What is blood pressure and why is it measured as a vital sign?

Blood pressure describes the lateral pressures produced by blood upon vessel walls. It is a vital sign obtained routinely in hospital and outpatient settings and is one of the most common medical assessments performed worldwide. Blood pressure measurement helps clinicians evaluate cardiovascular function and detect potential health issues.

What are the two main methods for measuring blood pressure?

Blood pressure can be determined directly using an intra-arterial catheter or indirectly through non-invasive methods. Indirect measurement is most commonly used because it is safe, easily reproducible, and can be performed automatically by oscillometry or manually by auscultation utilizing a sphygmomanometer with an inflatable cuff and manometer.

How does the auscultatory method work for blood pressure measurement?

The examiner places a stethoscope over the brachial artery, inflates the cuff above expected systolic pressure, then auscultates while deflating. Korotkoff sounds—audible turbulent blood flow in the partially squeezed artery—indicate systolic pressure at first sound and diastolic pressure when sounds cease. The auscultatory method relies on listening for these characteristic sounds to determine both pressure values.

What do Korotkoff sounds represent during blood pressure measurement?

Korotkoff sounds are audible turbulent blood flows produced when the cuff pressure falls below arterial pressure, partially squeezing the artery. The first audible sound signifies maximum arterial pressure during systole, while the point where sounds cease indicates minimal arterial pressure during diastole, representing systolic and diastolic blood pressure respectively.

Why is pulse obliteration performed before auscultation?

Determination of the pulse-obliterating pressure by palpation is performed prior to auscultation to give a rough estimate of the target systolic pressure. This preliminary step helps the examiner know approximately how high to inflate the cuff, ensuring accurate measurement and efficient technique during the subsequent auscultatory phase.

How is blood pressure recorded and why is repeated measurement important?

Blood pressure is measured in millimeters of mercury (mmHg) and recorded as a fraction with systolic pressure over diastolic pressure. Repeated measurement is especially important given potential measurement errors and blood pressure variations. The physician may repeat vital signs after the patient interview to ensure accuracy and account for natural fluctuations.

Who typically measures blood pressure in clinical settings?

In most instances, vital signs including blood pressure are initially measured by a health care assistant or registered nurse (RN). The physician may choose to repeat the vital signs and blood pressure measurement after completion of the patient interview to verify findings and ensure clinical accuracy.

Blood Pressure Measurement

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1. Preparation

Assess for any contraindications to BP measurement in the upper arm including arteriovenous fistula, history of axillary lymph node dissection, or evident lymphedema.
Make sure the patient has changed into a gown and rested for at least 5 min prior to obtaining blood pressure and other vital signs.
Ask the patient to sit comfortably with their feet uncrossed and resting on the floor.
Have the stethoscope and sphygmomanometer ready.
Confirm proper sizing of the BP cuff (when wrapped around the limb the index line on the cuff should fall within the marked arm circumference range limits). A small cuff may falsely elevate the readings and potentially lead to misdiagnosis.

2. Determination of pulse-obliterating pressure by palpation

Obtaining the pulse-obliterating pressure prior to measurement of blood pressure by auscultation allows avoiding measurement error due to the auscultatory gap. An auscultatory gap is an intermittent disappearance of Korotkoff sounds after their initial appearance before the true diastole, which may seriously underestimate the systolic pressure or overestimate the diastolic pressure.

Place the cuff on the patient's arm about 2.5 cm above the antecubital fossa.
Make sure the patient's arm is free of clothing and resting at their side with the brachial artery at the level of the heart.
Identify the radial pulse with your index and middle fingers.
Close the valve on the pressure bulb (by turning it clockwise with your thumb) and inflate the cuff by squeezing the pressure bulb rapidly.
Inflate until the radial pulse cannot be felt anymore and note the measurement on the manometer.
Continue to inflate until the pressure increases by an additional 30 mmHg. This is done to avoid over-inflation of the cuff on subsequent readings.
Open the valve slowly by rotating it counterclockwise with your thumb.
Deflate the cuff at 2 mmHg/sec until the radial pulse returns.
Record the manometer reading when the radial pulse reappears (obliterating pressure) on the vital signs flow sheet.

3. Obtaining blood pressure with auscultation

Place the stethoscope over the brachial artery (medial aspect of antecubital fossa).
Inflate the cuff again at a level of 30 mmHg above pulse-obliterating pressure and make sure no sounds are present.
Slowly deflate the cuff at a rate of 2 mmHg/sec.
Note the value on the manometer when the Korotkoff sound, indicated by the first two consecutive beats, can be heard. The manometer reading at that moment corresponds to the systolic blood pressure.
Continue slowly deflating the cuff while listening for the sounds to completely disappear, which signifies the diastolic blood pressure.
Make sure to deflate the cuff entirely so as not to miss the diastolic pressure.
Record the systolic and diastolic blood pressure measurements on the vital signs sheet.
Repeat the process in both arms (unless contraindicated).

4. Testing for pulsus paradoxus

Normally, the systolic blood pressure is lower on inspiration due to decreased intrathoracic pressure. An abnormally large fall (more than 10 mmHg) in systolic blood pressure on inspiration is defined as pulsus paradoxus and is most commonly associated with cardiac tamponade or severe chronic obstructive pulmonary disease .

Inflate the cuff to 30 mmHg higher than the systolic pressure determined during blood pressure measurement.
Deflate at 2 mmHg/sec until the first Korotkoff sound is audible on expiration (sound should be intermittent rather than every heartbeat, corresponding to higher blood pressure on expiration). Note the measurement.
Continue to deflate the cuff at 2 mmHg/sec until the Korotkoff sounds are audible on both expiration and inspiration (every heartbeat). Lower blood pressure on inspiration is due to a decrease in intrathoracic pressure.
Calculate the difference between systolic blood pressure on expiration and inspiration.

5. Orthostatic or Postural blood pressure measuring

An orthostatic hypotension is an abnormal decrease in systolic blood pressure of 20 mmHg or a decrease in diastolic blood pressure of 10 mmHg within 3 min of standing compared with blood pressure in supine or sitting position. This can result from compromised venous return and subsequent decrease in cardiac output. Orthostatic hypotension can happen transiently in people of all ages, but occurs most commonly in elderly patients. Some potential causes include blood loss, medications, and disease of the autonomic nervous system.

Place the patient in a supine position. Wait for a minimum of 5 min before obtaining the reading.
Obtain a blood pressure measurement as described.
Record the measurement on the vital signs sheet. Make sure to note the position of the patient.
Have the patient stand and repeat the BP measurement after 3 min of standing.
Calculate the difference in pressures. If there is a decrease of 20 mmHg or greater in the systolic pressure, or 10 mmHg or greater in the diastolic pressure, the patient has orthostatic hypotension.