Bench Talk

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Fitness bands, smartwatches, and other wearables for health and fitness routinely measure heart rate, estimate calorie expenditure, and track distance and elevation during the user's exercise routine. As exercise trackers, these devices go a long way in satisfying users' desires for more information about exercise intensity and fitness level, but what's next? Can wearables go further in enhancing their users' overall state of health and wellness? Can they do so without compromising their current convenience?

In their current form, consumer wearables have gained acceptance as easy-to-use devices that can provide a great deal of information with little or no effort on the part of their users. Although wearables hold great promise for providing more detailed health data, including vital signs measurements, fulfilling that promise is both a technological and usability challenge. Attempts to offer clinical-grade blood pressure (BP) measurements, for example, could introduce some friction in the convenient, mostly transparent way wearables currently operate. Still, the availability of devices that provide continuous BP and vital signs measurements would have huge implications on public health. The following examines how effective these devices are in measuring BP.

According to the Centers for Disease Control and Prevention (CDC), nearly half of adults in the US are diagnosed with hypertension. Less than a quarter of those have their hypertension under control despite the fact that it increases the risk of heart disease and stroke. High blood pressure costs the US about $131 billion (USD) each year and nearly 1,300 lives each day in 2017, according to a study.

Having blood pressure taken by your primary care physician (PCP) is so routine that we hardly think about it. Accurately measuring blood pressure is nearly as complex as understanding cardiovascular implications buried in subtle changes in systolic values. They're measured when arterial pressure rises to its highest levels as cardiac muscle contracts and diastolic when arterial pressure falls to its lowest levels as cardiac muscle contracts. The difficulty of obtaining accurate BP results is perhaps reflected in the fact that the gold-standard for clinical blood-pressure measurement is catheterization, which measures instantaneous BP by placing a strain gauge in fluid contact with blood in an artery.

Catheterization is certainly not what you'd expect to undergo during a routine visit with your PCP. Instead, your PCP probably uses a technique called auscultation. In this standard clinical method, a physician uses a stethoscope to listen for distinctive heart sounds—known as Korotkoff sounds—and watch a manometer while deflating a previously inflated cuff placed on the upper arm. Home BP devices use a simpler—and less accurate—method called oscillometry, which estimates BP using known relationships found in the oscillating pressure amplitudes measured with a cuff with a built-in pressure sensor.

Even with home BP monitors, the use of a cuff effectively precludes continuous BP measurement, referred to clinically as ambulatory blood pressure monitoring (ABPM). ABPM is considered vital for finding an individual's true BP, particularly for the estimated one-third of hypertensive patients who suffer from misclassified hypertension because of temporarily high BP (known as white coat hypertension) or temporary low BP (masked hypertension), when measured by a healthcare provider in a doctor's office, clinic, or hospital. ABPM devices are expensive and typically not covered by health insurance, effectively preventing their routine use. The availability of relatively low-cost cuffless BP wearables could be a game-changer, enabling routine ABPM needed to identify and treat hypertension.

Next-generation cuffless BP wearables rely on techniques that are dramatically different from conventional BP measurement methods. These wearables extract BP measurements using data from the same optical sensors used for photoplethysmography (PPG) in consumer heart-rate monitors and fitness wearables.

PPG relies on optical changes in the skin caused by the periodic increase in blood volume associated with each heart contraction. In principle, the same optically measured changes in blood volume can be used to track the arterial pressure waves associated with ventricular contractions that pulse blood throughout the body. By measuring the time required for this pulse pressure wave to pass along a known distance, the corresponding pulse wave velocity (PWV) can be calculated. Using known hemodynamic relationships, PWV can, in turn, be used to calculate blood pressure. Nonetheless, using these principles in a wearable design is challenging on multiple levels.

For the designer of a cuffless BP wearable, the first challenge lies in arriving at an accurate PWV measurement. Researchers have evolved two primary approaches for PWV determination:

• Pulse transit time (PTT), which calculates PWV using PPG measurements taken from two different parts of the body separated by a known distance,
• Pulse arrival time (PAT), which calculates PWV by comparing the arrival of a pulse wave measured using PPG with the same pulse's associated R wave, which is the peak in an electrocardiogram (ECG) QRS complex.

Researchers have made great progress in measuring PTT and PAT, and new methods continue to emerge. For example, machine-learning methods are being used to estimate BP by analyzing the shape of the pulse waveform itself. Even so, a significant challenge remains in interpreting the results. Besides dynamic characteristics such as arterial wall elasticity, blood viscosity, and others, variation in the cardiovascular system from individual to individual significantly complicate the task of calculating absolute blood pressure. As a result, researchers have found that all current approaches require periodic use of a cuff to calibrate the otherwise cuffless device.

Despite the sheer difficulty of cuffless BP measurement, researchers remain confident that suitable solutions can be found—particularly if a middle ground can be found between healthcare providers' desire for clinical accuracy and users' desire for utmost convenience. Indeed, next-generation health-based wearable designs are likely to evolve into two tracks: one emphasizing accuracy for clinical-grade vital signs measurements and another emphasizing convenience for consumer health and wellness.

Future consumer devices for health and fitness are likely to push for more subjective metrics focusing more on helping users adopt healthy behaviors. Besides estimated vital signs measurements, these devices will incorporate more types of sensors and sensory modalities such as voice analysis, sleep tracking, and body composition offered in Amazon's Halo wearable. By emphasizing user convenience and breadth of information, emerging consumer wearables can indeed go a long way in fostering wellness.