Blood pressure is the force of circulating blood against the walls of the arteries. It is recorded as two numbers: systolic pressure (the higher number during the heart’s contraction) and diastolic pressure (the lower number when the heart is at rest between beats). While automated devices are common, the auscultatory method, which uses a manual cuff and a stethoscope, remains the traditional standard for accurate measurement. This technique relies on listening for specific sounds generated by the blood flow within the artery.
Essential Equipment and Setup
Accurate blood pressure measurement requires an aneroid or mercury sphygmomanometer and a stethoscope. The sphygmomanometer consists of an inflatable cuff and a pressure gauge, used to temporarily stop and then control the flow of blood in the artery. The stethoscope, positioned over the artery, amplifies the faint sounds created by the turbulent blood flow as the pressure is released.
Patient preparation is crucial for an accurate reading. The individual should sit comfortably with their back and feet fully supported, keeping their legs uncrossed. The arm selected for measurement should be bare, and supported at the level of the heart.
The blood pressure cuff size must be appropriately selected; a cuff that is too small can lead to a falsely high reading. The cuff is wrapped snugly around the upper arm, about 2 to 3 centimeters above the crease of the elbow (antecubital fossa). Ensure the artery indicator mark is centered directly over the brachial artery.
The Step-by-Step Measurement Technique
The process begins by locating the brachial artery, typically found by palpating the pulse in the antecubital fossa. This is the site where the stethoscope will be placed. It is recommended to use the palpatory method first, which involves feeling the radial pulse while inflating the cuff to estimate the systolic pressure.
After estimating the systolic pressure, the stethoscope’s bell or diaphragm is positioned gently over the brachial artery. Apply light pressure with the stethoscope head, avoiding contact with the cuff or tubing, which could create extraneous noise. Excessive pressure can distort the sounds and affect the accuracy of the reading.
The cuff is then inflated rapidly to a pressure approximately 20 to 30 millimeters of mercury (mmHg) above the point where the radial pulse disappeared during the initial estimation. This inflation pressure completely occludes the artery, temporarily halting blood flow.
Once the inflation target is reached, the valve on the sphygmomanometer is slowly opened to begin the deflation process. The deflation rate must be consistent and slow, ideally maintained at 2 to 3 mmHg per second. This controlled rate is necessary to precisely correlate the pressure gauge reading with the auditory events. The observer must watch the pressure gauge throughout the slow deflation process while simultaneously listening intently for the first sounds to emerge.
Understanding the Korotkoff Sounds
The sounds heard during the controlled deflation of the cuff are called Korotkoff sounds, generated by the turbulent flow of blood forcing its way through the partially compressed artery. These sounds are categorized into five distinct phases, but only the first and fifth phases are used to determine the standard blood pressure reading.
The systolic pressure is identified by the first appearance of faint, repetitive, clear tapping sounds (Korotkoff Phase I). At this pressure, the blood pressure inside the artery is momentarily greater than the pressure exerted by the cuff, allowing a small volume of blood to push through.
As the cuff pressure continues to drop, the sounds progress through swishing (Phase II) and thumping (Phase III) before they become muffled (Phase IV). The diastolic pressure is determined by the point at which all sounds completely disappear (Korotkoff Phase V). This disappearance signifies that the artery is no longer compressed during the heart’s resting phase, and blood flow has returned to its normal, silent, laminar state. The final measurement is then recorded as a fraction, with the systolic pressure over the diastolic pressure, such as 120/80 mmHg.