The evolution of photoplethysmography (PPG) within smart devices

As a technology, photoplethysmography (PPG) has been explored for almost a century, but it is only in the last few years that it has had a broader application beyond specialised medical settings.

That has generally been driven by the evolution of smartphones, smartwatches and even smaller wearable devices such as smart rings.

PPG is most commonly used in pulse oximetry in clinical settings for measuring oxygen saturation but as the technology becomes more readily available, and inexpensive, the integration into portable devices has risen significantly.

Wearable devices

Advances in the development of smartphones and wearable devices have seen PPG use broaden to other areas, including helping detection of heart irregularities such as atrial fibrillation (AF).

One example is Sky Labs CART-I, which is the world’s first ring-type smart wearable heart rhythm monitoring medical device on the market and provides PPG signals to measure heart rate to help spot potential AF.

Having screened the bloodstream 24/7 through the user’s finger to measure irregular pulse waves, it then transmits the data to a cloud platform where AI technology detects and analyses AF. It also uses electrocardiogram signals to provide additional information to the user’s doctor, often without user intervention.

Optical absorption

Challenges still exist in the use of PPG within smart devices, but in its simplest form it is a method for measuring the amount of light that is absorbed or reflected by blood vessels in living tissue.

Since the amount of optical absorption or reflection depends on the amount of blood that is present in the optical path, the PPG signal is responsive to changes in the volume of the blood.

Another application in clinical practice, is the routine use of PPG to monitor cardiac-induced blood volume changes in microvascular beds at peripheral body sites.

PPG is regarded as an effective low-cost technology that can be applied to various aspects of cardiovascular monitoring.

Monitoring hypertension

More recently, it has been investigated for its potential for assessing hypertension. This will have value as high blood pressure is indicative of various chronic disease conditions.

PPG may have applications for being able to accurately and frequently measure blood pressure outside of a clinical setting, using mobile or wearable devices.

The advancement of digital sensors, signal processing, machine-learning algorithms, and improved physiologic models, pulse waveform analysis using PPG for the assessment of blood pressure has become more feasible.

Further applications

While PPGs can be obtained from the finger, and is valid outside of a clinical setting, there are occasions where within a clinic, such as in cases of shock or hypothermia, blood flow to the periphery can be reduced. This meaning PPG becomes less effective in the peripheral regions of the body, because of a less discernible cardiac pulse.

In these instances, a PPG can be obtained from a pulse oximeter on the head, with the most common sites being the ear, nasal septum, and forehead, with other locations on the body also being considered by researchers.

Along with monitoring heart rate, cardiac cycle and respiration, it can also play a role in highlighting the depth of anaesthesia. If a patient is not sufficiently anesthetized, the sympathetic nervous system response to an incision can generate an immediate response in the amplitude of the PPG.

Yet because of the nature and sensitivity of PPG, it is an application proving well-suited to small, discreet, devices such as smart rings.