Auscultatory method of blood pressure measurement offered by N.A. Korotkoff is well know all over the world. Insrumentation options of this method used in ABPM are based on the same principle. During clinical measurement we control cuff pressure by a manometer and during ABPM cuff pressure is controlled by sensor and processor; in the first case we listen to sounds with our ears via stethoscope, in the second case the sounds are captured by a microphone into a memory chip.
See figure: there is
- blood pressure in brachial arteria (1);
- cuff pressure (2);
- Korotkoff sounds (3).
The cuff pressure affects the arm tissues and as a result blood vessels shrink and their inner diameter is reduced. While the pressure in the cuff is higher than systolic BP blood doesn't flow through brachial artery (no sounds).
At the moment when cuff pressure becomes lower than systolic BP some part of blood obtains the posibility to pass through arteries squeezed by the cuff. As far as blood flows through partly pressed vessels this flow becomes turbulent (Not laminar as under usual condition). Turbulent flow causes the appearence of sound whish can be detected. Sounds of different character are auscultated during all time when the pressure in a cuff is between systolic and diastolic BP.
The processor determines the values of systolic and diastolic BP by analyzing the Korotkoff sounds at different levels of cuff pressure.