Wello: an iPhone case that measures your blood pressure?

Activity/medical trackers come in a variety of form-factors. They range from smartphone apps to smartphone attachments to wristbands to headbands to clipped-on devices. The newly introduced Wello (Video 1) takes a different approach: a smartphone (iPhone) case. The feature list is impressive; it purports to measure heart rate, blood pressure, temperature, blood oxygen levels, and even the EKG waves from your heart.

I was intrigued by the blood pressure measurement; how would Wello monitor your blood pressure?

Blood pressure arises from the pumping action of your heart that pushes blood through the blood vessels (arteries, capillaries, veins) which create resistance to the blood flow resulting in pressure. There are two phases to the heart beat: (1) First the heart ventricles contract forcing blood out of the heart and into the blood vessels of the body, and then (2) they relax allowing the atria to refill the ventricles. As a result, there are two pressures: The higher systolic pressure during ventricle contraction, and the lower diastolic pressure during ventricle relaxation. Pressure is measured the standard way in terms of millimeters of Hg (mercury). A typical healthy person has a blood pressure of 120/80 (systolic/diastolic); high blood pressure (hypertension) is considered to be a reading that exceeds 140/90.

The classic sphygmomanometer (blood pressure measuring device) uses an inflatable cuff on your upper arm to compress the brachial artery. As the cuff is inflated (and its pressure measured by a manometer), the artery is collapsed during both systole and diastole so that no blood flows. When the pressure is gradually released, then blood pushes through the pressure exerted by the cuff during systole but not diastole, and there should be a thumping sound (systolic beat) when the doctor listens to the brachial artery below the cuff. As the pressure drops further (below the diastolic pressure), then the blood vessel is not occluded at all and blood should flow freely so there is no noise. Thus, as the cuff deflates, the systolic pressure is when you first hear thumping, and the diastolic pressure is when it stops. This process is automated in home blood pressure measuring devices which inflates a cuff and then uses a digital monitor to sense the motion (thumping/beating) of the brachial artery.

It is possible to measure blood pressure from the finger using the same principle. A cuff is put on the finger and then the blood flow is monitored by shining light through the finger and measuring the blood volume (similar to the method used for measuring the heart rate in the Galaxy S5). The pressure in the finger cuff can be adjusted to prevent changes in blood volume during systole or diastole resulting in the two pressure measurements.

What these two techniques have in common is an inflatable cuff for the arm or finger. Wello possesses no cuff and so it is unclear how pressure can be used to impede blood flow.

My hypothesis (speculation) is that when you press your finger down on the iPhone case, it measures the pressure exerted by your finger while simultaneously monitoring changes in blood volume in the finger. Then it relates these two measurements to arrive at an estimate of blood pressure. I am quite curious about the accuracy of the reading; the FDA will check that the device is actually measuring blood pressure and not just throwing out some random numbers. Regardless, Wello is an interesting and potentially impressive addition to the category of activity/medical trackers.

Video 1. Measuring blood pressure with Wello?