Both automatic and manual non-invasive blood pressure (NIBP) meters involve the blood pressure cuffs (or bp cuffs) for the measurement of the individual’s blood pressure. Regardless whether automatic blood pressure cuffs, or manual bp cuffs, or best professional blood pressure monitor, or a home bp monitor type, the bp cuffs operate on the same physical principle.
The basic principle of NIBP operation
The principle of a NIBP meter using an air pressured cuff can be stated in simple terms by saying that, when the air force in the cuff from the outside of the artery wall is higher than the blood force on the inside of the artery, the artery will shrink (to the point of being closed up), and when the air force in the cuff onto the wall of the artery is less than the blood force onto the inside wall of the artery, the artery will expand to its normal size, unaffected by the force of the bp cuff.
The NIBP device will have a cuff which will be filled with pressurized air whose force onto the artery gradually increases as it pushes against the artery in the upper arm. The mechanics of the NIBP monitor is the interplay between the air pressure in the cuff and the blood pressure in the artery. There are two principal pressures in the artery, the systolic, and the diastolic. The systolic is the maximal blood pressure in the artery, and the diastolic is the minimal blood pressure.
The pulsation of the artery
Since the blood pressure in a human artery constantly fluctuates between the low value, or diastolic pressure, and the high value, or the systolic pressure, it is easy to see that, as we rise, or lower the air pressure in the cuff gradually, we will observe the pulsation of the artery in between the systolic and the diastolic pressure of the air in the cuff, which can be both controlled and measured.
No pulsation above the systolic or below the diastolic pressure
This is due to the fact that, as the air force in the cuff reaches the systolic force and higher, the artery will be squeezed to size zero and there will be no blood flow, and no pulsation.
As the air pressure is lowered below the diastolic pressure, there will be no effect on the artery, therefore the blood will flow through it as if there were no cuff. Again, there will be no additional pulsation.
In the interesting region between the systolic pressure of the cuff and the diastolic pressure, the artery will be squeezed from the expanded state all the way to the completely contracted state, following the pressure extended by the heart muscle. The shrinking and expanding of the artery will cause increased pulsation in the blood flow in the arm, which can be detected easily.
So, as the cuff air pressure is gradually increased up to the diastolic pressure, we begin to hear pulsation, and by further increasing the cuff air pressure we come to the higher, systolic pressure point, where the pulsation disappears again.
The exact opposite effect will be noticed as we gradually decrease the air pressure from above the systolic pressure, to below the diastolic pressure.
By noticing the two air pressures where the pulsation starts and stops, we have noted the systolic and the diastolic pressure of the artery.