Auscultated blood pressure measurement is a manual clinical procedure for assessing circulatory health by listening to specific sounds within an artery. This measurement provides a snapshot of how hard the heart is working to pump blood, and understanding these forces offers insights into cardiovascular function. The method’s accuracy is directly tied to the skill of the person performing it, making proper training important.
The Tools for Measurement
The primary instrument for this procedure is the sphygmomanometer, which consists of an inflatable cuff, a pressure-inflating bulb with a release valve, and a gauge to read the pressure. The cuff itself is a fabric wrap containing an inflatable rubber bladder. The size of this cuff is an important factor in obtaining an accurate reading and must be appropriate for the patient’s arm circumference.
A cuff that is too small for the patient’s arm will compress the artery unevenly, leading to a falsely high blood pressure reading. Conversely, a cuff that is too large will not provide adequate compression, resulting in a reading that is artificially low. Alongside the sphygmomanometer, a stethoscope is used to amplify the sounds of blood flow within the artery. The stethoscope’s diaphragm is used as it covers a wider area, making it easier to detect the necessary sounds.
Understanding the Sounds and Readings
When the blood pressure cuff is inflated, it squeezes the brachial artery in the upper arm to temporarily stop blood flow. As the cuff pressure is slowly released, blood begins to move turbulently through the constricted artery. This turbulent flow creates vibrations in the arterial wall, known as Korotkoff sounds, which are audible through a stethoscope.
Korotkoff sounds determine the two main figures in a blood pressure reading. Systolic pressure represents the maximum pressure on artery walls when the heart contracts. This is recorded at the moment the first faint, tapping sound is heard as the cuff deflates.
As cuff pressure decreases, it eventually falls below the artery’s resting pressure. At this point, the blood flow becomes smooth and silent, a state known as laminar flow. The moment the Korotkoff sounds disappear marks the diastolic pressure, which is the minimum pressure in the arteries between heartbeats.
The Measurement Process
The individual should rest comfortably in a seated position for at least five minutes, with their back supported and feet flat on the floor. The arm used for measurement should be bare, supported, and positioned so the cuff is at heart level. Speaking or moving during the measurement should be avoided as it can alter the results.
Once the patient is positioned, the cuff is applied snugly to the upper arm, about an inch above the elbow’s crease. The clinician locates the brachial artery by feeling for the pulse in the crook of the elbow and places the stethoscope directly over this spot. The cuff is then inflated rapidly to a pressure approximately 30 mmHg higher than the point where the pulse can no longer be felt at the wrist. This ensures the artery is fully occluded.
The air is then let out of the cuff at a controlled rate of 2 to 3 mmHg per second. While listening through the stethoscope and watching the pressure gauge, the clinician notes the pressure at which the first tapping sound appears (systolic) and the pressure at which the sounds vanish entirely (diastolic). After the final sound is heard, the cuff should be fully deflated.
Ensuring Accuracy and Its Continued Importance
Human error is a variable in the auscultatory method. Several common mistakes can lead to inaccurate readings, including:
- Deflating the cuff too quickly, which can lead to missing the true first or last sounds.
- Using an incorrectly sized cuff.
- Placing the stethoscope with too much pressure, which can distort the artery.
- Patient conditions, such as having a full bladder or recently smoking.
The auscultatory gap can also complicate readings. This is a period where Korotkoff sounds temporarily disappear between the systolic and diastolic pressures before reappearing. If an operator does not inflate the cuff high enough, they might mistake this reappearance for the true systolic pressure, leading to a significant underestimation. Awareness of this gap is important for accuracy.
Despite the prevalence of automated monitors, the manual auscultatory method remains a benchmark for measurement. Automated devices can be less reliable in patients with very high or low blood pressure or irregular heart rhythms. Clinicians often use the auscultatory method to verify questionable readings from an automated device, making skill in this technique a part of modern patient care.