Blood pressure is the force exerted by the circulating blood against the walls of the body’s arteries. It is recorded as two numbers: systolic pressure, the force when the heart beats, and diastolic pressure, the force when the heart rests between beats. The most established method for obtaining this measurement is the auscultatory technique, which relies on listening to sounds produced by blood flow in the arteries. This manual approach uses a sphygmomanometer and a stethoscope, and it remains the standard reference for accurate blood pressure determination.
Gathering Supplies and Preparing the Patient
Accurate manual blood pressure measurement requires three primary pieces of equipment: a sphygmomanometer, a blood pressure cuff, and a stethoscope. The sphygmomanometer is the device that measures the pressure, typically featuring an aneroid gauge or a mercury column, connected to an inflatable cuff. The stethoscope is used to listen to the sounds of blood flow in the artery, acting as the auditory part of the process.
Selecting the appropriately sized cuff is important, as using one that is too small can lead to a falsely high reading. The cuff’s inflatable bladder should wrap around 75% to 100% of the arm’s circumference and be wide enough to cover at least 40% of the distance from the shoulder to the elbow. The patient must be properly positioned and rested before the measurement begins to avoid temporary pressure spikes.
The patient should be seated comfortably with their back supported, feet flat on the floor, and legs uncrossed. The arm being used should be bare, with no clothing rolled up that might constrict circulation, and should be supported at the level of the heart. It is important the patient avoids talking and is allowed to rest quietly for at least five minutes before the assessment.
The Step-by-Step Measurement Process
The first step in the procedure is to correctly place the blood pressure cuff on the upper arm, with the lower edge positioned about one inch (2.5 cm) above the bend of the elbow. The cuff’s artery marker should be aligned over the brachial artery, which is located on the inner side of the arm. To locate the correct spot for the stethoscope, one must first palpate, or feel, the brachial artery pulse just above the elbow crease.
After the pulse is located, the bell or diaphragm of the stethoscope is placed over the brachial artery, just below the edge of the cuff. The cuff is then quickly inflated to a pressure approximately 20 to 30 millimeters of mercury (mmHg) above the expected systolic pressure. If the expected pressure is unknown, inflate the cuff until the radial pulse is no longer palpable, and then add 30 mmHg.
Once the artery is fully occluded by the inflated cuff, the air is released slowly. The valve on the inflation bulb is opened slightly, allowing the pressure to drop at a rate of about 2 to 3 mmHg per second. The examiner must maintain a steady rate of deflation while listening intently for the sounds that signal the return of blood flow.
Identifying Systolic and Diastolic Pressure
The pressure values are determined by listening for the specific sounds created by the turbulent flow of blood, known as Korotkoff sounds, as the cuff deflates. As the pressure drops, the first faint but clear, rhythmic tapping sound heard through the stethoscope marks the systolic pressure, which is Korotkoff Phase I. This sound indicates the moment the pressure in the cuff falls below the peak pressure generated by the heart’s contraction.
As deflation continues, the sounds change, becoming softer and then more muffled before finally disappearing completely. The point at which the Korotkoff sounds vanish entirely is known as Korotkoff Phase V, and this reading corresponds to the diastolic pressure. The corresponding numbers on the sphygmomanometer gauge are recorded at the moment the first sound is heard (systolic) and the moment the final sound disappears (diastolic). The systolic reading is recorded as the top number and the diastolic reading is the bottom number, such as 120/80 mmHg.