The photoplethysmogram (PPG) signal provides an efficient measure of the blood oxygen levels using comfortable and non-invasive form factors. Pioneered clinically in the 1930s by Alfréd Hertzman, the PPG signal was originally used to study the changes in blood flow in human tissues. Clinical use of the PPG signal expanded in the 1970s in the form of pulse oximetry, coined by Dr. Takuo Aoyagi (Nihon Kohden), which made the measurement of oxygen saturation possible.
Today, PPG is deeply integrated into modern healthcare as part of a larger trend to incorporate machine learning algorithms and Software as a Medical Device (SaMDs) ubiquitously across healthcare verticals as a means of measuring healthcare vitals. Given the breadth of information gleaned from the PPG channel, its use in healthcare SaMDs is expected to continue growing in 2025 and beyond.
Increasing Clinical Uses for the PPG Signal
Pulse oximeters (pulse ox) are clinically active across many healthcare verticals, including but not limited to cardiology, radiology, pulmonology, neurology, and sleep medicine. They are deployed in intensive care units (ICUs), operating rooms, and emergency departments. And they are used to monitor patients under anesthesia and remotely across various disease states, including COPD and sleep apnea.
This ubiquitous use can be partially attributed to the accessibility of the PPG signal. Pulse oximeters shine light on skin to measure the amount of light being absorbed and reflected with each heartbeat, outputting an information rich waveform. In turn, this allows pulse oximeters to continuously monitor health vitals, including the SpO2 and heart rate, with a growing list of others as AI analysis of the PPG signal evolves. Let’s take a closer look at some of these active PPG use cases across verticals.
Cardiology Applications
In cardiology, PPG is extensively used to gather a consistent measurement of heart rate, in the identification of arrhythmias, and related measures like blood pressure and SpO2. For years, clinical probes, like the SET Pulse Oximeter from Masimo, have offered accurate readings across challenging cardiovascular scenarios. The SET provides accurate readings, even if a patient is in motion, has low perfusion, or has a darker skin pigmentation.
Biobeat, a non-invasive, medical grade and FDA cleared chest patch that offers 24/7 monitoring, analyzes the PPG waveform to estimate blood pressure. This advancement is particularly beneficial in continuous monitoring scenarios where traditional methods are impractical across cardiology and in other specialities, including radiology and in sleep medicine.
Radiology, Pulmonology and Vascular Health
Similarly to the applications in cardiology, PPG’s capability to assess blood circulation makes it a valuable tool in radiology, specifically for gaining additional insights into a patient’s current overall health. By analyzing the waveform characteristics of the PPG signal, clinicians can assess arterial stiffness and detect peripheral vascular diseases.
Again, this non-invasive assessment aids in early diagnosis and management of conditions affecting blood flow in peripheral arteries. This can be applied to future PPG-based screening tools, as patient use of pulse ox devices along with SaMD tools has the possibility of identifying other underlying health issues and comorbidities, like FibriCheck for remote atrial fibrillation, Cardiio’s camera-based pulse gathering solution, and Welltory’s all-in-one AI-powered wellness platform.
In pulmonology, PPG is also a highly-utilized signal to monitor respiratory parameters. The technology can detect respiratory-induced variations in the PPG signal, allowing for the estimation of respiratory rate and detection of respiratory effort. This application is particularly useful in monitoring high-risk patients in low acuity settings for respiratory disorders and providing continuous, non-invasive respiratory assessment.
Neurology and Autonomic Function
PPG also finds applications in neurology, particularly in assessing autonomic nervous system function. Per Malak Abdullah Almarshad et al., the features of the PPG signal can reflect autonomic activity, providing insights into conditions like anxiety and depression. Mina Namvari et al. also highlighted the strong capability of using the PPG signal as a benchmark for mental health and stress detection.
And further research from Lynnette Nathalie Lyzwinski et al. indicates that PPG holds promise as a complementary technology for detecting changes in mental health status. Across neurology use cases, non-invasive data collection simplifies the monitoring of mental health, improving patient comfort and compliance.
PPG in Sleep Medicine
Like other healthcare specialties, the PPG signal has become an invaluable tool for diagnosing and monitoring sleep disorders. Per Chicago ENT/Chicago Sleep Center (CENT/CSC) sleep staff, the workflow with PPG devices is much easier than the home testing model prior to PPG-devices. Manual work – cleaning, servicing, uploading, visiting offices, etc. – requires both staff and patients to spend more time and increase the chance for mistakes. Patients often waited for weeks.
However, new algorithmic-based tools, like EnsoSleep PPG, offer a more accessible option by offering non-invasive home sleep apnea tests (HSATs) through wearable devices that collect the PPG signal, like watches, rings, wrist/finger combos, and patches. These comfortable devices capture the PPG signal during sleep and provide accurate data on par with traditional flow-based sleep studies. These form factors are far less expensive, while still offering an equally accurate way to detect sleep stages and sleep-disordered breathing events.
Chris R. Fernandez, MS et al. have demonstrated that EnsoData’s PPG-based systems can also effectively analyze these conditions across skin tones, a challenge that was recently tackled, again by the EnsoSleep PPG algorithm, as presented at SLEEP 2024. As sleep medicine continues to pioneer the use of the PPG signal, the opportunity to continue expanding and condensing specialities with multi-morbidity diagnoses, patients will be the primary benefactors. Easier to get screened. Easier to get tested. And easier to get monitored for long-term therapy success.
All of this data and potential stems from the powerful, accessible, non-invasive PPG signal. As a form factor, using the PPG signal allows for newer screening, testing, and monitoring devices to cost less while maintaining similar clinical quality.
