Blood pressure (BP) is the force exerted by circulating blood against the walls of the arteries, representing a fundamental measure of cardiovascular health. While automated devices are convenient, the manual, auscultatory method remains the established standard for obtaining the most accurate clinical readings. This technique uses an inflatable cuff, a pressure gauge (sphygmomanometer), and a stethoscope to listen for specific sounds over the brachial artery. Understanding what to listen for during cuff deflation is paramount to correctly determining the systolic and diastolic readings.
The Origin of Auscultatory Sounds
The sounds heard during manual blood pressure measurement are acoustic phenomena known as Korotkoff sounds, not the heart’s valves. Normally, blood flows silently through the brachial artery in a smooth, layered pattern called laminar flow. When the cuff is inflated above the systolic pressure, it completely compresses the artery, stopping the flow and resulting in silence. As the cuff pressure is slowly released, blood pulses through the narrowed artery, creating chaotic, non-layered movement known as turbulent flow. This turbulence causes the arterial walls to vibrate, generating the distinct sounds picked up by the stethoscope.
Identifying the Systolic Pressure
The systolic pressure, the higher number, represents the peak pressure when the heart is actively contracting. This reading is identified by the first of the Korotkoff sounds, specifically Phase I. When the cuff pressure drops enough to allow the first small spurt of blood past the constriction, a faint, clear tapping sound is heard. This initial sound marks the highest pressure point in the artery during the cardiac cycle. The manometer reading must be noted precisely at this first detection, as the tapping sounds typically increase in intensity shortly thereafter.
Identifying the Diastolic Pressure
The diastolic pressure, the lower number, represents the pressure within the arteries when the heart is at rest between beats. This value focuses on the very last sound heard, which corresponds to Korotkoff Phase V in adults. As the cuff continues to deflate, the sounds soften, become muffled (Phase IV), and then abruptly vanish. The disappearance of all sound indicates that the cuff pressure is no longer constricting the artery, allowing blood flow to return to its smooth, silent laminar state. Phase V (complete silence) is the standard endpoint for the diastolic pressure reading in most adults because it is consistently reproducible.
Techniques for Clear Hearing and Accuracy
Achieving an accurate manual blood pressure reading depends heavily on proper technique and minimizing interference. The stethoscope’s diaphragm should be placed lightly over the brachial artery in the inner bend of the elbow, ensuring it is not tucked under the cuff edge. The environment must be quiet, as external noise can obscure the low-frequency Korotkoff sounds, especially the faint Phase I and Phase V endpoints. Using a correctly sized cuff is fundamental, as improper sizing leads to erroneously low or high readings. The pressure should be released slowly and steadily, at approximately 2 to 3 millimeters of mercury (mmHg) per second, allowing the listener time to accurately identify the manometer reading for the first and final sounds.