How do blood pressure monitors really work?

It is not very difficult to measure a person’s blood pressure. The classic method, which can be traced back to Italian physician Scipione Riva-Rocci’s auscultatory technique, requires only a stethoscope and an arm cuff hooked up to a manometer. 

The arm cuff is pumped full of air, cutting off the circulation in the brachial artery, and the pressure is then released gradually. If it drops below the systolic blood pressure level, the blood flow through the artery will return. However, the blood flow will be turbulent for as long as the artery remains partially compressed. This results in a muffled, pulsating noise phenomenon known as Korotkoff sounds, which can be counted by placing a stethoscope on the brachial artery or by integrating a microphone into the measurement cuff. Systolic blood pressure readings can be taken from the manometer as soon as they occur. The Korotkoff sounds disappear as soon as compression of the artery is released completely and the blood can flow again smoothly. The cuff pressure as measured at this point corresponds to the diastolic blood pressure.

However, the auscultatory technique has how been widely replaced by automatic blood pressure monitoring devices. These are not only used in hospitals and doctors’ surgeries, they are also very popular for home use. The automatic devices work on the same principle as the method developed by Riva-Rocci, yet, in contrast to the auscultation technique, they usually do not record the resultant sounds, but rather the vibrations or oscillations of the vascular wall. 

As with the auscultatory method, blood flow through the artery only returns when the cuff pressure falls below systolic blood pressure level. The turbulent blood flow causes vibrations of the artery wall, the maximum of which correlates with what is known as ‘mean arterial pressure’, or ‘MAP’. These vibrations are detected by the sensors in the arm cuff. If the cuff pressure, in turn, falls below the diastolic pressure, they come to a virtual standstill. An oscillogram is generated based on the changing oscillation amplitude between systolic and diastolic blood pressure.

Since it is easier to detect the oscillation maximum than it is to start and stop it, the MAP constitutes the most reliable figure for automatic blood pressure measurement. Using appropriate algorithms, systolic and diastolic blood pressure can also be calculated using the oscillogram and the MAP. However, there is still no uniform formula for performing this calculation. Consequently, differences in measurement between individual devices and manufacturers are possible. 

Regardless of whether they are performed digitally or in the old fashioned way, blood pressure checks are an important aspect of routine healthcare practices. A significant proportion of the population will develop undetected arterial hypertension at some point during their life. If left untreated, this can cause permanent damage to the cardiovascular system, as well as to other organs such as the eyes and kidneys. High blood pressure increases the risk of heart attacks and strokes, which are the most common causes of premature death and disability. If detected early enough, however, arterial hypertension can be treated effectively with physical exercise, a change in diet and weight loss or medication, and the development of secondary diseases can be prevented.