Several years ago when I started to study blood pressure measurement theory, I found an interesting thing – every BPM manufacturer has its own method to guestimate the blood pressure, so called algorithm. Is there any measurement tool in the world like this? For example, does a ruler have an algorithm to measure the length? I dig further to understand this question. Blood pressure measurement with a cuff is indirect measurement. The pressure sensor measures the pressure in the cuff during cuff deflation but the sensor can’t tell which point is the blood pressure. A doctor find the two points based on auscultatory sounds from their stethoscope during cuff deflation – when the sound appears, it is systolic pressure and when the sounds disappear, it is diastolic pressure. So, doctors don’t need any algorithm and they just need their ear. But there is no stethoscope in a normal BPM, how does a BPM find the blood pressure? A BPM analyzes the characteristics of the pressure oscillation in the cuff. The pressure oscillation is not similar to auscultatory sounds – the oscillation is always there no matter the pressure is above the systolic pressure or below the diastolic pressure, so there is no direct determination like the sounds. However, the pressure oscillation does change with blood pressure, so the BPM guestimates the blood pressure just based on the oscillation amplitude, slope or other changes. For instance, some manufacture may use half the largest osicllation amplitude as systolic pressure. That charateristics on the relation between blood pressure and pressure oscillation may vary from people to people. That means, maybe for some people, the corresponding amplitude is half but some are 0.6 or 0.55. If such characteristics of person is different from the characterics that algorithm can capture, the measurement of that person could be inaccurate.
So, there comes another question: with different algorithms, can different BPMs give the same reading? The answer is NO. Researchers from Germany has found two automatic BPMs from two different manufacturers gave very different readings based on a study (here is the link: http://journals.lww.com/…/Disagreement_of_the_two_oscillome…). A person may be diagnosted as hypertension with a BPM from this manufacturer while normotension with a BPM from another manufaturer. Will the person be happy with the BPM that tell him his blood pressure is good?