2. Plan of presentation
Basic concepts of hemodynamics and
it’s clinical application in arterial disease
3. Primary physiology of arterial
disease
obstruction of the lumen
atherosclerosis, emboli, thrombi, fibromuscular
dysplasia,trauma or external compression
disruption of the vessel wall
aneurysms or trauma
4. BASIC PRINCIPLES OF ARTERIAL
HEMODYNAMICS
Fluid Pressure:
The pressure in a fluid system is defined as force per
unit area (in dynes per square centimeter).
(1) the dynamic pressure produced by contraction of
the heart
(2) the hydrostatic pressure
(3) the static filling pressure
5. Fluid Energy
Potential energy can be expressed as follows:
predominant component of potential energy is
the pressure produced by cardiac contraction.
Kinetic energy :
predominant component of kinetic energy is velocity.
6. Bernoulli’s Principle
When fluid flows steadily(no acceleration or
deceleration)from one point in a system to another its
total energy content along any given streamline
remains constant,provided that there are no frictional
losses’
This is in accordance with the law of conservation
of energy .
7. total fluid energy
remains constant
increse in potential energy, (higher pressure)
(lower velocity )
proportional loss of kinetic energy .
This situation is not observed in human arteries
because the ideal flow conditions specified in the
Bernoulli relationship are not present.
8. Fluid Energy Losses
Energy losses in flowing blood
1. viscous losses:
viscosity= hematocrit of blood
2. inertial losses:
changes in the velocity or direction of flow.
9. viscosity
The viscosity (η) is defined as the
ratio of shear stress (τ) to shear rate (D):
η= τ/D
Shear stress :
proportional to the energy loss owing
to friction between adjacent
fluid layers,
shear rate:
the relative velocity of adjacent fluid layers.
10. Rheologic Agents and viscus loss
.
1. Low-molecular-weight dextran.
2. pentoxifylline
are most often used in the immediate postoperative
period to increase flow through a reconstructed arterial
segment for lowering blood viscosity
11. inertial loss andPoiseuille’s law
This law states that the pressure gradient along a tube
(P1 – P2, ) is directly proportional to the mean
flow velocity (V,)or volumeflow (Q), the tube
length(L), and the fluid viscosity (η) and it is inversely
proportional to either the second or fourth power of
the radius (r) .
Therefore, radius has a profound
influence on energy losses
12. Stenosis Length and Multiple
stenosis
Doubling the length of a stenosis results in a doubling of the
associated energy losses.
But reducing the radius by half increases energy losses by a
factor of 16 .
Thus separate short stenosis tend to be more significant than a
single longer stenosis.
multiple subcritical stenosis may have the same effect as a
single critical stenosis
13. Inertial Energy Losses
variations in lumen diameter
at points of curvature and branching.
the acceleration and deceleration of pulsatile flow,
inertial energy losses result
Energy losses related to inertia (ΔE) are proportional to a
and the square of blood velocity .
15. Poiseuille’s law and inertial loss
the pressure drop is negligible until the radius is
reduced to 0.3 cm or 3mm.
if the radius less than 0.2 cm or 2mm, the pressure
drop increases rapidly.
These observations may explain the preference of
autogenous vein bypass grafts of not less than 3-4 mm
in diameter .
16. Vascular Resistance
Hemodynamic resistance (R) can be defined as the ratio of the
energy drop between two points along an artery (E1 – E2) to the
mean blood flow (Q):
The standard physical units of hemodynamic resistance
peripheral resistance unit (PRU)
1PRU= 8 × 10^4 dyne-s/cm.^5 .
17. total vascular resistance
arteries and capillaries=90%,
arterioles and capillaries=> 60%
large- and medium-sized arteries=15%
venous flow=10% .
atherosclerotic occlusive disease occur normally in vessels with
low resistance .
The large and medium arteries are most commonly affected
19. Flow in Parallel Graft and
Stenotic Artery
Surgeons occasionally express
concern over the possibility that
continued patency of a stenotic
artery might lead to thrombosis
of a parallel graft.
To allay this fear; they either
avoid end-to-side anastomoses or
ligate the stenotic artery.
20. Theoretical considerations strongly suggest that
such concerns are not valid, provided that the arterial
segment is sufficiently diseased to merit bypass
grafting
21. vein Grafts with Double Lumens
saphenous veins bifurcate into two
separate and parallel channels that
rejoin after a variable
distance to reconstitute a single
lumen.
When this situation is
encountered, the surgeon must
decide whether or
not to include both channels in the
graft
22. the combined resistance of the
two parallel channels exceeds that of the undivided
vein .
24. theoretical advantages and
disadvantages .
