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Measuring Blood Pressure

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A pressure cuff attached to an arm.

Blood pressure is the force with which blood flows through vessels. Think of a garden hose. If you were to point a hose directly in the air and measure how far the water flew upwards, you could reliably estimate the pressure of the water. The pressure with which blood moves through the arteries uses the same principle.

Pressure is a property of physics. For the purpose of clarity, our pressure will simply refer to the 'strength' of a moving fluid. In measuring blood pressure, the 'strength' is used to move blood through a vessel that is being squeezed. Good blood pressure 'strength' can push blood through a vessel that is really being pushed closed. Similarly, a garden hose that has enough pressure will be able to push water through a knot in the hose. For example, would you be able to knot a fire hose? No, it has too much 'strength' or pressure.

When people refer to blood pressure, they typically mean arterial blood pressure. The arteries carry blood from the heart with a great deal of pressure. The heart, as everyone knows, is a big, four-chambered pump. When you are looking at a person, the bottom right part of the heart is pumping blood to the entire body (except the lungs). This part of the heart is the left ventricle. Blood flows out of the left ventricle into the aorta. The aorta supplies blood to the brain and the body1. There is a valve inside the heart where it meets the aorta. Conveniently it's named the aortic valve. This valve has three flaps and is called a semilunar valve. If the pressure in the left ventricle is greater than the pressure in the aorta, the valve opens. The pressure created by the ventricle is strong enough to force the valves to open and push the blood out. If the pressure in the aorta is greater than the pressure in the left ventricle, then the aorta forces the valve closed and no blood flows out.

The blood rushing into the aorta is carried throughout the body. The pressure of the blood changes depending upon what type of blood vessel you are looking at. In the big arteries, the pressure is fairly consistent. Measuring the blood pressure in the arm is a fair estimate of measuring the pressure right at the beginning of the aorta. This pressure is an indication of how hard the heart must work to pump blood. The other aspect of blood pressure is the pressure of the vessel when the heart is filling with blood and not pumping out the blood: the pressure left over in the vessel even when the blood isn't flowing. Back to our friend, the garden hose: when you turn off the valve by the house, the hose still has water inside.

Many things affect blood pressure, such as:

Though there are many problems when blood pressure is either too high or too low, this Entry will not be discussing those problems. The above explanation will help you understand how taking a blood pressure measurement works. Click here to learn more about blood pressure disorders.

How to Measure Blood Pressure

Blood pressure is measured by using a sphygmomanometer (a blood pressure cuff). The cuff is attached to either a dial or a stand that is enumerated. In the models with stands, bars of mercury will line up with the numbers. The models with dials have an arrow that will point to numbers as the pressure changes. Also attached to the cuff is a source for creating pressure. Usually these are hand held balloons that the examiner squeezes to inflate the cuff. In this model the balloon has a valve to release air from the cuff. When the valve is closed and the balloon is squeezed, the cuff will inflate and increase its pressure. To lower the pressure, turning the valve releases air. Another device is sometimes found in hospitals with automatic bedside monitors. In these monitors, a computer will inflate the cuff at a given time (eg, every 15 minutes) and will display the pressure on a TV monitor.

Using the proper cuff is essential. The first mistake made in measuring blood pressure is using the incorrect size of cuff. The common cuff sizes are small, medium, large and pediatric.

Once the appropriate size is found, the cuff is placed over bare skin above the elbow. The arm is passively raised to the level of the heart. That is, the arm should not be lifted by the examinee. Moreover, the patient should be as relaxed as possible throughout the exam. This means both physical and emotional relaxation. At this point the cuff is inflated to a pressure at which the radial pulse cannot be palpated in the wrist. The examiner then listens to the radial artery as the pressure is slowly lowered in the cuff. The point at which the heart exerts enough pressure to overcome the pressure applied via the cuff is called the Systolic Blood Pressure (SBP). This pulse2 will still be heard as the pressure is slowly lowered. At a lower pressure the heart easily overcomes the pressure of the cuff and no pulse can be heard, this is the Diastolic Blood Pressure (DBP).

