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5-4. WHAT EQUIPMENT IS NEEDED TO MEASURE A PATIENT'S BLOOD PRESSURE?

 

TECHNOLOGY UPDATE

Digital Blood Pressure Wrist Cuff

Click this image to see the device in greater detail, learn  more about the specifications, and find an opportunity to purchase on for your class or personal use.

You will need a sphygmomanometer (sfig'-mo-mah-NOM-e-ter) and a stethoscope (STETH-ah-skop).

a. Sphygmomanometer. The sphygmomanometer (figure 5-2) is usually called the "blood pressure cuff." There are several different types of blood pressure cuffs in use. Some are made to be attached to a wall (next to a patient's hospital bed, for example), but most are portable. All blood pressure cuff devices work basically in the same way and have the same parts--a bladder, a handbulb with release valve, a tube connecting the handbulb to the bladder, and a gauge (either mercury or aneroid) for measuring pressure.

(1) Bladder. The bladder (also called the "cuff") is a long rubber bag about 6 inches wide and 24 inches long that is covered with fabric. The bladder is wrapped around the patient's arm and filled with air when taking the patient's blood pressure. Parts of the fabric are made of non-slip material, such as Velcro. When the fabric is overlapped, the two pieces of fabric adhere to each other and will not slip when the bladder is inflated. Some sphygmomanometers use snaps or other devices to secure the bladder instead of non-slip fabric.

(2) Handbulb. The handbulb is a device for inflating the bladder. When squeezed, the handbulb forces the air through an opening connected to the tubing. When the bulb is released, it refills with air from the environment. The handbulb is designed so that air from the tubing cannot flow back into the handbulb.

(3) Rubber tubes. One tube connects the bladder and handbulb. Air that is forced out of the handbulb travels through the tube and enters the bladder. A second tube connects the bladder and the gauge..

531_0502.jpg (22072 bytes)

Figure 5-2. Sphygmomanometers.

A Portable mercury sphygmomanometer.
B Portable aneroid sphygmomanometer.



A modern sphygmomanometer

Click  on this image to see the instrument in greater detail, important specifications, and an opportunity to purchase one for class, clinical service, or  personal use.

(4) Release valve. The release valve (screw) is a device for releasing air from the bladder. It is located between the handbulb and the tubing. One hand can operate both the handbulb and the release valve easily. The valve is controlled by a screw. When tightened, no air escapes. When unscrewed fully, the air escapes rapidly. (Note: The screw does not separate from the apparatus. "Unscrewed fully" means the screw is turned so that air will escape as fast as possible.) The screw can also be turned to any position between completely closed and full release. In this way, you can let air escape from the bladder as quickly or as slowly as you wish

(5) Gauge. The gauge measures the air pressure in the bladder. There are two types of gauges--the mercury gauge and the aneroid gauge.

(a) Mercury. The mercury gauge has a column of mercury in a glass tube. The column of mercury measures the air pressure in the bladder. The higher the pressure of air in the bladder, the higher the column of mercury. The height of the column of mercury is determined using a scale to the side of the tube containing the mercury. Usually there is a scale on each side of the glass tube in order to make it easier to read the height of the column. As air is released from the bladder, the air pressure drops and the column of mercury becomes shorter.

(b) Aneroid. The aneroid gauge is circular and has a dial. The greater the air pressure in the bladder, the farther the needle on the dial rotates. A scale on the dial is equivalent to the scale of the mercury gauge. Both scales measure the force of air pressure in the bladder in terms of mm Hg. As the air pressure is released, the needle moves in a counter-clockwise direction. The gauge will normally be designed so that it can be attached to the bladder. This frees the person taking the blood pressure from having to hold the gauge in one of his hands.

b. Stethoscope. The stethoscope is an instrument used for listening to sounds produced within the body. A stethoscope consists of a diaphragm, metal and rubber tubing, and earpieces (figure 5-3).

(1) Diaphragm. The diaphragm is normally a flat metal disk that is placed on the body area being examined. The diaphragm will pick up sounds produced within the body such as the heartbeat and breathing sounds. Sometimes a bell-shaped listening device is used instead of a flat disk. Some stethoscopes have combination (both flat disk and bell) listening devices.

