Blood pressure measurement provides two fundamental readings: systolic and diastolic pressure. The auscultatory method, which uses a sphygmomanometer (cuff and gauge) and a stethoscope, remains a standard for obtaining these values. The stethoscope amplifies specific sounds, known as Korotkoff sounds, generated by blood flow within the artery as the cuff pressure is released. Listening for the appearance and disappearance of these sounds determines a precise measurement.
Preparing the Patient and Equipment
Accurate blood pressure readings require proper preparation of both the patient and the equipment. The patient should be seated comfortably with their back supported and feet flat on the floor, avoiding crossed legs, for at least five minutes. The arm chosen must be bare, supported at heart level, and relaxed, as tension or a lowered arm can artificially alter the reading.
Selecting the correct cuff size is necessary to prevent inaccurate results. A cuff that is too small leads to a falsely high reading, while one too large may yield a falsely low one. The cuff is wrapped snugly around the upper arm, positioned about two to three centimeters above the antecubital fossa (the crease of the elbow). The cuff bladder must be centered over the main artery to ensure accurate compression.
Identifying the Auscultation Site
The precise location for placing the stethoscope diaphragm is directly over the brachial artery pulse in the antecubital fossa. This is the main artery of the upper arm, and its pulse is typically felt on the inner aspect of the elbow crease. Before placement, a healthcare provider gently palpates the area to confirm the exact location of this pulse point.
Placing the diaphragm directly over the palpated pulse ensures maximum transmission of the arterial sound waves. The sounds are generated by the turbulent flow of blood partially occluded by the inflated cuff. If the stethoscope is placed off-center, the sounds may be muffled or missed, leading to an incorrect reading. The stethoscope head must be placed just below the edge of the cuff, never underneath it.
Proper Diaphragm Technique and Pressure Application
Once the correct site is identified, the stethoscope diaphragm should be placed on the skin with light but firm pressure. The goal is to maintain a complete seal to transmit the sound effectively without pressing so hard that the underlying artery is compressed. Excessive pressure can distort the artery or create turbulence, artificially altering the Korotkoff sounds and leading to an inaccurate diastolic reading.
The stethoscope tubing should be held still to minimize artifact noise. Allowing the stethoscope head or tubing to rub against the cuff or clothing generates extraneous noise that can obscure the true Korotkoff sounds. The diaphragm is generally preferred over the bell for blood pressure measurement due to its larger surface area.
Understanding the Korotkoff Sounds
The sounds heard through the stethoscope are known as Korotkoff sounds, which are divided into five distinct phases. These sounds are produced when the cuff pressure is slowly released, causing the compressed brachial artery to open and close with each heartbeat, creating turbulent blood flow.
The first sound heard, designated as Phase I, is a sharp, repetitive tapping sound, and the pressure reading at this moment is recorded as the systolic blood pressure. As the cuff pressure continues to drop, the sounds progress through Phases II (a soft swishing sound) and Phase III (a crisper tapping sound). Phase IV is marked by a distinct muffling or softening of the sound. Finally, the point at which all sounds completely disappear is Phase V, and this reading is recorded as the diastolic blood pressure, signifying the return to smooth, unobstructed blood flow.