Auscultation involves using a stethoscope to hear internal body sounds. For blood pressure measurement, it refers to listening to the specific sounds generated in the brachial artery as a cuff is inflated and then slowly deflated. This method requires a sphygmomanometer and a stethoscope, and it is considered the gold standard technique for non-invasive blood pressure measurement in a clinical setting.
The auscultatory method relies on a human observer detecting the appearance and disappearance of specific arterial noises known as Korotkoff sounds. While automated oscillometric devices estimate blood pressure using algorithms based on pressure oscillations, auscultation directly correlates audible physiological events to pressure readings. This direct connection to hemodynamics is why the method remains the benchmark against which other non-invasive devices are validated.
Essential Equipment and Patient Positioning
The manual sphygmomanometer consists of three primary parts: a compression cuff, an inflation bulb with a controlled exhaust valve, and a manometer. The manometer displays the pressure in millimeters of mercury (mmHg) and can be an aneroid dial or a digital display. Aneroid manometers require regular calibration to ensure accuracy. The stethoscope is used to hear the arterial sounds; the diaphragm is commonly used in practice as it is often easier to secure.
Selecting the correct cuff size is essential, as an improperly sized bladder frequently causes error. The rubber bladder inside the cuff should cover approximately 80% of the arm’s circumference and be roughly 40% of the arm’s circumference in width. If the cuff is too small, the pressure reading will be falsely high, and if it is too large, the reading will be falsely low.
Before measurement, the patient must be seated comfortably with their back supported and their feet flat on the floor. The arm being used should be bare, supported at heart level, and free of restrictive clothing. A rest period of three to five minutes prior to the measurement is required to allow the patient’s blood pressure to stabilize.
The Step-by-Step Auscultation Procedure
The procedure begins by palpating the brachial artery, typically found on the inner aspect of the upper arm near the elbow crease. Once the maximal pulse location is determined, the cuff is applied smoothly around the upper arm, ensuring the center of the bladder is directly over the artery. The lower edge of the cuff should be positioned about one inch (two to three centimeters) above the antecubital crease.
To avoid the auscultatory gap, the estimated systolic pressure must first be determined by palpation. The cuff is inflated rapidly until the radial pulse is no longer palpable, and this pressure is noted. The cuff is then inflated an additional 20 to 30 mmHg above this point to ensure complete arterial occlusion.
With the artery fully compressed, the stethoscope is placed lightly over the brachial artery pulsation point, just below the edge of the cuff. Applying heavy pressure can distort the artery and cause sounds to be heard below the true diastolic pressure. The exhaust valve is opened to allow the cuff pressure to drop at a controlled rate of two to three mmHg per second.
The first clear, repetitive tapping sound heard marks the systolic blood pressure. The observer continues to listen as the pressure falls, noting the pressure at which the sounds completely disappear, which is recorded as the diastolic blood pressure. This controlled deflation rate is necessary because deflating too quickly can underestimate the true pressure.
Understanding Korotkoff Sounds
The sounds heard during the auscultation process are termed Korotkoff sounds, named after the Russian physician Nikolai Korotkoff. These distinctive sounds are produced by the turbulent flow of blood through the brachial artery as the cuff pressure is gradually released. When the cuff is inflated above systolic pressure, the artery is fully collapsed, and no sound is detectable.
The first phase, K1, begins when the cuff pressure drops just below the systolic pressure, creating clear, repetitive tapping sounds that define the systolic reading. As the pressure falls, the sounds transition through K2 (swishing) and K3 (crisper and louder). K4 is marked by an abrupt muffling or softening of the sound. Finally, the complete disappearance of all sound marks K5, the point where the artery is fully open and blood flow is laminar. For adults, K1 is the systolic pressure, and K5 is the definitive diastolic pressure.
Common Sources of Measurement Error
The auscultatory method is susceptible to several sources of human and technical error. Using a cuff that is too small is a frequent technical error, leading to an inaccurately high reading. Conversely, placing the cuff over clothing or failing to position the arm at heart level can introduce errors, often resulting in artificially elevated readings if the arm is low.
Procedural mistakes, such as deflating the cuff too quickly or applying excessive pressure with the stethoscope, can make identifying the Korotkoff sounds difficult. Patient factors also contribute to variability; talking, moving, or having a full bladder can temporarily raise the pressure. The auscultatory gap, where Korotkoff sounds temporarily disappear, can lead to underestimation of the true systolic value if not anticipated by palpation.