The manual technique for measuring blood pressure, known as the auscultatory method, offers a high degree of precision when performed correctly. This traditional method relies on using a stethoscope and a sphygmomanometer to listen for specific sounds created by blood flow in the artery. The approach remains highly valued in clinical settings for its accuracy and portability compared to automated devices. Understanding the proper use of the stethoscope is central to obtaining a reliable measurement, involving precise equipment preparation, correct positioning, and careful auditory discrimination.
Necessary Tools and Positioning
Successful manual blood pressure measurement requires a stethoscope and a manual sphygmomanometer, which consists of an inflatable cuff, a pressure gauge, and a rubber inflation bulb. For auscultation, the stethoscope’s diaphragm is typically used over the brachial artery because it is best suited for hearing the higher-pitched Korotkoff sounds. Selecting the correct cuff size is important, as a cuff that is too small can artificially elevate the pressure reading, while one that is too large can underestimate it.
The individual being measured should be seated comfortably with their back supported and their feet flat on the floor, without their legs crossed. The arm used for the measurement must be bare and supported at the level of the heart, usually resting on a table. The cuff should be wrapped snugly around the upper arm, with its lower edge placed approximately 2 to 3 centimeters above the elbow crease.
The center of the cuff’s bladder must be positioned directly over the brachial artery, located on the inner side of the arm. Palpating the pulse in the antecubital fossa (the crease of the elbow) helps identify the correct spot for stethoscope placement. Proper positioning ensures the pressure exerted by the cuff accurately reflects the pressure within the artery at heart level, preventing hydrostatic errors.
Step-by-Step Measurement Technique
The first step is to locate the brachial artery pulse by gently pressing your fingers into the elbow crease. Once the pulse is found, the stethoscope head should be placed lightly over that spot. Pressing too firmly on the stethoscope can artificially compress the artery, which may interfere with the sounds heard.
Next, the cuff must be inflated rapidly using the rubber bulb until the pressure gauge reads about 20 to 30 millimeters of mercury (mmHg) above the estimated systolic pressure. Inflating the cuff to this level ensures the complete occlusion of the brachial artery, temporarily stopping blood flow. This initial high pressure is necessary to avoid missing an “auscultatory gap,” a silent interval that can occur between the first and second phases of sounds, potentially leading to an inaccurate measurement.
The valve on the inflation bulb should be opened slightly to allow the cuff pressure to drop at a steady rate of approximately 2 to 3 mmHg per second. This slow, controlled release is necessary to accurately identify the pressures at which the sounds appear and disappear. A deflation rate that is too fast can lead to significant errors, potentially underestimating the systolic reading.
As the pressure drops, the listener must maintain focus on the sounds transmitted through the stethoscope and simultaneously watch the manometer needle. This coordinated action allows the user to correlate the specific sounds with the pressure readings on the gauge. The physical action of slowly turning the thumbscrew valve while listening intently is the defining skill of the auscultatory method.
Identifying Systolic and Diastolic Pressure
The interpretation of the sounds heard through the stethoscope, known as Korotkoff sounds, directly determines the pressure readings. These sounds are generated by the turbulent flow of blood through the partially compressed artery. Systolic pressure is identified at the moment the first clear, repetitive tapping sounds are heard, which corresponds to Korotkoff Phase I.
This initial tapping indicates that the cuff pressure has dropped just below the peak pressure generated by the heart’s contraction, allowing blood to begin surging past the compression. The pressure reading on the gauge at this exact point is recorded as the systolic blood pressure. Listening carefully for at least two consecutive beats confirms the presence of this initial sound.
As the cuff continues to deflate, the sounds will change in quality, becoming softer and acquiring a swishing quality (Phase II), then becoming crisper and louder again (Phase III). The diastolic pressure is identified when the sounds finally disappear completely, which corresponds to Korotkoff Phase V. This silence occurs because the cuff no longer constricts the artery, allowing smooth, non-turbulent blood flow to resume.
The pressure reading on the manometer at the point of complete sound disappearance is recorded as the diastolic blood pressure. It is important to continue deflating the cuff for another 10 to 20 mmHg after the sounds disappear to confirm that they do not return. Distinguishing the true Korotkoff sounds from artifact noise, such as movement sounds or the sounds of the cuff rubbing, is a skill that develops with practice and ensures an accurate measurement.