Blood pressure (BP) is the force exerted by circulating blood against the walls of the arteries. It is always reported as two numbers representing two different phases of the heart’s cycle. Systolic pressure is the maximum pressure recorded when the heart contracts. Diastolic pressure is the minimum pressure recorded when the heart is at rest between beats. The manual method, using a stethoscope and a cuff, remains a reliable standard for obtaining these values.
Preparing the Patient and Equipment
Accurate measurement requires careful preparation of the patient and equipment. The patient should sit with their back and feet supported, legs uncrossed, and rest quietly for at least five minutes prior to the measurement. The arm used for the reading must be bare, extended, and fully supported at the level of the heart, as an arm hanging lower can artificially elevate the reading. Selecting the correct cuff size is important; a cuff that is too small results in a falsely high reading, while a cuff that is too large produces a falsely low one. The nurse locates the brachial artery in the antecubital fossa (bend of the elbow), where the stethoscope will be placed.
The Physics of Cuff Inflation and Deflation
The process relies on a sphygmomanometer, which consists of an inflatable cuff and a manometer (pressure gauge). The nurse uses a bulb to inflate the cuff to a pressure high enough to completely stop blood flow through the brachial artery. This occlusion pressure is typically inflated to about 20 to 30 millimeters of mercury (mmHg) above the estimated systolic pressure. At this point, the artery is fully collapsed, and no sound can be heard through the stethoscope.
The cuff is then slowly deflated at a steady, controlled rate of approximately 2 to 3 mmHg per second. As the external pressure drops, the arterial wall begins to open and close with each heartbeat, creating turbulent blood flow that generates audible vibrations.
Identifying Systolic and Diastolic Pressure Sounds
The sounds heard during controlled deflation are called Korotkoff sounds (K-sounds), caused by turbulent blood flow through the compressed artery. The first distinct tapping sound heard is Korotkoff Phase 1 (K1), which marks the systolic pressure. This sound signifies the moment the heart’s contraction pressure overcomes the external cuff pressure, allowing the first spurt of blood to pass. As the cuff pressure continues to decrease, the sounds progress through three more phases, becoming softer, swishing, and then crisper.
Korotkoff Phase 4 (K4) is characterized by a sudden muffling of the sounds, but this phase is only used to determine diastolic pressure in specific populations, such as children or pregnant patients. For adults, the diastolic pressure is defined by Korotkoff Phase 5 (K5), the point where all sounds completely disappear. The disappearance of sound indicates that the cuff pressure has dropped low enough for the artery to be fully open, and blood flow has returned to a smooth, silent pattern. The nurse records the pressure reading on the manometer at the appearance of the first sound (K1) and the point of complete disappearance (K5) to obtain the systolic and diastolic values.
Common Sources of Measurement Variability
Several factors can introduce variability, leading to inaccurate or inconsistent blood pressure readings. A common technical error is deflating the cuff too quickly, which can cause the nurse to miss the precise pressure point of the first sound (K1) or the final disappearance (K5). Another phenomenon is the “auscultatory gap,” a silent interval that sometimes occurs between the first and second Korotkoff phases, which can cause the systolic pressure to be underestimated if the nurse does not first estimate the pressure by palpating the radial pulse. Patient anxiety, often called “white coat syndrome,” can temporarily elevate blood pressure readings due to the clinical setting. If the patient’s arm is not supported at the level of the heart, the hydrostatic effect of gravity can artificially raise the reading by several millimeters of mercury.