Latest patent in Fitbit’s push for wearable blood pressure tech

Fitbit’s patent portfolio in the blood pressure monitoring game just got a little thicker. With the ink barely dry on their latest filing—published on August 1, 2024, and stamped with the snazzy code US 2023/0293028 A1—they’re diving headlong into a novel way of measuring blood pressure from the wrist.

Blood pressure monitoring is still a developing feature in the wearable tech industry. The majority of mainstream wearables have yet to incorporate this functionality, making it a field ripe for innovation. And the handful of solutions that are out there, mostly come with limitations. This includes the lack of continuous monitoring and cumbersome calibration processes.

The PPG-pressure sensing solution

This latest patent describes a smartwatch that measures blood pressure from the wrist by analyzing how the PPG signal changes with varying applied pressure. That’s something we have not seen before.

The smartwatch would be equipped with a PPG sensor, typically consisting of green LEDs and photodetectors, positioned on the back of the watch where it contacts the user’s wrist. Adjacent to this, there would be a force or pressure sensor capable of detecting how firmly the watch is pressed against the skin.

To take a measurement, the user would be instructed to press the watch against their wrist with gradually increasing pressure. As they do this, the PPG sensor continuously monitors blood volume changes in the arteries beneath the skin, while the pressure sensor tracks the force being applied.

The watch’s processor would analyze how the amplitude of the PPG signal (which represents the pulsatile blood volume) changes as the applied pressure increases. Initially, as pressure increases, the PPG amplitude typically rises as the arterial wall is compressed, making the pulsations more pronounced. However, at a certain point, further pressure begins to occlude the artery, causing the PPG amplitude to decrease.

This relationship between applied pressure and PPG amplitude creates a characteristic curve. The shape and key points of this curve (such as the pressure at maximum amplitude) correlate with the user’s blood pressure. The watch’s software would use algorithms to interpret this curve and estimate systolic and diastolic blood pressure values.

This method allows for cuff-less blood pressure estimation in a compact, wearable form factor. However, it’s worth noting that such a system would likely require periodic calibration against a traditional cuff-based measurement to maintain accuracy, as factors like the watch’s position on the wrist and the user’s technique in applying pressure can influence the readings.

Fitbit’s previous blood pressure patents

It’s also essential to consider Fitbit’s prior work in this area. As mentioned, the company has a number of other patents to do with blood pressure monitoring.

In December 2021, Fitbit filed a patent focusing on estimating arterial stiffness by measuring pulse wave velocity (PWV) from the wrist. Experts considered this technique promising for continuous, cuffless blood pressure measurements.

In July 2023, Fitbit filed yet another patent, this time for a finger-based blood pressure monitor. The filing describes a device designed to overcome the limitations of traditional oscillometric blood pressure measurement (OBPM) systems by being less cumbersome and more accurate. The device features a miniaturized and inflatable bladder, which fits inside a hole specifically designed to accommodate a human finger. This design offers a more comfortable and precise measurement process.

Pulse Transit Time

A patent in late 2023 discusses Pulse Transit Time (PTT) as a method for continuous, unobtrusive blood pressure measurement. In essence, PTT gives you the time duration for the pulse wave to travel a certain distance, while PWV tells you the speed of that pulse wave. Both are used for estimating blood pressure, but they offer different perspectives on arterial health.

The paperwork goes into lots of detail on the issue of calibration. This has been a significant challenge in the adoption of PTT technology. The patent mentions the use of maneuvers such as the Valsalva or cold pressor test to obtain reliable calibration data.

Comparative insights

Given Fitbit’s series of patents on this topic, it’s evident that the company is working diligently to integrate blood pressure monitoring in their devices. While the technology is still in its infancy, these patents indicate a move towards more user-friendly and reliable blood pressure monitoring solutions in wearables.

Essential reading: Best fitness trackers and health gadgets

We are hopeful Fitbit will have at least one new device in late August or early September. In time for IFA in Berlin. The company always has something new to show around this time. And let’s not forget the Google Pixel Watch 3 which is widely expected to land on August 13th.

But don’t hold your breath for blood pressure monitoring to make its debut on any of these devices. The smart money says we’ll see our wrists doubling as sphygmomanometers more commonly in 2025.

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