Hemodynamics

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Hemodynamics

• Physics of Blood flow in the circulation

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Circulatory System

• Heart
• Has 2 collecting chambers - (Left, Right
  Atria)
• Has 2 Pumping chambers - (Left, Right
  Ventricles)

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Circulation Schematic
Left Side of Heart
Pulmonary Vein
Aorta
A
V
Aortic Valve
Mitral Valve
Tissues
Lungs
Tricuspid Valve
Pulmonary Valve
V
A
Pulmonary Artery
Right Side of Heart
Sup. Inf. Vena Cava
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Heart Valves

• Atrioventricular (A-V) valves - separate Atria
  from Ventricles
• Bicuspid (Mitral) - Left Side
• Tricuspid - Right Side
• Semi-Lunar Valves - separate ventricles from
  Arteries

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Opening, Closing of Valves - Depends on
Pressure differences between blood in
adjacent areas
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Heart Sounds

• Lubb (1st sound) - Closure of A-V valves
• Dupp (2nd sound) - Closure of S-L valves
• Caused by Turbulence on closing.
• Anything extra ?Murmur (swishing of blood)
• Could be due to
• Stenosis of Valves (calcification)
• Valves not closing properly
• (Incompetence, Insufficiency)

Increases Pressure on heart
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Heart Sounds and Phonocardiography

• Heart sounds are vibrations or sounds due to the
  acceleration or deceleration of blood during
  heart muscle contractions, whereas murmurs (a
  type of heart sounds) are considered vibrations
  or sounds due to blood turbulence.
• Phonocardiographyis the recording of heart
  sounds.

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• Heart Sounds
• The auscultation of the heart provides valuable
  information to the clinician concerning the
  functional integrity of the heart.

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Basic heart sounds
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• The first heart sound is generated at the
  termination of the atrial contractions, just at
  the onset of ventricular contraction. This sounds
  is generally attributed to movement of blood into
  the ventricles, the artioventricular (AV) valves
  closing, and the sudden cessation of blood flow
  in the atria. Splitting of the first heart sound
  is defined as an asynchronous closure of the
  tricuspid and the mitral valves.
• The second heart sound is a low frequency
  vibration associated with the closing of the
  semilunar valves - the aortic and pulmonary
  valves. This sound is coincident with the
  completionof the T wave of the ECG.

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• The third heart sound corresponds to the sudden
  cessation of the ventricular rapidfilling. This
  low-amplitude, low frequency vibration is audible
  in children and in some adults.
• The fourth heart sound occurs when the atria
  contracts and propel blood into the ventricles.
  This sound with very low amplitude and low
  frequency is not audible, but may be recorded by
  the phonocardiography (PCG).

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• The sources of most murmurs, developed by
  turbulence in rapidly moving blood, are known.
  Murmurs are common in children during early
  systolic phase they are normally heard in nearly
  all adults after exercise.
• Abnormal murmurs may caused by stenoses and
  insufficiencies (leaks) at the aortic, pulmonary,
  and mitral valves. They are detected by noting
  the time of their occurrence in the cardiac cycle
  and their location at the time of measurement.

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Auscultation and Stethoscopes

• Heart sounds travel through the body from the
  heart and major blood vessels to the body
  surface.
• The physician can hear those sounds with a
  stethoscope.
• Basic heart sounds occur mostly in the frequency
  range of 20 to 200 Hz.
• Certain heart murmurs produce sounds in the
  1000-Hz region, and some frequency components
  exist down to 4 or 5 Hz.
• Some researchers even reported that heart sounds
  and murmurs have small amplitudes with
  frequencies as low as 0.1 Hz and as high as
  2000Hz.

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Stethoscopes Historical and Current

• Has been used for almost 200 years, and still
  being used nowadays for screening and diagnosis
  in primary health care.

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The typical frequency-response curve for a
stethoscope
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• Many types of electronic stethoscopes have been
  proposed by engineers. These devices have
  selectable frequency-response characteristics
  ranging from the "ideal" flat-response case and
  selected bandpass to typical mechanical-stethoscop
  e responses.
• Physicians, however, have not generally accepted
  these electronic stethoscopes, mainly because
  they are unfamiliar with the sounds heard with
  them. Their size, portability, convenience, and
  resemblance to the mechanical one are other
  important considerations.

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Phonocardiography

• Phonocardiography is an mechano-electronic
  recording technique of heart sounds and murmurs.
• It is valuable in that it not only eliminates the
  subjective interpretation of these sounds, but
  also makes possible an evaluation of the heart
  sounds and murmurs with respect to the electrical
  (such as ECG) and mechanical (carotid pulse
  recorded in the midneck region) events in the
  cardiac cycle.
• It is also valuable in locating the sources of
  various heart sounds.

