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Understanding How Blood Pressure Is Taken Note: Do not use the sphygmomanometer until you have completed this section. It is very important to understand what happens when you use a sphygmomanometer. A sphygmomanometer allows you to detect the pressure in the artery when the heart is contracting the systolic pressure) and when the heart is resting (the diastolic pressure). Before the cuff is inflated, no sounds can be heard in the artery, because the blood is flowing smoothly through the vessel. When the cuff is inflated it puts pressure on the artery and constricts it until no blood can flow through. If you were to listen for a pulse using a stethoscope at this point, there would be no sound in the artery below the cuff level, since no blood is flowing. As you release the pressure (by slowly turning the release valve), the artery begins to expand each time the heart beats. At some point (around 120 mm Hg) the blood vessel is fully open when the heart beats but collapsed when the heart is at rest. When the blood pressure at contraction (systole) is stronger than the cuff pressure, you can hear the turbulence of the blood in the artery through the stethoscope. The moment sound is heard, the first reading on the manometer is taken. This is the systolic reading and represents the highest pressure in the artery. The pressure in the cuff continues to decrease as air flows out. Eventually, the pressure in the cuff is weaker than the pressure in the artery when the heart is at rest (diastole). At this point, because the artery is completely open, no sounds can be heard in the stethoscope. Now the second reading is taken. This is the diastolic reading and represents the lowest pressure in the artery. Answer the following questions before you go on: 1. Why is there no sound from the artery before the cuff is inflated? 2. Why is there no sound from the artery after the cuff is inflated? 3. Why do you hear sound as you release the cuff pressure? 4. Why does the sound go away as you release the cuff pressure? 5. Match the state of the lumen of the artery as the cuff is inflated and deflated. a. No pressure in cuff 1. Artery fully closed b. Cuff pressure at 180 mm Hg 2. Artery fully open c. Cuff at systolic pressure 3. Artery opens then closes d. Cuff at diastolic pressure Testing Your Recall before taking Blood Pressure Make sure you have the sphygmomanometer and stethoscope ready. Read through the full instructions, and answer these questions before you begin. 1. Why can’t you leave a pressurized cuff on a person’s arm?
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