Sphygmomanometer

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Introduction

A sphygmanomometer is used to indirectly measure arterial blood pressure. Sphygmanomometry is the process of manually measuring one's blood pressure.

This is the blood pressure cuff that one would see in the Doctor's office, or in a medical clinic/setting. This is seen in the picture on the right.

The sphygmanomometer consists of  :

• An inflatable arm cuff
• A bulb pump for pumping air into the cuff, and a valve for letting air out of the cuff
• A column of mercury, to display the changing pressure
• A pressure meter/dial (manometer), which measures the air pressure, with the help of the rising mercury

In addition to the sphygmanomometer, a stethoscope is also used to listen to the sounds of blood flowing through the brachial artery. See the auscultation page for guidelines on proper placement and usage of the stethoscope.

Definitions

Blood Pressure: Pressure exerted on the walls of blood vessels while blood is moving through the body. This is measured in mmHg and displayed as 'systolic blood pressure/diastole blood pressure' mmHg.

Normal blood pressure values are around 120/80mmHg . If values are below 90/60mmHg, that would be considered low blood pressure (hypotension). Above 140/90 mmHg would be considered high blood pressure, and is classified as hypertension. If it is 180/120mmHg, that is dangerously high and would require medical attention.

Systole: The contraction phase of the cardiac cycle, when the heart fills with blood.

Diastole: The relaxation phase of the cardiac cycle, when the heart empties the blood.

Systolic Pressure: Pressure that is exerted on the arteries as blood is leaving the heart during ventricular systole.

Diastolic Pressure: Pressure that is exerted on the arteries during ventricular relaxation (diastole).

Guidelines

Here are some rules and guidelines for when having your blood pressure measured:

• No food or drink within 30 minutes
• Empty your bladder beforehand
• Ensure that your body is relatively warm (not cold) to avoid inaccurate results
• Ensure that you are relaxed, and not nervous or anxious
• No talking during the procedure
• The cuff is against your bare skin, sleeves loosely rolled up
• Sitting up straight on a chair with your back supported - sit quietly for a few minutes prior to measuring
• Feet flat on the floor in front of you, with your legs uncrossed
• Rest your left forearm with the cuff on a table/surface at chest height, your palm should be facing up - use the left arm since it is closer to the heart, but you can also compare both sides
• Ensure that the cuff fits well around your arm - you don't want it too loose, or too tight that it is uncomfortable and restricting

Procedure

Locate the brachial pulse in the elbow crease (cubital fossa), with the index and middle fingers. If it is not easily located, it ca be found using the head of the stethoscope.

The cuff is wrapped around the middle of the upper arm, so it surrounds the brachial artery. The bottom of the cuff should be 1 inch from the cubital fossa. It should be placed relatively snug, but not too tight to make the person uncomfortable.  

Before inflating the cuff with the bulb pump, ensure that the airflow valve on the bulb pump is closed, by screwing it clockwise.

With the bulb pump, pump air into the cuff until the pressure exceeds arterial pressure by about 30mmHg . Due to this pressure being greater than arterial pressure, the brachial artery is squeezed closed and blood flow is stopped.

When you place the head of the stethoscope over the brachial artery (in the cubital fossa), there would be no sound due to the absence of blood flow.

Gently open the airflow valve. You will notice the pressure in the cuff begins to decline, and will fall below the arterial pressure. There should only be a change of 2-3mmHg per heartbeat when you first open the air valve.

The blood will be able to flow through the artery and will create a turbulent flow. This turbulent flow will create sharp sounds, which are heard through the stethoscope. These sharp sounds are known as Korotkoff sounds.

The pressure at which the first tapping sound (Korotkoff sound) is heard represents systolic blood pressure. This can be read on the manometer/dial. This occurs when the cuff pressure decreases and is unable to keep the brachial artery closed during systole. As the artery is gradually opening and blood flow is gradually increasing, turbulence is reduced and produces sounds.

Once the sounds can be heard, the valve is slowly released to enable the blood flow to increase, and the cuff pressure to decrease. There will be a series of louder, more pronounced sounds, at varying pitches, as the turbulence decreases and blood flow increases.

Diastolic pressure will be reached when the sounds are no longer heard. This occurs because the turbulent flow diminishes.

Once the test is complete, fully release air pressure valve to deflate the cuff before taking it off the patient's arm.

Korotkoff sounds

The Korotkoff sounds are the loud thumping sounds you'll hear through the stethoscope while deflating the sphygmanomometer.

Systolic blood pressure is indicated by the first Korotkoff sound that is heard.

Diastolic blood pressure is indicated after all Korotkoff sounds have been heard through the stethoscope.

Factors affecting arterial Blood Pressure

Factors that can directly affect your true blood pressure reading include:

• Being nervous or anxious, therefore raising one's blood pressure
• Having eaten or drank 30 minutes before the blood pressure reading, which includes alcohol or caffeine, raising the blood pressure
• Having exercised within 30 minutes of the blood pressure reading
• Having smoked within 30 minutes of the blood pressure reading
• Your posture/the way you are sitting - crossing your legs, and letting your arm hang at your side can cause the blood pressure to go up - *see "Guidelines" above for details on posture

Note the definition White Coat Syndrome - this is when someone's blood pressure is consistently/purposely higher in the lab/Doctor's office setting, most likely due to nervousness, anxiety, or similar factors.

Internal factors that can affect your blood pressure include:

• ≥40 years of age
• Overweight or obese
• Family history of heart disease or diabetes
• Birth control pills
• Ethnicity of Black/African American

Risks

If you did not manage to get the measurement while conducting the test, and need to redo it, take off the cuff, and wait a few minutes before starting over, to minimize any discomfort of impeded blood flow to the patient.

There are no risks to having your blood pressure taken with sphygmanomometry. However, there may be some discomfort when the cuff is inflated.

Here is a brief video displaying the steps of measuring blood pressure with a sphygmanomometer  :

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