Is Pulse Transit Time Needed for Accurate Blood-Pressure Monitoring from Wearables?

Is Pulse Transit Time Needed for Accurate Blood-Pressure Monitoring from Wearables?

Cardiovascular disease has been the leading cause of death for Americans since 1920. Also alarming is the fact that related medical as well as indirect costs that totaled $555 billion in 2016 are expected to reach $1.1 trillion by 2035, according to the American Heart Association1.

It's a good thing that wearables—with their ability to provide continuous, real-time monitoring of vital signs—are getting increasingly sophisticated.

Designed with advanced sensors and algorithms, along with powerful yet efficient processors, wearable devices that monitor parameters such as heart rate, blood-oxygen levels, sleep quality, and stress levels are already on the market. Research and work is underway on products for the non-invasive detection or screening of such conditions as diabetes, *** cancer, atrial fibrillation, and UV exposure. Wearables can empower people to more closely manage chronic conditions and also be more proactive about addressing previously undetected health issues (Figure 1).

Let's consider blood pressure—an important indicator of cardiovascular health and a vital sign that must be managed. The American Heart Association recommends that people with high blood pressure engage in home monitoring as a way to evaluate whether their treatments are effective. There are already plenty of home-monitoring tools available. However, most of them are cuff-based and require dedicated time sitting down to capture the measurement (and these measurements aren't made continuously).

Fortunately, wrist-based devices that monitor blood pressure continuously and non-invasively are becoming available. Omron Healthcare's HeartGuide is, for instance, the first wearable blood-pressure monitor; it has the approval of the U.S. Food and Drug Administration (FDA). HeartGuide is essentially a smart watch and uses the oscillometric cuff method, which is the standard for medical-grade personal blood-pressure measurement2. An accompanying app provides insight on readings and allows sharing with the user's doctor.

Figure 1: With wearables, like smartwatches, that monitor vital signs, users are empowered to be more proactive about managing their health and wellbeing.

The Challenge of Accurate, Wrist-Based Blood Pressure Monitoring

Capturing precise blood-pressure readings from the wrist is challenging because the arteries there are more narrow and not as deep under the skin as upper arm arteries. Also, the arm and wrist must be at heart level to capture a correct reading3. Pulse transit time (PTT), which denotes the time that it takes a pulse pressure waveform to propagate through the length of the arterial tree, provides a means to measure blood pressure. This pulse pressure waveform occurs when blood is ejected from the left ventricle, and it moves with a greater velocity than the forward movement of the blood itself. In addition to a measurement of blood pressure, PTT also provides an indicator of arterial stiffness.

PTT can be derived from calculations on electrocardiogram (ECG) and photoplethysmography (PPG) signals; it is based on tightly defined characteristics of the shape of the pressure pulse waves in blood vessels4. ECG provides a measurement as well as graphical representation, in terms of time, of the electrical signals associated with heart muscles. PPG provides an optical measurement of the volumetric change of blood in tissue during the cardiac cycle. Optical sensors that utilize PPG and ECG are used in wearables to measure heart rate. Measuring PTT involves calculating the time between the R-peak of the ECG and a reference point on the pressure pulse wave measured using PPG5.

Studies have shown, however, that measuring PTT alone to assess blood pressure may not be sufficient.6 Researchers on one study found that integrating PTT, heart rate, and a previous blood pressure estimate results in a more accurate current blood pressure value. In addition to general blood-pressure monitoring, PTT has been examined for use in other applications, such as a means to detect sleep ailments via measurement of respiratory effort or as an indicator of the onset of low blood pressure during spinal anesthesia.

Maxim offers biosensor modules and biopotential analog front-ends (AFEs) for health wearables. One example is the MAX30001 ultra-low-power, single-channel integrated biopotential and bioimpedance AFE. The solution consists of a single biopotential channel that provides ECG waveforms and heart-rate and pacemaker edge detection and a single bioimpedance channel that measures respiration.

Examinations have been undertaken that utilize a two-chip solution consisting of an ECG AFE and a PPG sensor to measure, via a wearable, pulse arrival time (PAT), which is equal to the sum of PTT and the pre-ejection period. PAT is simpler to measure than PTT and has been proposed as a PTT surrogate. However, subsequent studies have shown that, due to accuracy issues, PAT is not an ideal replacement for PTT as a blood-pressure marker. However, it may potentially indicate wide trend blood-pressure changes in some cases7.

The interesting point here is, there is a means using a two-chip solution to measure PAT via a wearable. Now, imagine the potential solutions that could emerge by adding the capability to calculate PTT from the data gathered by the wearable form factors. Applying sensor fusion to bring together data from multiple sensors along with artificial intelligence (AI) to identify patterns and opportunities for action opens up many more possibilities. The sophisticated data analysis would happen in the background. But for the wearer, he or she simply has a device that provides continuous monitoring, comfortably and conveniently.

This blog post was adapted from an article, "What Can We Do with Pulse Transit Time Calculations from Wearables?", that was published in Electronic Design on August 29, 2019.