measurement of blood pressure

Practical medical physiology measurement of arterial blood pressure
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4- Measurement of arterial blood pressure
Introduction:
The blood pressure means the force of blood exerted against the blood vessel
wall. Arterial blood pressure (ABP) is one of essential parameters in cardiovascular
physiology. In young adult human the ABP fluctuates between systolic levels of 120
mmHg, and a diastolic level of 70 mmHg. The ABP is written as systolic pressure
over diastolic pressure (120/70 mmHg).
Systolic pressure is the maximum pressure exerted by the blood against the artery
walls.it results when the ventricles contract. Normally, it measures 120 mm Hg.
Diastolic pressure is the lowest pressure in the artery. It results when the ventricles
are relaxed and is usually around 80 mm Hg.
The mean blood pressure (MBP) is the average pressure throughout the cardiac
cycle which equals to the diastolic blood pressure (DBP) plus one third of the pulse
pressure (the difference between the systolic blood pressure and DBP).
MBP = DBP + 1/3 Pulse pressure.
Figure (4-1): Blood pressure curve.
The ABP is the product of the cardiac output (COP) and peripheral resistance (PR).
So increase in COP leads to increase the systolic blood pressure (SBP) whereas
increase in PR leads to increase the DBP.
Blood Pressure = Cardiac Output X Peripheral Resistance
Pulse Pressure
Systolic Dicrotic Notch Diastolic Mean PressurePractical medical physiology measurement of arterial blood pressure
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Classification of blood pressure
Category SBP DBP
Normal ?120 ?80
Prehypertension 121-139 81-89
Stage 1 hypertension 140-159 90-99
Stage 2 hypertension ?160 ?100
There are two methods to measurement of ABP:
1- Direct method:- A cannula or needle filled with anticoagulant is inserted in
artery. Then it is connected to the manometer.
2- Indirect method:
A- Palpatory method: The SBP can be determined by inflating an arm cuff and
then letting the pressure fall and determining the pressure at which the radial pulse
first becomes palpable.
B- Auscultatory method: It is standard method of taking a patient blood pressure
by use technique developed by Korotkoff in 1905. The arterial pressure in human is
routinely measured by this method by using instrument which is called a
sphygmomanometer. Inflate the bag of instrument by means of a rubber squeeze
bulb to pressure above the expected systolic pressure so no sound is heard with the
stethoscope. The pressure in the cuff is then lowered slowly by open release valve.
When the inflation pressure falls, the small spurt of blood escapes through the cuff
and slight tapping sound heard. The pressure at which the sound is first heard
(phase 1 of korotkoff sound) represents the systolic blood pressure (SBP). The
sounds become louder, then dull, muffled and finally they disappear. The point at
which the sound becomes muffled is taken as the diastolic pressure in pregnancy,
children, and adult after exercise. This is also true in diseases such hyperthyroidism
and aortic insufficiency. Diastolic pressure in resting adult correlates best with the
pressure at which the sound disappears.
Korotkoff sounds are:-
1- Phase 1: tapping sound.
2- Phase 2: louder sound.
3- Phase 3: dull sound.
4- Phase 4: muffled sound.
5-Phase 5: disappeared sound.
The ausculatory method is accurate when used properly, because of the difficulty
in determining exactly when the first beat is felt. Pressure by palpatory method is
usually 2 – 5 mmHg. lower than those measured by the ausculatory method.Practical medical physiology measurement of arterial blood pressure
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The number of precautions must be observed:-
1- The cuff must be at heart level to obtain a pressure that is uninfluenced by
gravity (The blood pressure increase or decrease about 0.77 mmHg/cm above or
below heart level).
2- Using standard arm cuff A cuff that is too small will produce a falsely high
reading; one that is too large, a falsely low reading. The American Heart Association
recommends comparing the cuff with subject arm. The length of bladder should be
at least 80% of the arm circumference.
3- Compare blood pressure in both arms, when examining an individual for first
time. Presences of difference between them indicate vascular obstruction.
4- Tell the subject not to talk during measurement of pressure.
5- A void using an arm with I.V, edema, injury or paralysis.
6- Smoking and drinking alcohol within last 15 minutes alter reading.
7- Pain, anxiety and discomfort give a falsely high pressure.
Normal value:- The average ABP in young adult is about 120/70 mmHg. In sitting or
lying position at rest. The normal range is 100 – 140 / 60 – 90 mmHg. It falls at night
and in women than in men. In healthy human both SBP and DBP rise with age.
Exercise, anxiety and discomfort can lead to a transient rise in BP.
Physiological variation:-
1- Age: The ABP increase with age.
2- Sex: in women is slightly less than men.
3- It is more at evening and less in morning.
4- It is more after meal exercise and well-built person.
5- It is less in sleep and in lying position.
Sphygmomanometer:- See figure 4-2.
1- Graduated vertical limb which open to atmosphere. It has marking from 0 – 250
mmHg from below upward.
2- Rubber bag covered with linen cuff. See figure 4-3.
3- Rubber bulb with valve.
4- Release screw.
5- Mercury reservoir.
6- Rubber tubes.Practical medical physiology measurement of arterial blood pressure
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Figure (4-2): Sphygmomanometer.
Figure (4-3): Rubber bag covered with linen cuff.
Objective:-
To measure the ABP in human beings.
Materials and instruments:-
1- Sphygmomanometer.
2- Stethoscope.
3- Subject.
Procedure figure 4-4:-
1- Subject should be relaxed, sitting or lying for five minutes.
2- Manometer is placed at level of observer s eyes.
3- All clothing should be removed from upper arm.
4- The arm of subject should be supported because failing of it causing isometric
contraction which leads to false measurement.Practical medical physiology measurement of arterial blood pressure
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5- Inflatable arm cuff is applied around the upper arm not too tightly, leaving one
to two inches between the lower end and the cubital fossa, at l
6- Cuff is connected with a mercury or aneroid manometer.
7- The bell of stethoscope is placed slightly on the brachial artery. Don’t put too
much pressure on bell that may occlude arterial flow. Point ear pieces forward.
8- Cuff is inflated by a rubber squeeze bulb to pressure above expected SBP (no
sound is heard) 20 to 30 mmHg. wait 15 – 30 seconds.
9- The pressure in the cuff is lowered slowly to hear soft sound by open release
valve. So pressure falls at 2 -3 mmHg per second.
10- When the first sound (step 1 of Korotkoff s sound) is heard, the SBP is
measured. Listen for the onset of at least two consecutive beats.
11- When the sound disappears, at this level DBP is measured in adult. In some
conditions like pregnancy, exercise…etc we use muffling of sound (phase 4 of
korotkoff s sound) for measurement of DBP.
12- Wait 1 - 2 minutes before repeating pressure on the same arm.
Measuring od Pressure
Pressure in cuff closes off the arm’s
The pressure is gradually released.
The first sound is heard when
pressure of left ventricle is more than
pressure in cuff or systolic pressure
More air is released from cuff.
When sounds stop, it is diastolic
pressure (80 mm Hg).
Figure 4-4: procedure of arterial blood pressure measurement