At this pressure, the blood flows without being interrupted by the low pressure of the cuff. With our buddy, the garden hose, you can hear 'spurts' when you put your finger over the end of the hose. As you remove your finger, the water stops 'spurting' and flows more smoothly. In fluid mechanics it's called 'laminar flow'. The 'laminar flow' has less turbulence and makes less noise.

Though a detailed description of hypertension is not the goal of this Entry, an idea of blood pressure measurement is quite useful. The range for blood pressures and their diagnoses are:

Adult blood pressures (in mm of Hg):

  • Normal - SBP less than 130 and DBP less than 85
  • High Normal SBP 130-139 or DBP 85-89
  • Stage I (mild) SBP 140-159 or DBP 90-99
  • Stage II (moderate) SBP 160-179 or DBP 100-109
  • Stage III (severe) SBP 180-209 or DBP 110-119
  • Stage IV (very severe) SBP greater than 210 or DBP greater than 120

For proper diagnosis the blood pressure must be read on three separate occasions.

Several factors may cause an inaccurate blood pressure measurement:

  • Any error in the above procedure (wrong cuff size, incorrect placement, patient elevating own arm, for example)

  • Recent smoking can cause a transient high blood pressure

  • Emotional stress, anxiety, fear of the examination can all elevate the blood pressure

The Four Stages of Heartbeat and Blood Pressure

Stage One

The ventricle squeezes creating pressure, the aortic valve opens and blood rushes with a lot of force into the artery.

Stage Two

The blood, having filled the aorta, exerts its pressure onto the aortic valve in the opposite direction. If you blow up a balloon far enough, and try to put in one... more... breath... the balloon will push the air back into your mouth. The pressure of the air already in the balloon is more than the pressure in your lungs.

The aortic valve is closed, the ventricle is relaxed, but there is still blood left over in the ventricle and in the aorta. The pressure of the blood left in the ventricle is called the 'Preload'. This means that before the heart fills there is blood already left over inside. A very efficient heart has less left over, and gets a high percentage of blood out on every squeeze.

Stage Three

The ventricle is quite soft, but the atrium (the part of the heart where blood is stored before it fills the ventricle) is really full and creating some pressure of its own. Once the atrium gets filled, the valve between the atrium and ventricle (on the left it's called the 'mitral valve') opens and blood starts to fill the ventricle.

Stage Four

The ventricle gets filled, and it stretches out as it fills. There are neurons that sense this filling and start to squeeze. At this point the aortic valve is still closed and the mitral valve is open. Very soon after the ventricle starts to close, the ventricle exceeds the pressure of the atrium and the atrial valve closes. Now, the heart is squeezing, but the blood isn't going anywhere. The pressure is building up. When the pressure gets high enough, the aortic valve opens and we go back to step one.

In this fashion, Step One can be thought of as Systole, the source of systolic blood pressure. The aortic valve, already filled with blood, is getting blood pushed into it by a heart with all this built up pressure. Similarly, Step Three can be thought of as Diastole, the source of diastolic blood pressure. This is the low point as far as pressure goes. This is the pressure the arteries work at when they are filled with blood, but more blood is not getting forced into the vessel.

What Is this 'Auscultory Gap' I've Been Hearing so Much About?

The auscultory gap is a phenomenon during which the Korotkoff sounds disappear at a point between the SBP and DBP and then reappear. This gap usually occurs at a highish pressure and can be mistaken for the silence of a pressure greater than the SBP, except that the pulse can still be palpated. This is why it is greatly recommended to both palpate and auscultate the SBP.

1Including the heart itself, but not including the lungs. The blood travels to the lungs from the right ventricle.2The 'pulse' is actually called the Korotkoff sounds, which is the sound as blood spurts through the vessel, as opposed to flowing freely as it would without the external pressure.

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