Transcript Blood pressure

5.4.12
Blood flow through stenosis
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Bernoulli’s principle states that when the fluid flow
through a tube is constant, the total fluid energy –the
sum of kinetic energy and potential energy-remains
constant
It explains why fluid pressure is low in blood vessels at
places where its radius is less
Blood flow through stenosis
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Proximal to the structure of
stenosis, blood flow is laminar.
Blood passing through stenotic
orifice also remains laminar,
but its velocity increases to
maintain the volumetric rate of
flow. Here the pressure
against the walls is least and
hence there is a pressure drop
across the narrower area of a
vessel
Blood Pressure
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Blood pressure (BP) is the pressure exerted by
circulating blood upon the walls of blood vessels
The blood pressure in the circulation is principally due to
the pumping action of the heart. Differences in mean
blood pressure are responsible for blood flow from one
location to another in the circulation
The rate of mean blood flow depends on the resistance
to flow presented by the blood vessels. Mean blood
pressure decreases as the circulating blood moves
away from the heart through arteries, capillaries and
veins due to viscous losses of energy
Mean blood pressure drops over the whole circulation,
although most of the fall occurs along the small arteries
and arterioles
Forces acting on blood during
circulation
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The main forces acting on blood during circulation
Inertial force due to acceleration
Flowing blood has inertia and energy is expended
in setting the blood into motion. Inertial force has
two components (i) time acceleration due to
pulsatile ejection from heart (ii) spatial acceleration
e.g. at entry into a circulation
Viscous force (Fv)
Pressure force FP (systolic force)
Gravitational force FG
Forces acting on blood during circulation
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According to Newton’s law of motion which also governs
the motion of blood
F = FV + FP+ FG
Energy of fluids moving in horizontal and vertical vessels
differ
Energy per unit volume in horizontal vessels:
Sum of kinetic energy (dynamic pressure) + Potential
energy/Hydrostatic pressure = 1/2ρv2 + P
Energy
per
unit
volume
in
vertical
vessels:
Sum of kinetic energy+ Potential energy/Hydrostatic
pressure + gravitational energy = 1/2ρv2 + P + ρgh
ρ is the fluid density in kg/m3, v is the linear velocity in m/s,
h is the height above or below the heart
Blood Pressure Profile
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Blood pressure is highest in the arteries.
It decreases as we move to arterioles,
capillaries and then to veins
Reference points for measuring blood
pressure
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While measuring pressures in cardiovascular system,
ambient atmospheric pressure is used as zero
reference point. Thus a blood pressure of 90mmHg
means that pressure is 90mmHg above atmospheric
pressure
The second reference point for measuring blood
pressureis anatomical and is the position of heart. For
example, the usual convention is to measure blood
pressure in the brachial artery above elbow i.e.
approximately at hearts level when patient is seated
If the blood pressure measurements are to be made in
the legs, the patient is brought to lying down position.
In this position vessel is approximately at cardiac level
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Blood Pressure
Venous pressures
-35 mm Hg
Effect of gravity on pressure
 Distance heart-head~ 0.4 m
1 mm Hg
 Heart-feet ~ 1.4 m
 DP = rgh
105 mm Hg
Arterial pressures
55 mm Hg
95 mmHg
100 mmHg
95 mm Hg
100 mm Hg
195 mm Hg
The pressure in any vessel above heart
level is decreased by the effect of
gravity
 The arterial pressure is increased by
0.77mmHg for every centimeter below
the right atrium and similarly decreased
for each cm above the right atrium
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Blood pooling and skeletal muscles of leg
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The increased hydrostatic pressure in the veins of the legs
upon standing pushes outward upon the veins walls
causing marked distension with pool-leg of blood
Gravitational force also increase capillary pressure. This in
turn causes increased filtration of fluid out of capillaries into
interstitial spaces
The tendency is counteracted by the skeletal muscles of the
leg which contract and compress the veins thereby
compressing the column of venous blood from feet to heart
The column of venous blood is also interrupted. In the neck.
This interruption helps in neutralizing the effect of gravity
and the venous pressure in the neck becomes
approximately zero
Posture effects on the blood
pressure
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A sudden change in posture from horizontal to vertical
can have a very marked effect on the pressure pattern in
the body. Consequences can be faintness one can
experience on sudden rising. This is known as “postural
hypotension syncope (syncope= fainting)”
The reason is that veins are most distensible . On rising
suddenly the pressure of blood in veins rises markedly.
