Measurement of hydrostatic blood pressure.
A traditional method of measuring the hydrostatic blood pressure is the auscultation method
, in which an inflatable cuff connected to a pressure meter (mercury manometer) known as a sphygmomanometer
, and a stethoscope are used. This method takes advantage of the fact that when an artery is partially constricted by a pressure cuff, blood flow through that artery becomes turbulent at and immediately beyond the point of constriction and, hence, leads to the production of sounds that can be heard by placing a stethoscope on the skin over the constriction area (see laminar versus turbulent blood flow
). The pressure in the cuff (blue line in top panel) is initially raised to a value above the expected arterial systolic pressure (red line in top panel). This will ensure complete obstruction of flow in the brachial artery. The cuff pressure is then gradually reduced at a rate of 2-3 mm Hg per second. At all cuff pressures above the systolic pressure, flow in the brachial artery remains obstructed. However, as soon as the cuff pressure falls below that of the peak systolic pressure, a small volume of blood is forced through the obstruction. The vessel constriction caused by the pressure cuff causes flow of this volume of blood to be turbulent. The turbulent flow causes a tapping sound known as the Korotkoff’s sound
, which can be heard by the stethoscope. Therefore, the tapping sound is in synch with the left ventricular systole. As the cuff pressure is further reduced, more and more blood is able to flow past the constriction during each left ventricular systole resulting in louder tapping Korotkoff’s sounds. However, as the constriction is relaxed further, turbulence in the flow decreases, and the Korotkoff’s sounds become dull and muffled. Thus, as long as the cuff pressure is below the systolic pressure, but above the diastolic pressure, flow past the constriction site will be turbulent, and the sounds of Korotkoff will be heard. When the cuff pressure is below the diastolic pressure, laminar flow once again resumes, and the Korotkoff’s sounds disappear. Thus, as the cuff pressure is reduced, the pressure that corresponds in time to the first tapping sound is the systolic pressure, and the pressure that corresponds in time to the last muffled sound is the diastolic pressure. It is also noteworthy to emphasize that as the cuff pressure is reduced gradually from the systolic to diastolic pressure, flow through the brachial artery is intermittent (bottom panel). Blood spurts through the constricted site during pressure peaks of the ventricular systole only when these pressures are greater than the cuff pressure. During each ventricular diastole, the pressure again falls below the cuff pressure, the brachial artery collapses, and no blood flows through. When the cuff pressure is again below the diastolic pressure, blood flow is unimpeded and laminar, however, it continues to be pulsatile (driven by the contractions and relaxations of the left ventricle).