(2) Tubing. The hollow and metal tubes transmit the sounds from the diaphragm to the earpieces. The rubber tubing provides flexibility.

531_0503.jpg (21190 bytes)

Figure 5-3. Stethoscope.

(3) Earpieces. The earpieces are twisted metal tubes with plastic ends. The plastic pieces protect the ears from the metal. The twisting helps to improve the quality of sound heard through the stethoscope. The stethoscope should be worn with the earpieces forward (figure 5-4) to help prevent the sounds picked up by the diaphragm from being distorted.

531_0504.jpg (8013 bytes)

Figure 5-4. Earpiece of stethoscope in place for use.

 

5-5. HOW DOES THE AIR PRESSURE INSIDE THE BLADDER PROVIDE INFORMATION ABOUT THE BLOOD PRESSURE?

Paragraph 5-1 stated that blood pressure is the force with which the blood pushes against the walls of the blood vessel. However, paragraph 5-3a(5) states that the gauge on the sphygmomanometer measures the air pressure inside the bladder!

a. Indirect Measurement. Some things cannot be measured directly without difficulty. For example, the height of a building can be measured by climbing to the top of the building, holding on to one end of a very long tape measure, and dropping the other end to a friend on the ground who reads off the height. This method may work for a building that is not very high, but is not recommended for determining the height of the Empire State Building. The height of a building, however, can be determined indirectly, such as by measuring its shadow. (Method: Put a stick in the ground so that it is straight up and down. Measure the height of the stick, the length of the stick's shadow, and the length of the building's shadow. The height of the building is equal to the length of the building's shadow times the height of the stick divided by the length of the stick's shadow.)

b. Blood Pressure Measurement. Just as the height of the building was determined by measuring something else (its shadow), the pressure of the blood at its highest (systolic) and normal (diastolic) levels can be determined by measuring the air pressure in the bladder.

(1) When the bladder is first placed around the arm and not inflated, the artery beneath the bladder functions normally (figure 5-5 A ).

(2) When the bladder is inflated, the bladder squeezes the arm. If the bladder is inflated to a pressure greater than the systolic pressure of the artery, the artery beneath the bladder will collapse (figure 5-5 B ). The artery will remain collapsed, thus shutting off blood flow below the bladder, even when there is a heartbeat. Thus, when there is no blood flow in the artery below the bladder, you know that the air pressure in the bladder is greater than the systolic blood pressure.

(3) When the bladder is inflated to a pressure that is less than the systolic pressure but greater than the diastolic pressure, blood will flow beneath the bladder only when the (blood) pressure within the artery is greater than the (air) pressure within the bladder. This occurs when the force of the heartbeat increases the pressure within the artery. Once the additional force of the heartbeat has passed (the artery returns to diastolic pressure), the artery will collapse again (figure 5-5 C ). Thus, when blood suddenly passes through the artery beneath the bladder, stops, starts again, and stops again, you know that the pressure within the bladder is less than the systolic pressure but more than the diastolic pressure.

(4) If the bladder is inflated, but the pressure within the bladder is less than the lowest level of pressure within the artery (diastolic pressure), then the bladder cannot collapse the artery. The pressure of the bladder may interfere somewhat with the blood flow, but it cannot stop the blood flow (figure 5-5 D ). Thus, when the blood continues to flow through the artery beneath the bladder without stopping, you know that the pressure within the bladder is less than the lowest (diastolic) pressure of the blood within the artery.

531_0505.jpg (79299 bytes)
A Artery without the bladder--artery expands during heartbeat, returns to normal.
B Air pressure in the bladder is greater than the systolic pressure--the artery stays collapsed.
C Air pressure in bladder is between systolic pressure and diastolic pressure--artery collapsed except during heartbeat.
D Air pressure in bladder is less than diastolic pressure--artery does not collapse.

 
Figure 5-5. Effects of an inflated bladder on an artery.

David L. Heiserman, Editor

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Revised: June 06, 2015