BLOOD PRESSURE

The determination of an individual's blood pressure is one of the most useful clinical
measurements that can be taken. By "blood pressure" we mean the pressure exerted
by the blood against the vessel walls, the arterial blood pressure being the most useful,
and hence the most frequently measured pressure. One should become familiar with
the following pressures used in cardiovascular physiology.

· Systolic blood pressure. The highest pressure in the artery, produced in the
heart's contraction (systolic) phase. The normal value for a 20-year-old man is
120 mm Hg.

· Diastolic blood pressure. The lowest pressure in the artery, produced in the
heart's relaxation (diastolic) phase. The normal value for a 20-year-old man is
80 mm Hg.

· Pulse pressure. The difference between the systolic and diastolic pressures.
The normal value is 40 mm Hg.

· Mean blood pressure. Diastolic pressure plus one third of the pulse pressure.
This is the average effective pressure forcing blood through the circulatory
system. The normal value is 96 to 100 mm Hg.

IMPORTANCE OF BLOOD PRESSURE MEASUREMENT:

The mean blood pressure is a function of two factors - cardiac output (CO) and total
peripheral resistance (TPR). Peripheral resistance depends on the calibre (diameter) of
the blood vessels and the viscosity of the blood.

Mean BP = Cardiac output (ml/sec) x TPR

Cardiac output (ml/min) = Heart rate/min x Stroke volume (ml)

Thus, the measurement of blood pressure provides us with information on the heart's
pumping efficiency and the condition of the systemic blood vessels. In general, we
say that the systolic blood pressure indicates the force of contraction of the heart,
whereas the diastolic blood pressure indicates the condition of the systemic blood
vessels (for instance, an increase in the diastolic blood pressure indicates a decrease in
vessel elasticity).

MEASUREMENT OF BLOOD PRESSURE:

Blood pressure can be measured by several techniques. Basically they are categorized
into two methods,

1. Direct method
2. Indirect method

1. Direct method

The direct method involves directly inserting a tube or catheter into a blood vessel.
The catheter is connected to a blood pressure transducer, which generates an electrical
signal.

2. Indirect method

In this method, we measure the arterial blood pressure using two different methods:

1. The first method uses the sense of touch: it is thus called the palpatory
method.
2. The second method uses the sense of hearing: it is thus called the
auscultatory method.

In either of these indirect methods, pressure is applied to the artery using an
instrument called the sphygmomanometer.

A sphygmomanometer, an instrument that measures pressure, is needed in both
methods. Each sphygmomanometer consists of a cuff which is connected by lengths
of tubing to an inflating bulb with a needle valve and to a mercury manometer.

PALPATORY METHOD:

1. Have the subject seated, with his or her arm resting on a table. Wrap the
pressure cuff snugly around the bare upper arm, making certain that the
inflatable bag within the cuff is placed over the inside of the arm where it can
exert pressure on the brachial artery. Wrap the end of the cuff around the arm
and tuck it into the last turn, or press the fasteners together to secure the cuff
on the arm. Close the valve on the bulb by turning it clockwise.

2. With one hand, palpate (feel) the radial pulse in the wrist. Slowly inflate the
cuff by pumping the bulb with the other hand and note the pressure reading
when the radial pulse is first lost. Then increase the pressure to around 20 mm
Hg above this point. Slowly reduce the pressure in the cuff by turning the
valve counterclockwise slightly to let air out of the bag. Note the pressure
when the radial pulse first reappears. This is systolic blood pressure, the
highest pressure in the systemic artery.

3. Let all the air out of the cuff, allow the subject to rest, and then run a second
determination. Do not leave the cuff inflated for more than 2 minutes, because
it is uncomfortable and will cause a sustained increase in blood pressure.

4. The systolic pressure recorded with the palpatory method is usually around 5
mm Hg lower than that obtained using the auscultatory method. A major
disadvantage of the palpatory method is that it cannot be used to measure the
diastolic pressure.

AUSCULTATORY METHOD:

1. Place the bell of the stethoscope below the cuff and over the brachial artery
where it branches into the radial and ulnar arteries. Use your fingers, rather
than your thumb, to hold the stethoscope over the artery; otherwise you may
be measuring the thumb arterial pressure rather than the brachial artery
pressure. With no air in the cuff no sounds can be heard.

2. Inflate the cuff so the pressure is above diastolic (80-90 mm Hg), and you will
be able to hear the spurting of blood through the partially occluded artery.
Increase the cuff pressure to around 160 mm Hg; this pressure should be
above systolic pressure so that the artery is completely collapsed and no
sounds are heard.

3. Now, open the valve and begin to slowly lower the pressure in the cuff. As
the pressure decreases you will be able to hear four phases of sound changes;
these were first reported by Korotkoff in 1905 and are called Korotkoff
sounds.

· Phase 1. Appearance of a fairly sharp thudding sound that increases in
intensity during the next 10 mm Hg of drop in pressure. The pressure
when the sound first appears is the systolic pressure.
· Phase 2. The sounds become a softer murmur during the next 10 to 15
mm Hg of drop in pressure.
· Phase 3. The sounds become louder again and have a sharper thudding
quality during the next 10 to 15 mm Hg of drop in pressure.
· Phase 4. The sounds suddenly become muffled and reduced in
intensity. The pressure at this point is termed the diastolic pressure.
This muffled sound continues for another drop in pressure of 5 mm
Hg, after which all sound disappears. The point where the sound
ceases completely is called the end diastolic pressure. It is sometimes
recorded along with the systolic and diastolic pressures in this manner:
120/80/75.

ANALYSIS:

The auscultatory method has been found to be fairly close to the direct method in the
pressures recorded; usually the systolic pressure is about 3 to 4 mm Hg lower than
that obtained with the direct method.

Blood pressure varies with a person's age, weight, and sex. Below the age of 35, a
woman generally has a pressure 10 mm lower than that of a man. However, after 40 to
45 years of age, woman's blood pressure increases faster than does a man's. The old
rule of thumb of 100 plus your age is still a a good estimate of what your systolic
pressure should be at any given age. After the age of 50, however, the rule is invalid.
The increase in blood pressure with age is caused largely by the overall loss of vessel
elasticity with age, part of which is due to the increased deposit of cholesterol and
other lipids in the blood vessel walls.