The manual reading of blood pressure, known as the auscultatory method, remains a highly accurate procedure for assessing cardiovascular health. This technique requires two fundamental tools: a sphygmomanometer, which includes the inflatable cuff and the pressure gauge, and a stethoscope. The process relies on detecting Korotkoff sounds, which are generated by the turbulent flow of blood within the brachial artery. By correlating the first appearance and final disappearance of these sounds with the pressure indicated on the gauge, the observer determines the systolic and diastolic pressures.
Understanding the Sphygmomanometer Scale
The pressure gauge, or manometer, displays the measurement in millimeters of mercury (mm Hg), which is the standard unit for blood pressure. The face of the gauge is marked with larger lines, typically representing increments of 10 or 20 mm Hg, and these are usually numbered. Between these major markings, smaller lines represent a 2 mm Hg change in pressure. This two-millimeter interval is the smallest unit of measurement, meaning that all manual blood pressure recordings will be even numbers. The needle’s position must be noted exactly at the moment the acoustic changes occur. The pressure inside the cuff is controlled by a valve, and the needle falls as the air is slowly released.
The Reading Process: Linking Sound to Scale
The manual reading begins after the cuff has been inflated to a pressure high enough to completely stop blood flow in the brachial artery, usually around 30 mm Hg above the estimated systolic pressure. The stethoscope is placed over the artery, and the observer slowly opens the control valve to allow the pressure to drop at a controlled rate. The recommended deflation speed is approximately 2 to 3 mm Hg per second.
The systolic pressure is identified by the first clear, repetitive tapping sound heard (Korotkoff Phase 1 or K1). This sound marks the point where the arterial pressure overcomes the cuff pressure during the heart’s contraction phase. The observer must simultaneously note the precise number on the gauge as the sound is heard.
As the cuff pressure continues to fall, the sounds change in quality, eventually becoming muffled (K4) and then completely disappear (K5) as the artery fully reopens, marking the diastolic pressure. The diastolic reading is the exact gauge number where the last audible sound is heard before silence begins.
A temporary silence known as the auscultatory gap can occur between the systolic and diastolic sounds, potentially leading to an underestimation of the true systolic pressure. To avoid this error, the initial inflation must be high enough to pass through this gap. Once the diastolic pressure is recorded, the cuff should be rapidly and fully deflated to prevent discomfort and venous congestion in the arm.
Recording and Classifying the Results
Blood pressure measurements are always recorded in the format of systolic over diastolic pressure, such as 120/80 mm Hg, with the unit of measure (mm Hg) included. The recorded numbers are then referenced against established health guidelines to provide context for the reading. Guidelines from organizations like the American Heart Association (AHA) and American College of Cardiology (ACC) define several categories for adult blood pressure.
Blood Pressure Categories
- Normal: Systolic pressure less than 120 mm Hg and diastolic pressure less than 80 mm Hg.
- Elevated: Systolic reading between 120 and 129 mm Hg and diastolic pressure less than 80 mm Hg.
- Stage 1 Hypertension: Systolic pressure is 130 to 139 mm Hg or the diastolic pressure is 80 to 89 mm Hg.
- Stage 2 Hypertension: Systolic pressure is 140 mm Hg or higher, or a diastolic pressure is 90 mm Hg or higher.
These classification categories serve as a framework for understanding the measurement relative to general health standards. Accurate recording and classification are necessary for healthcare providers to track trends and make informed decisions.
Ensuring Accuracy: Minimizing Reading Errors
Maintaining an ideal cuff deflation rate of 2 to 3 mm Hg per second is a strict requirement for accuracy; deflating the cuff too quickly can cause the observer to miss the exact K1 and K5 points. Another common mistake is called parallax error, which occurs when the observer reads the gauge from an angle, making the needle appear to be on a different line than its true position. To prevent this, the gauge must be viewed straight-on, with the eye level with the needle. It is also important to avoid rounding the measurement to the nearest 5 or 10 mm Hg, as the reading must be noted to the nearest 2 mm Hg line on the scale. Environmental noise can easily obscure the faint Korotkoff sounds, so the procedure should be performed in a quiet room to ensure the K1 and K5 points are not missed. The patient’s physical state, such as having a full bladder or talking during the measurement, can also introduce temporary increases in pressure that lead to an artificially high reading.