Continuous blood pressure monitoring is becoming a reality

In a previous blog post, I discussed the potential of Apple's AirPods to function as a wearable, continuous blood pressure monitor. In the long run, Apple envisions AirPods not just as entertainment devices but also as health monitors, similar to how the Apple Watch has evolved. Rumors date back to 2014 about health sensors in AirPods using ballistocardiogram (BCG, i.e. measuring mechanical recoil generated by the beating heart) technology to gather detailed cardiovascular data, including blood pressure. Measuring blood pressure via earbuds is quite challenging, as traditional methods like a sphygmomanometer rely on arterial compression that earbuds cannot provide.

On the other hand, photoplethysmography (PPG) is a more mature technology compared to BCG. For example, the Apple Watch uses PPG to measure heart rate by employing a light-emitting diode (LED) as a light source and a photodiode sensor to detect reflected or scattered light. The device shines bright light onto the skin, and as blood absorbs more light than surrounding tissues, variations in light absorption correlate with the fluctuating volume of blood with each heartbeat. More specifically, because blood is red, it reflects red light and absorbs green light. When your heart beats, the blood flow in your wrist increases, resulting in greater absorption of the green light, whereas between beats, the blood flow decreases and absorption drops. By flashing green LEDs hundreds of times per second, the watch detects these rapid fluctuations in light intensity to distinguish individual pulses and calculate your heart rate.

An example of a wearable blood pressure monitor is the Omron HeartGuide (QH). It is a wristband device that measures blood pressure using a pressurized cuff similar to a conventional sphygmomanometer, but worn on the wrist instead of higher up on the arm. The HeartGuide functions by inflating the cuff to compress the radial or ulnar artery, capturing blood pressure data as the cuff gradually deflates just like a miniaturized sphygmomanometer.  It detects changes in the amplitude of pressure oscillations to determine the blood pressure values. Because the blood vessels in the wrist are not as large as the brachial artery in the arm, the signal is less robust. Nevertheless in clinical testing, the Omron device was roughly equivalent in accuracy to the standard upper arm monitor, and as a result, it has been cleared by the FDA as a medical device.

Although wearable, the HeartGuide is not a continuous monitor, i.e. it takes time to inflate and deflate which is a single measurement. Thus, the prospect of a continuous BP monitor has generated excitement. The Aktiia bracelet is a cuffless, wrist-worn blood pressure monitor designed to capture blood pressure trends passively throughout the day and night (Figure 1). Instead of inflating a cuff for each reading, it uses our good friend photoplethysmography (PPG) to record pulse wave signals at the wrist and applies algorithms to estimate blood pressure, resulting in more frequent and convenient measurements than traditional cuff-based approaches.

Compared to the Apple Watch, the Aktiia bracelet measures detailed features of the pulse wave (shape, amplitude, etc.) rather than just the frequency of the pulses (i.e. pulse rate). Through Pulse Wave Analysis (also known as wave intensity analysis), Aktiia’s proprietary algorithms examine the unique characteristics of this waveform to estimate systolic and diastolic blood pressure without ever applying physical pressure to the arm.

To convert the raw PPG waveform into mmHg pressure readings (e.g. 120 or 80), there is a calibration step (initialization) using a conventional cuff. During setup you take three measurements with the included cuff, on the other arm so the system can align the bracelet’s optical signal to your cuff-based blood pressure (BP) reference. In day-to-day use, the bracelet primarily produces BP estimates during periods when you are still. A single valid reading typically requires a window of approximately 30 to 60 seconds of uninterrupted stillness to process the optical signals accurately, and the app presents the results as periodic summaries roughly every 2 hours.

So how accurate is this technology? In a key 1-month validation study versus reference measurements, the mean error was about 0.46 ± 7.75 mmHg for systolic and 0.39 ± 6.86 mmHg for diastolic, meeting ISO (International Organization for Standardization) accuracy expectations for noninvasive BP monitors. The error standard deviation of ~7 mmHg gives one an idea of the accuracy level. Based on these trial data, the Aktiia received FDA 510(k) clearance for over-the-counter use in the U.S., and the equivalent CE Mark in Europe. It should be noted that accuracy/precision can be worse outside ideal conditions (e.g. with different body positions or motion), and the system relies on periodic cuff-based calibration to maintain performance.

On a full charge, the bracelet typically lasts several days to about two weeks. There are ~150–375 readings per charge, and the device captures BP about 25 times per day (once an hour with the summary being produced every other hour).

The Aktiia bracelet has been rebranded as the Hilo Band, and is already sold in Europe. The price varies by country, listing at £209.99 in the UK, €229.99 in the Eurozone, and C$335 in Canada. It is expected to launch in the U.S. later this year (2026) at about $280. Included in this price is 12 months of app access (subscription). After the first year, there will be a $60-$80 annual fee.

As an aside, the Samsung Galaxy smartwatch also uses PPG to measure blood pressure, but it has not been cleared by the FDA. Newer models of the Apple Watch detect patterns of high blood pressure (hypertension) and send alerts, but do not give you specific numbers. Aktiia's clearance in July 2025 is the first OTC (Over-the-Counter) clearance for a cuffless optical blood pressure monitor.

Why is continuous blood pressure monitoring important? Blood pressure is dynamic, and a single clinic or home measurement can miss clinically important patterns. For example, White Coat Syndrome is when a medical setting (i.e. doctor’s check up) causes a temporary spike in BP due to anxiety. On the other hand, Masked Hypertension is when a patient may be relaxed at the doctor's office (showing normal readings) but have dangerously high BP at work or home due to life stressors. Finally, continuous monitoring can reveal “nocturnal dipping”. In a healthy person, blood pressure should drop by 10–20% while sleeping. Some people are "non-dippers" (their BP stays high at night) or "reverse dippers" (it actually rises). This is a major independent risk factor for stroke, heart attack, and kidney failure, even if daytime blood pressure appears normal.

In summary, the Aktiia bracelet (aka Hilo Band) does not provide a continuous, real-time, second-by-second blood pressure "live feed." Instead, it operates passively in the background, capturing discrete measurements when the user is still, and then averages these readings over a two hour period which is displayed in the app. However, these regular, around-the-clock snapshots furnish valuable information especially at night. We are not quite there yet, but we are getting closer and closer to a wearable continuous real-time blood pressure monitor.

Figure 1. The Aktiaa bracelet (also known as Hilo Band) is an important step toward continuous blood pressure monitoring becoming a reality (https://healthcare.aktiia.com/).