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• A PCG machine is usually consist of four main
  parts
• A microphone or PCG transducer,
• filtering (mechanical and electrical),
• processing unit, and
• display.

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• There are optimal recording sites for the various
  heart sounds or PCG signals. Because of the
  acoustical properties of the transmission path,
  heart sound waves are attenuated but not
  reflected. Figure shows four basic chest
  locations at which the intensity of sound from
  the four valves is maximized.
• Auscultatory areas on the chest A, aortic P,
  pulmonary T, tricuspid and M, mitral areas.

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Blood Vessels

• Arteries
• Capillaries
• Veins
• Systemic Pathway
• Left Ventricle Aorta Arteries
  Arterioles
• of Heart
• Capillaries
• Venules Veins Right Atrium
• of Heart

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Blood

• Composition
• Approx 45 by Vol. Solid Components
• Red Blood Cells (12?m x 2 ?m)
• White Cells
• Platelets
• Approx 55 Liquid (plasma)
• 91.5 of which is water
• 7 plasma proteins
• 1.5 other solutes

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Blood Functions

• Transportation
• of blood gases, nutrients, wastes
• Homeostasis (regulation)
• of Ph, Body Temp, water content
• Protection

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As a Result .

• Blood behaves as a simple Newtonian Fluid when
  flowing in blood vessels
• i.e. Viscous stresses ? Viscosity, strain rate

y
u(y)
No slip at wall
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• Viscosity of Blood 3 3.5 times of water
• Blood acts as a non-newtonian fluid in smaller
  vessels (including capillaries)

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Cardiac Output

• Flow of blood is usually measured in l/min
• Total amount of blood flowing through the
  circulation Cardiac Output (CO)
• Cardiac Ouput Stroke Vol. x Heart Rate
• 5 l/min
• Influenced by Blood Pressure Resistance

Force of blood against vessel wall

• Blood viscosity
• Vessel Length
• Vessel Elasticity
• Vasconstriction / Vasodilation

? with water retention ? with dehydration,
hemorrage
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Overall

• Greater Pressure ? Greater Blood
• Differences Flow
• Greater Resistance ? Lesser Blood Flow

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Blood Pressure

• Driving force for blood flow is pressure created
  by ventricular contraction
• Elastic arterial walls expand and recoil
• continuous blood flow

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• Blood pressure is highest in the arteries and
  falls continuously . . .
• Systolic pressure in Aorta 120 mm Hg
• Diastolic pressure in Aorta 80 mm Hg

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Typical values of circulatory pressures SP is
the systolic pressure, DP the diastolic pressure,
and MP the mean pressure.
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• Ventricular pressure difficult to measure
• arterial blood pressure assumed to indicate
  driving pressure for blood flow
• Arterial pressure is pulsatile
• useful to have single value for driving pressure
  Mean Arterial Pressure
• MAP diastolic P 1/3 pulse pressure

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• Pulse Pressure systolic pressure - ??
• measure of amplitude of blood pressure wave

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MAP influenced by

• Cardiac output
• Peripheral resistance
• MAP CO x Rarterioles
• Blood volume
• fairly constant due to homeostatic mechanisms
  (kidneys!!)

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BP too low

• Driving force for blood flow unable to overcome
  gravity
• O2 supply to brain ?
• Symptoms?

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BP too high

• Weakening of arterial walls - Aneurysm
• Risk of rupture hemorrhage
• Cerebral hemorrhage ?
• Rupture of major artery

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BP estimated by Sphygmomanometry

• Auscultation of brachial artery with stethoscope
  
• Laminar flow vs. turbulent flow

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Typical indirect blood-pressure measurement
system The sphygmomanometer cuff is inflated by a
hand bulb to pressures above the systolic level.
Pressure is then slowly released, and blood flow
under the cuff is monitored by a microphone or
stethoscope placed over a downstream artery. The
first Korotkoff sound detected indicates systolic
pressure, whereas the transition from muffling to
silence brackets diastolic pressure.
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Principles of Sphygmomanometry

• Cuff inflated until brachial artery compressed
  and blood flow stopped what kind of sound?

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Slowly release pressure in cuff
turbulent flow
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Pressure at which . . .

