conclusion
Despite entering the consumer market only a decade ago, PPG-based devices have gained widespread adoption among consumers and healthcare providers. An example of this advancement is Viatom's FDA-approved PPG-based continuous oximeter, which makes sleep apnea tests affordable, accessible, convenient, and reliable. This trend creates increasing opportunities for real-life data collection, supporting research on sleep health outcomes, and the impact of timely interventions.
References:
[1] Photoplethysmogram - Wikipedia (2024)
[2] A Review of Methods for Non-Invasive Heart Rate Measurement on Wrist (2021)
[3] The Impact of Contact Force on Signal Quality Indices in Photoplethysmography Measurements (2024)
[4] Photoplethysmography (PPG) - News Medical (2019)
[5] Towards Photoplethysmogram Based Non-Invasive Blood Pressure Classification (2018)
[6] Evaluating changes in pulse transit time drop index in patients with obstructive sleep apnea before and during CPAP therapy (2022)
[7] Quantifying peripheral sympathetic activations during sleep by means of an automatic method for pulse wave amplitude drop detection (2019)
[8] The application of esophageal pressure measurement in patients with respiratory failure (2014)
[9] Investigating relative respiratory effort signals during mixed sleep apnea using photoplethysmogram (2013)
[10] Statistical investigation of heart rate variable derived from photoplethysmography signal in sleep staging process (2017)
[11] Insights into vascular physiology from sleep photoplethysmography (2023)
[12] Sleep Deprivation Deteriorates Heart Rate Variability and Photoplethysmography (2021)
[13] The gold standard of sleep tracking: Polysomnography (2023)
[14] Multi-stage sleep classification using photoplethysmographic sensor (2023)
[15] Photoplethysmography—new applications for an old technology: a sleep technology review (2023)
[16] Feasibility and Long-Term Compliance to Continuous Positive Airway Pressure Treatment in Adults With Down Syndrome, a Genetic Form of Alzheimer's Disease (2022)
[17] Cost-effectiveness and safety of continuous pulse oximetry for management of undiagnosed obstructive sleep apnea in bariatric surgery: a nationwide cohort study (2024)
[18] Lab Sleep Study (Polysomnography) versus Home Sleep Test (HSAT) (2024)
[19] Diagnosis of Obstructive Sleep Apnea Using Pulse Oximeter Derived Photoplethysmographic Signals (2014)
[20] Review of Pulse Oximeters and Factors that can Impact their Accuracy - FDA Executive Summary (2022)
[21] Ensosleep PPG: FDA-Cleared, AI-Powered Sleep Diagnosis Using Pulse Oximeters (2024)
[22] Impact of the PPG Sampling Rate in the Pulse Rate Variability Indices Evaluating Several Fiducial Points in Different Pulse Waveforms (2021)
[23] Keeping a Pulse on Oximetry (2008)
[24] The Impact of Averaging Window Length on the “Desaturation” Indexes Obtained Via Overnight Pulse Oximetry at High Altitude (2015)
[25] Alarms, oxygen saturations, and SpO2 averaging time in the NICU (2016)
[26] Averaging Times for Pulse Oximeter Measurements – A Review of Manuscripts Published in the Top Five Sleep Medicine Journals (2024)
[27] Dependence of SpO2 signal noise on the pulse oximeter averaging time (2021)
[28] Night-to-night variability of obstructive sleep apnea (2017)
[29] The Application of Deep Learning Algorithms for PPG Signal Processing and Classification (2021)