Flow rates in femorotibial grafts should theoretically
be lower than those in the proximal segment of
sequential grafts but higher than those in the distal
segment .
Femorotibialgraft may be more likely to fail than the
sequential graft.
25. Blood Flow Patterns
Laminar Flow
In steady-state conditions the flow pattern is laminar.
All motion is parallel to the walls of the tube and the
fluid is arranged in a series of concentric layers or
laminae.
velocity profile that is parabolic in shape .
26. PARABOLA- Laminar Flow
with velocity being highest in the center of the stream
and becoming progressively lower towards the vessel
wall.
The layer of fluid or blood in contact with the wall is
stationary .
27. TURBULENCE
In contrast to the linear streamlines of laminar flow,
turbulence is an irregular flow state in which velocity
varies rapidly with respect to space and time.
These random velocity changes result in inertial
energy .and dissipation of fluid energy as heat.
To measure --- Reynolds number, Re
28. Reynolds number,
The point at which flow changes from laminar to
turbulent is best defined in terms of a dimensionless
quantity known as the Reynolds number, Re.
Re exceeds 2000= turbulence
laminar if Re s <2000.
29. Because Re values are well below 2000 in most
peripheral arteries, turbulence is unlikely to occur
under normal circumstances.
However, turbulence does appear to develop in the
ascending aorta during the peak systolic ejection
phase .
30. Normal Pressure and Flow
As the arterial pressure pulse moves distally, the
systolic pressure rises, the diastolic pressure falls, and
the pulse pressure becomes wider.
The decrease in mean arterial pressure between the
heart and the ankle is normally less than 10 mm Hg .
normal individuals, (ankle-brachial index) has a
mean value of 1.11 ± 0.10
in the resting state.
31. Boundary Layer Separation
In fluid flowing through a tube, the portion of fluid
adjacent to the tube wall is referred to as the boundary
layer.
This layer is subject to both frictional interactions with
the tube wall and viscous forces generated by the more
rapidly moving fluid toward the center of the tube.
.
32. When the tube geometry changes suddenly, such as at
points of curvature, branching, or variations in lumen
diameter,small pressure gradients are created that
cause the boundary layer to stop or reverse direction.
This change results in a complex, localized flow
pattern known as an area of boundary layer separation
or flow separation.
33. Flow Separation and Shear
The clinical importance
of boundary layer
separation
is that these localized
flow disturbances may
contribute
to the formation of
atherosclerotic plaques
35. Anastomotic Configuration
To reduce energy losses due to flow disturbances, the
transition from graft to host vessel should be as
smooth as possible
End-to-end anastomoses, therefore, most
closely approximate the ideal. End-to-side or side-to-
end anastomoses always result in alterations in flow
direction
36. Tailoring the anastomosis to enter the recipient artery
or leave the donor artery at an acute angle will
minimize but can never eliminate flow disturbances.
37. Bifurcations and Branches
The branches of the arterial system produce sudden
changes in the flow pattern that are potential sources
of energy loss.
Flow patterns in a bifurcation are determined
mainly by the area ratio and the branch angle.
38. Area Ratio
The area ratio is defined as the combined area of the
secondary branches divided by the area of the primary
artery.
For efficient transmission of pulsatile energy across a
bifurcation, the vascular impedance of the primary
artery should equal that of the branches,
39. ideal area ratio of 1.15 for larger arteries and 1.35 for
smaller arteries.
infrarenal aorta
an area ratio of 0.8, approximately 22% of the incident
pulsatile energy is reflected .
Hence atherosclerosis and aneurysms are common in
this arterial segment
40. Despite their theoretical
disadvantages,
commercially
available bifurcation
grafts have functioned
extremely well in
a variety of clinical
applications.
41. The Branch Angle.
Flow disturbances are minimized when the angle is
narrow and are exaggerated when the limbs are widely
spread
The average angle between the human iliac
arteries is 54 degrees;
however, with diseased or tortuous
iliac arteries, this angle can approach 180 degrees.
43. Bruits
Stenosis or irregularities of the vessel lumen produce
turbulent flow patterns that set up vibrations in the
arterial wall.
These vibrations generate displacement waves that
radiate through the surrounding tissues and can be
detected as audible sounds.
44. Generally, a soft, midsystolic bruit is associated with a
relatively minor lesion that does not significantly
reduce flow or pressure.
A bruit with a loud diastolic component
suggests a stenosis severe enough to reduce flow and
produce a pressure drop.
A bruit may be absent when an artery is
nearly occluded or when the flow rate is extremely low
45. Poststenotic Dilatation
The most likely explanation for this phenomenon
is that arterial wall vibrations result in structural
fatigue of elastin fibers.
In a series of animal model studies, poststenotic
dilatations did not develop unless a bruit was
present distal to the stenosis.
