An Open Source and Accurate Blood Pressure Monitor

Regular blood pressure measurements are critical for the maintenance of many different health conditions and are a smart idea even for people who believe that they’re in good health, too. There is, after all, a reason that a nurse will measure your blood pressure pretty much any time you step into any kind of doctor’s office. But while there are numerous affordable blood pressure monitors on the consumer market, their accuracy is questionable and it is difficult to determine just how inaccurate they are. That’s why element14 Presents' Milos Rasic built an open source blood pressure monitor that prioritizes transparency.

Automatic blood pressure monitors are common, but you may have noticed that a lot of doctors — especially cardiologists — still measure your blood pressure the old-fashioned way: with a stethoscope and manual pressure cuff. That’s because automatic machines can sometimes struggle with the intricacies of our messy anatomy. The machine designed by Rasic, which is automatic, gets around that by collecting measurements with a few different sensors and comparing them to each other, verifying that they align as expected. The machine can also be calibrated for use and, because it is open-source, anyone can examine the algorithms used and make sure they’re sensible.

The machine has three distinct sensor systems: conventional blood pressure measurement (with a cuff and electronic stethoscope), a PPG (Photoplethysmography) finger clip-style pulse oximeter, and an ECG (Electrocardiogram). A Raspberry Pi Pico W development board monitors those and transmits the data to a PC running custom software programmed in Python. Because that Pico has a wireless adapter, it would be possible to transmit data to a PC or smartphone wirelessly in the future.

A 3D-printed enclosure contains a custom PCB that hosts the Pico, the amplifiers for the sensors, and a power system isolated for safety. That power can come from an internal 18650 lithium battery or from an external power adapter. The enclosure also contains the air pump and motor for the pressure cuff.

Rasic built a separate calibration rig for the pressure cuff that can produce known pressures, giving him the ability to verify the readings supplied by his monitor.

That, in addition to the redundant sensors, proved to be pretty accurate. In his limited testing, Rasic compared his machine’s readings to those from a commercial machine, as well as to measurements taken manually with a stethoscope. They were all in the same neighborhood and the differences can be explained — at least partially — by the interval between measurements.

It is difficult to determine how accurate the machine truly is without the help of a cardiologist and proven medical equipment, but this design has promise for people who don’t want to rely on the options on the market today.