In the absence of correcting mechanisms, the veins
expand markedly and the blood ‘pools’ there
The venous return to the heart drops drastically and the
blood circulation to the brain consequently falls- resulting
in dizziness or even loss of consciousness
Posture effects on the blood
pressure
Normally the body possesses three mechanisms for the
maintenance of good blood return to the heart. These
are:
1. Pressure reflexes which induce constriction of diameter
of arteries and arterioles
2. reflex acceleration of the heart rate when receptors in
the aorta or the carotid artery sense a drop in pressure
3. muscular activity in the limbs which help to maintain the
diameter of veins and act to reduce pooling of the blood
 Dysfunction of these reflexes or too rapid motion which
produces blood pooling before the postural reflexes
come into play , can produce dizziness and/or fainting
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Blood pressure measurement
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Direct method
This is an invasive method in which artery or vein is cannulated or
catheterised. Pressure measured by direct method is known as “end
pressure” Here the kinetic energy of blood flow is measured in terms
of pressure. Direct method is used in patients of ‘shock’ where
indirect measurements may be inaccurate or indeed impossible
Indirect methods of blood
pressure measurement
 Indirect
method
(non-invasive,
measures lateral/side pressure)
Auscultatory
Oscillometric
Auscultatory Method
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The auscultatory method uses a
stethoscope and a sphygmomanometer
An inflatable cuff encircles the arm.
Pressure in the cuff is transmitted
through the tissue to compress brachial
artery and can be viewed on a
manometer
A stethoscope is used to listen to
sounds in the artery distal to the cuff.
The sounds heard during measurement
of blood pressure are not the same as
the heart sounds 'lub' and 'dub' that are
due to vibrations inside the ventricles
that are associated with the snapping
shut of the valves
Auscultatory Method
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If a stethoscope is placed over the brachial artery in a
normal person no sound should be audible. As the
heart beats, pulses (pressure waves) are transmitted
smoothly via laminar (non-turbulent) blood flow
throughout the arteries, and no sound is produced
Similarly, if the cuff of a sphygmomanometer is placed
around a patient's upper arm and inflated to a pressure
above the patient's systolic blood pressure, there will be
no sound audible. This is because the pressure in the
cuff is high enough such that it completely occludes the
blood flow This is similar to a flexible tube or pipe with
fluid in it that is being pinched shut
Korotkoff sounds
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If the pressure is dropped to a level equal to that of the
patient's systolic blood pressure, the first Korotkoff sound
will be heard. As the pressure in the cuff is the same as
the pressure produced by the heart, some blood will be
able to pass through the upper arm when the pressure in
the artery rises during systole. This blood flows in spurts
as the pressure in the artery rises above the pressure in
the cuff and then drops back down beyond the cuffed
region, resulting in turbulence that produces an audible
sound
Korotkoff sounds
As the pressure in the cuff is allowed to fall further,
thumping sounds continue to be heard as long as the
pressure in the cuff is between the systolic and diastolic
pressures, as the arterial pressure keeps on rising above
and dropping back below the pressure in the cuff.
 Eventually, as the pressure in the cuff drops further, the
sounds change in quality, then become muted, and finally
disappear altogether. This occurs because, as the
pressure in the cuff drops below the diastolic blood
pressure, the cuff no longer provides any restriction to
blood flow allowing the blood flow to become smooth
again with no turbulence and thus produce no further
audible sound. The pressure where sound just
disappears is the diastolic pressure
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Oscillometric method
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The
oscillometric
method
was
first
demonstrated in 1876 and involves the
observation
of
oscillations
in
the
sphygmomanometer cuff pressure[ which are
caused by the oscillations of blood flow, i.e.
the pulse
It uses a sphygmomanometer cuff, like the
auscultatory method, but with an electronic
pressure sensor (transducer) to observe cuff
pressure
oscillations,
electronics
to
automatically interpret them, and automatic
inflation and deflation of the cuff.
Oscillometric method
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The cuff is inflated to a pressure initially in excess of the
systolic arterial pressure and then reduced to below
diastolic pressure over a period of about 30 seconds.
When blood flow is nil (cuff pressure exceeding systolic
pressure) or unimpeded (cuff pressure below diastolic
pressure), cuff pressure will be essentially constant.
When blood flow is present, but restricted, the cuff
pressure, which is monitored by the pressure sensor,
will vary periodically in synchrony with the cyclic
expansion and contraction of the brachial artery, i.e., it
will oscillate. The values of systolic and diastolic
pressure are computed, results are displayed.
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