How To Take Blood Pressure with a Sphygmomanometer

What is a Sphygmomanometer

A sphygmomanometer is a device that measures blood pressure. It is composes of an inflatable rubber cuff, which is wrapped around the arm. A measuring device indicates the cuff's pressure. A bulb inflates the cuff and a valve releases pressure. A stethoscope is used to listen to arterial blood flow sounds.

As the heart beats, blood forced through the arteries cause a rise in pressure, called systolic pressure, followed by a decrease in pressure as the heart's ventricles prepare for another beat. This low pressure is called the diastolic pressure.

The sphygmomanometer cuff is inflated to well above expected systolic pressure. As the valve is opened, cuff pressure (slowly) decreases. When the cuff's pressure equals the arterial systolic pressure, blood begins to flow past the cuff, creating blood flow turbulence and audible sounds. Using a stethoscope, these sounds are heard and the cuff's pressure is recorded. The blood flow sounds will continue until the cuff's pressure falls below the arterial diastolic pressure. The pressure when the blood flow sounds stop indicates the diastolic pressure.

Systolic and diastolic pressures are commonly stated as systolic 'over' diastolic. For example, 120 over 80. Blood flow sounds are called Korotkoff sounds.

Try it! Practice Taking Blood Pressure

Definitions

There are three types of sphygmomanometers. Digital sphygmomanometers are automated, providing blood pressure reading without needing someone to operate the cuff or listen to the blood flow sounds. However, digital types are less accurate. Some healthcare providers use digital for screening but use manual sphygmomanometers to validate readings in some situations.

Manual sphygmomanometers consist of aneroid (dial) and mercury (column) devices. Operating these aneroid and mercury sphygmomanometers is nearly the same, except that aneroid devices require periodic calibration.

How To Measure Blood Pressure with a Sphygmomanometer

Procedures

• To begin blood pressure measurement, use a blood pressure cuff of the correct size. The length of the cuff's bladder should be at least equal to 80% of the circumference of the upper arm.
• Wrap the cuff around the upper arm with the cuff's lower edge one inch above the antecubital fossa.
• Lightly press the stethoscope's bell over the brachial artery just below the cuff's edge. Some health care workers have difficulty using the bell in the antecubital fossa, so we suggest using the bell or the diaphragm to measure the blood pressure.
• Rapidly inflate the cuff to 180mmHg. Release air from the cuff at a moderate rate (3mm/sec).
• Listen with the stethoscope and simultaneously observe the dial or mercury gauge. The first knocking sound, is the subject's systolic pressure. When the knocking sound disappears, that is the diastolic pressure (such as 120/80).
• Record the pressure in both arms and note the difference; also record the subject's position (supine), which arm was used, and the cuff size (small, standard or large adult cuff).
• If the subject's pressure is elevated, measure blood pressure two additional times, waiting a few minutes between measurements.
• A BLOOD PRESSURE OF 180/120mmHg OR MORE REQUIRES IMMEDIATE ATTENTION!

Precautions

• Aneroid and digital manometers may require periodic calibration.
• Use a larger cuff on obese or heavily muscled subjects.
• Use a smaller cuff for pediatric patients.
• For pediatric patients a lower blood pressure may indicate the presence of hypertension.
• Don't place the cuff over clothing.
• Flex and support the subject's arm.
• In some patients the Korotkoff sounds disappear as the systolic pressure is bled down. After an interval, the Korotkoff sounds reappear. This interval is referred to as the "auscultatory gap." This pathophysiologic occurrence can lead to a marked under-estimation of systolic pressure if the cuff pressure is not elevated enough. It is for this reason that the rapid inflation of the blood pressure cuff to 180mmHg was recommended above. The "auscultatory gap" is felt to be associated with carotid atherosclerosis and a decrease in arterial compliance in patients with increased blood pressure.