Blood pressure is the force exerted by circulating blood against the walls of the arteries. It is commonly expressed as two numbers: systolic pressure, the maximum pressure during a heartbeat, and diastolic pressure, the minimum pressure between heartbeats. While many people use convenient automatic devices, the manual method, known as auscultation, relies on a stethoscope and a pressure cuff. This acoustic technique, involving listening for specific sounds, is often considered the reference standard for non-invasive measurement in a clinical setting.
Required Equipment and Setup
The auscultatory method requires three items: an inflatable cuff, a pressure gauge, and a stethoscope. The cuff contains a bladder and is wrapped around the arm to temporarily stop blood flow in the brachial artery. The pressure gauge, or manometer, measures the pressure within the cuff and can be either an aneroid (dial) or a traditional mercury column.
Choosing the correct cuff size is important, as an incorrect size can artificially raise or lower the reading. The patient should be seated comfortably with their back and feet supported, and their bare arm supported at the level of the heart. Allowing the patient to rest for at least five minutes and ensuring they have not ingested caffeine or nicotine for 30 minutes contributes to an accurate baseline measurement.
Performing the Measurement: The Step-by-Step Technique
The first step is correctly applying the cuff, ensuring the bladder is centered over the brachial artery on the inside of the upper arm. The bottom edge of the cuff should be positioned roughly 2 to 3 centimeters above the crease of the elbow (antecubital fossa). Next, the pulse should be palpated at the wrist or elbow, and the cuff is inflated rapidly until the pulse is no longer felt.
Once the pulse disappears, the cuff is inflated an additional 20 to 30 mmHg above that point to ensure complete occlusion of the artery. This step helps avoid the auscultatory gap, a common measurement error. The stethoscope’s diaphragm is then placed gently over the brachial artery at the elbow crease.
The air is released slowly and deliberately, at a controlled rate of 2 to 3 mmHg per second or per heartbeat, which is necessary for accurately identifying the subtle sounds. Releasing the pressure too quickly can lead to inaccurate systolic and diastolic readings. As the pressure drops, the listener focuses for the return of blood flow sounds through the stethoscope.
Decoding the Sounds: Identifying Systolic and Diastolic Pressure
The sounds heard during the measurement are called Korotkoff sounds, generated by the turbulent flow of blood as the artery opens and closes under the falling cuff pressure. The first distinct, faint, repetitive tapping sound heard is Korotkoff Phase I, and the pressure reading at this moment is recorded as the systolic blood pressure. This sound indicates the maximum pressure when the heart contracts.
As the cuff pressure continues to drop, the sounds progress through several phases, becoming swishing, then louder, and finally muffled. The final pressure noted is when the sounds completely disappear (Korotkoff Phase V), representing the diastolic blood pressure. The disappearance signifies that the cuff pressure has fallen below the pressure in the artery when the heart is at rest.
Ensuring a Reliable Reading
Accuracy relies heavily on proper technique and minimizing external factors. Estimating the systolic pressure by palpation before auscultation is a necessary precaution to avoid underestimating the reading due to the auscultatory gap.
Patient activities, such as talking, moving, or having an unsupported arm, can introduce significant errors in the reading. Ensure the patient remains still and quiet throughout the deflation process. To account for natural variations, two separate measurements should be taken, spaced by one to two minutes, and the average of the two recorded.