• . . . sound ( blood flow) first heard
• . . . sound disappeared

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Ultrasonic determination of blood pressure A
compression cuff is placed over the transmitting
(8 MHz) and receiving (8 MHz D ƒ) crystals. The
opening and closing of the blood vessel are
detected as the applied cuff pressure is varied.
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• Pressure can be stated in terms of column of
  fluid.
• Pressure Units
• mm Hg cm H2O PSI ATM
• 50 68 0.9 0.065
• 100 136 1.9 0.13
• 200 272 3.8 0.26
• 300 408 5.7 0.39
• 400 544 7.6 0.52

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• Pressure Height x Density
• or P ?gh
• If Right Atrial pressure 1 cm H2O in an open
  column of blood
• ? Pressure in feet 140 cm H2O
• ? Rupture
• ? Venous Valves

Density of blood 1.035 that of water
Incompetent venous valves ? Varicosities
Actual Pressure in foot 4-5 cm H2O
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Pressures in the circulation

• Pressures in the arteries, veins and heart
  chambers are the result of the pumping action of
  the heart
• The right and left ventricles have similar
  waveforms but different pressures
• The right and left atria also have similar
  waveforms with pressures that are similar but not
  identical

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3. As blood enters the aorta, the aortic pressure
begins to rise to form the systolic pulse
4. As the LV pressure falls in late systole the
aortic pressure falls until the LV pressure is
below the aortic diastolic press.
2. Pressure rises until the LV pressure exceeds
the aortic pressure
5. Then the aortic valve closes and LV pressure
falls to LA pressure
? The blood begins to move from the ventricle to
the aorta
1. The LV pressure begins to rise after the QRS
wave of the ECG
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• The first wave of atrial pressure (the A wave) is
  due to atrial contraction
• The second wave of atrial pressure (the V wave)
  is due to ventricular contraction

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Normal Pressures

• RV and pulmonary systolic pressure are 12-15 mm
  Hg
• Pulmonary diastolic pressure is 6-10 mm Hg
• LA pressure is difficult to measure because
  access to the LA is not direct

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AS produces a pressure gradient between the aorta
and LV

• The severity of AS is determined by the pressure
  drop across the aortic valve or by the aortic
  valve area
• The high velocity of blood flow through the
  narrowed valve causes turbulence and a
  characteristic murmur AS can be diagnosed with a
  stethoscope

i.e. For blood to move rapidly through a narrowed
aortic valve orifice, the pressure must be higher
in the ventricle
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(a) Systolic pressure gradient (left
ventricular-aortic pressure) across a stenotic
aortic valve. (b) Marked decrease in systolic
pressure gradient with insertion of an aortic
ball valve.
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Pressure Measurement

• Accurate pressure measurements are essential to
  understanding the status of the circulation
• In 1733 Steven Hales connected a long glass tube
  directly to the left femoral artery of a horse
  and measured the height of a column of blood (8
  feet, 3 inches) to determine mean BP
• Direct pressure measurements are made frequently
  in the cardiac catheterization laboratory, the
  ICU and the OR

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• A tube is inserted into an artery and connected
  to an electrical strain gauge that converts
  pressure into force that is sensed electrically
• The output of the transducer is an electrical
  signal that is amplified and recorded on a strip
  chart
• For correct pressure measurements the cannula and
  transducer must be free of air, the cannula
  should be stiff and short

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Extravascular pressure-sensor system A catheter
couples a flush solution (heparinized saline)
through a disposable pressure sensor with an
integral flush device to the sensing port. The
three-way stopcock is used to take blood samples
and zero the pressure sensor.
Flush solution under pressure
Sensing port
Roller clamp
Sample and transducer zero stopcock
Electrical connector
Disposable pressure transducer with an integral
flush device
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(a) Physical model of a catheter-sensor system.
(b) Analogous electric system for this
catheter-sensor system. Each segment of the
catheter has its own resistance Rc, inertance Lc,
and compliance Cc. In addition, the sensor has
resistor Rs, inertance, Ls, and compliance Cs.
The compliance of the diaphragm is Cd.
Sensor
Diaphragm
P
(a)
Catheter
Liquid
Incremental length
DV
Ls
Lc
Rs
Rc
Lc
Rc
Lc
Rc
(b)
DV
Cc
Cc
Cc
C
d
Cs
DP
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Cardiac Output (CO)Measurement

• The measurement of blood flow through the
  circulation is usually done clinically using
  either the Fick method
• The Fick method states that the cardiac output is
  equal to the oxygen consumption divided by the
  arterial-venous oxygen difference
• CO Oxygen consumption / A-V O2

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• The measurement is done by determining the oxygen
  consumption using respiratory gas measurements
  and the O2 content of arterial and mixed venous
  blood
• The mixed venous blood sample is obtained from a
  PA with a catheter
• The arterial sample can be drawn from any artery