The manual measurement of blood pressure, known as the auscultatory method, remains a reliable technique for assessing cardiovascular health. This traditional method involves listening for specific sounds in the artery using a stethoscope while controlling the pressure from a blood pressure cuff. This procedure is considered a reference standard against which automated devices are often checked. In certain clinical situations, such as when a patient has an irregular heart rhythm or very low blood pressure, manual measurement can provide an accurate reading when an electronic device may fail or be unreliable.
Necessary Tools for Manual Blood Pressure Measurement
The manual auscultatory method requires three distinct components. The first is the sphygmomanometer, which is the complete blood pressure apparatus consisting of the cuff, the inflation system, and the pressure gauge. Modern practice uses an aneroid sphygmomanometer, which features a circular dial gauge, replacing the mercury column devices phased out due to environmental concerns.
The inflatable cuff contains a rubber bladder that encircles the arm and must be properly sized to ensure an accurate reading. For an adult, the bladder should cover about 80% of the arm’s circumference to prevent artificially high or low readings. The inflation system includes a rubber bulb used to manually pump air into the cuff and a control valve to precisely release the pressure.
The third component is the stethoscope, which allows the operator to hear the subtle sounds of blood flow through the artery. The bell-side of the stethoscope’s chest piece is recommended for manual blood pressure measurement. It is better designed to pick up the low-pitched sounds created by the turbulent blood flow.
Proper Patient and Cuff Placement
Preparation and positioning prevent common errors and ensure measurement accuracy. The patient should be seated comfortably with their back and arm supported, and their feet flat on the floor with legs uncrossed. The arm selected for the measurement must be positioned so that the midpoint of the upper arm is level with the heart. An unsupported arm or one held too low can lead to an artificially high reading.
Before applying the cuff, the patient should rest quietly for at least five minutes without talking, as recent physical activity or stress can temporarily elevate blood pressure. The cuff should be applied directly to the bare skin, never over clothing, which can introduce measurement errors. The bottom edge of the cuff needs to be placed approximately 1 to 1.5 inches (2 to 3 cm) above the antecubital fossa, the crease of the elbow.
The artery marker printed on the cuff should be positioned directly over the brachial artery. This artery can be located by palpating for a pulse on the inner arm near the elbow crease. The cuff should be wrapped snugly around the arm, but not so tightly that it causes discomfort before inflation. Checking the cuff’s index line against its range markings confirms that the correct size is being used.
Step-by-Step Auscultation Technique
The measurement begins by estimating the systolic pressure through palpation to avoid missing an “auscultatory gap,” a silent interval that can occur between the systolic and diastolic pressures. The operator finds the radial pulse and inflates the cuff quickly until the pulse disappears, noting this pressure value. The cuff should then be inflated an additional 20 to 30 millimeters of mercury (mmHg) above that point to ensure complete arterial occlusion.
After establishing the target pressure, the stethoscope is placed lightly over the brachial artery at the antecubital fossa. The operator then slowly opens the control valve to deflate the cuff at a precise rate, ideally 2 to 3 mmHg per second. Deflating the cuff too quickly can cause the true blood pressure to be missed, leading to an inaccurate reading.
As the pressure is released, the operator listens intently for the first faint, repetitive tapping sound, which marks the start of blood flow past the cuff. This initial sound is the systolic pressure reading and must be noted immediately on the gauge. The sounds will continue and change in quality, becoming swishing, then crisper, and finally muffled, before disappearing completely. The point at which the sounds completely disappear is the diastolic pressure. Once the diastolic reading is obtained, the cuff should be deflated completely and rapidly.
Understanding Systolic and Diastolic Values
The two numbers recorded represent the maximum and minimum pressures exerted on the artery walls during a single cardiac cycle. The higher number, the systolic pressure, represents the force of the blood when the heart contracts and pushes blood out into the body. This reading corresponds to the first sound heard in the stethoscope, the Korotkoff Phase I sound—a distinct tapping noise indicating the turbulent return of blood flow through the partially compressed artery.
The lower number, the diastolic pressure, represents the pressure inside the arteries when the heart is at rest between beats and is refilling with blood. This value is determined by the point at which all sounds disappear, known as the Korotkoff Phase V sound. At this pressure, the artery is no longer compressed, and blood flow has returned to a smooth, non-turbulent state.
For most healthy adults, a normal blood pressure reading is considered to be less than 120 mmHg systolic and less than 80 mmHg diastolic. Pressures consistently above this range may indicate a risk for hypertension, while lower readings may indicate hypotension.