7.5 Blood Pressure Measurement

Understanding the Two Numbers

Blood pressure (BP) is the force of blood pushing on artery walls, written as systolic over diastolic in millimeters of mercury (mmHg).

A reading of 120/80 is spoken "one-twenty over eighty."

Adult Classifications (ACC/AHA)

The diagnostic threshold for hypertension is 130/80 mmHg, unchanged in the 2025 ACC/AHA guideline, which keeps the same categories below.

Equipment and Cuff Size

The sphygmomanometer has a cuff (with an inner bladder), a gauge, and a bulb-and-valve. The stethoscope's diaphragm sits over the brachial artery.

Critical rule: wrong cuff size means a wrong reading. A cuff too small reads falsely HIGH (it cannot compress the artery fully); a cuff too large reads falsely LOW.

Manual Technique, Korotkoff Sounds, and Errors

Step-by-Step Manual BP

1. Prepare: rest the patient 5 minutes, seated, feet flat, arm supported at heart level, sleeve not rolled into a tight band.
2. Place the cuff: center the bladder over the brachial artery, lower edge 1 inch above the elbow crease, snug enough that only two fingers slide under.
3. Find the brachial pulse in the antecubital space and rest the stethoscope diaphragm there.
4. Inflate with the valve closed to about 30 mmHg above the expected systolic (commonly 160-180).
5. Deflate slowly, releasing 2-4 mmHg per second, and listen.
6. Record both numbers, the arm used, and the patient's position.

Korotkoff Sounds

The first tap is systolic; the silence is diastolic.

Common Errors and Their Effect

Restricted Arms and Special Situations

Never take BP on an arm with an IV line, a dialysis access (AV fistula or shunt), a recent fracture or injury, or on the side of a mastectomy. Document which arm you used and stay consistent. Orthostatic (postural) BP is measured lying, sitting, then standing to detect drops that cause dizziness; a fall of more than 20 mmHg systolic on standing is reported.

When to Report

Report immediately if systolic is above 160 or below 90, diastolic is above 100 or below 60, the reading changed sharply from baseline, or the patient reports dizziness, a pounding headache, or chest pain.

Electronic Cuffs, the Two-Step Method, and Worked Scenarios

Electronic vs. Manual

Many facilities use an electronic (automatic) BP machine that inflates and reads on its own. Some states let candidates test on the electronic skill, but the classic NNAAP version uses the manual sphygmomanometer and stethoscope, so master listening for Korotkoff sounds. Even with an electronic machine the fundamentals do not change: correct cuff size, arm at heart level, a rested patient, and the same arm each time. If an electronic reading looks wildly off, the trained response is to repeat it manually rather than chart a number you do not trust.

The Two-Step (Palpated) Method

To avoid the auscultatory gap, a window where Korotkoff sounds briefly vanish, experienced staff first palpate the radial pulse while inflating: the pressure at which the pulse disappears estimates systolic. They then inflate 30 mmHg above that point before listening. This keeps you from under-inflating and recording a falsely low systolic, and it is why the procedure says inflate to about 30 above the expected systolic rather than a fixed number for everyone.

Worked Scenario

A CNA gets 168/96 on a resident who is normally 124/78. Before reporting an alarming spike, check the controllable variables: was the resident rested 5 minutes, was the arm supported at heart level, was the cuff the right size and snug, and was talking avoided during the reading? Suppose the resident had just hurried back from the dining room and the arm was hanging at the side. The CNA reseats the resident, supports the arm at heart level, waits 5 minutes, and re-measures, getting 130/82. The correct documentation notes both, and the nurse is informed of the rested value.

Orthostatic Readings and Safety

For orthostatic (postural) BP, measure lying, then sitting, then standing, waiting 1 to 3 minutes between positions, and stay with the patient because a large drop can cause fainting. A systolic fall greater than 20 mmHg or a diastolic fall greater than 10 mmHg on standing, especially with dizziness, is reported, and the patient is kept safe from a fall during the test.