46. CRITICAL STENOSIS
The degree of narrowing at which pressure and flow
begin to be affected has been called the “critical
stenosis.
appreciable changes in pressure and flow do not occur
until the cross-sectional area of a vessel has been
reduced by more than 75% or 50% reduction in
diameter.
47. VELOCITY OF BLOOD FLOW
Critical stenosis-varies with resistance of run off bed.
system with a high flow velocity (low
resistance) shows a reduction in pressure with less
narrowing than a system with low flow velocity.
48. Palpable Pulses
Motion of the arterial wall is responsible for the
palpable pulses that are so important in the physical
examination of a patient in whom arterial disease is
suspected.
However, a 7-mm femoral artery in a young subject
would expand only 0.2 mm under the influence of 50
mm Hg pulse pressure, and the stiffer arteries in an
older individual would expand even less.
49. circular cross-section into an ellipse.
It takes much less energy to bend the wall of an
elliptically shaped vessel than it does to stretch the
wall of a circular vessel.
Therefore, when the artery is partially compressed, its
expansion in the direction of the compression is
greatly augmented
50. The clinical value of pulse palpation is based on the
assumption that the strength of the pulse is directly
related to the pulse pressure, which should be
decreased distal to an obstruction.
However, stiff, calcified vessels may display little or no
palpable pulse even though there is no decrease in
pulse pressure.
52. Resistance
fixed , collateral system resistance/segmental resistance.
variable of a peripheral runoff bed resistance.
Normally, the resting segmental resistance is low and
the peripheral resistance is relatively high;
53. Peripheral Runoff Bed
terminal arterioles and precapillary sphincters
Because of their small diameter and heavily muscled
walls, these vessels are ideally suited for regulatory
function.
Their resistance is subject to control by
(1) the autonomic nervous system,
(2) circulating catecholamines,
(3) local metabolic products and
(4) myogenic influences
54. AUTOREGULATION
Autoregulation can compensate for a drop in perfusion
pressure only until it falls below a critical level (e.g.,
about 20 to 30 mm Hg for skeletal muscle ).
With pressures below this level, normal blood flow is
no longer maintained and flow responds passively to
changes in perfusion pressure
55.
56. Exercise Therapy
exercise therapy is best suited for patients with mild,
stable claudication who are not candidates for direct
intervention .
In severe grade patients, the
peripheral resistance has already been lowered to
compensate for the increased segmental resistance,
attempts to further reduce the peripheral resistance
are seldom beneficial.
57. Vasodilators
The rationale for the use of vasodilators is that they
lower peripheral vascular resistance and improve limb
blood flow.
Although this may occur in normal limbs, it is
unlikely to be beneficial in limbs in which peripheral
resistance is already decreased as a result of arterial
disease.
58. Sympathectomy
purpose of sympathectomy is to reduce
peripheral resistance by release of vasomotor tone.
has little or no effect on collateral resistance.
Clinical improvement can occur only if the ischemic tissues
are capable of further vasodilatation, as demonstrated by
reactive hyperemia testing.
59. VASCULAR STEAL
Vascular “steal” may arise when two runoff beds with
different resistances must be supplied by a limited
source of arterial inflow.
When an extraanatomic bypass is performed, a single
donor artery must supply several vascular beds.
60. In the case of a femoral-femoral crossover graft, one
iliac artery is the donor artery, the leg ipsilateral to the
donor artery is the donor limb, and the contralateral
leg is the recipient limb.
Studies of crossover grafts in animal models have
shown that the immediate effect of the graft is to
double flow in the donor artery.
When an arteriovenous fistula is created in the
recipient limb, graft flow may increase by a factor of 10
without any evidence of steal from the donor limb
61. The most important factor contributing to vascular
steal with a femoral-femoral graft is stenosis of the
donor iliac artery.
Occlusive disease in the arteries of the donor limb
distal to the origin of the graft does not result in steal,
provided that the donor iliac artery is normal.
These principles also apply to other types of
extraanatomic bypass grafts, including axillary-
axillary, carotid-subclavian, and axillofemoral grafts.
62. GRAVITY
Many patients with severe lower extremity ischemia
discover that hanging their feet over the edge of the
bed or walking a few steps often provides some relief
from ischemic rest pain.
The improvement in peripheral perfusion that
accompanies dependency can be documented
objectively with measurement of transcutaneous
oxygen tension, which may be increased severalfold
over levels measured when the patient is supine
63. pressure in the dependent arteries, veins, and capillaries is
increased by gravity commensurate with the vertical
distance from the foot to the heart.
Although there is no increase in the arteriovenous pressure
gradient,
the increased hydrostatic pressure dilates capillaries and
microvascular vessels, thereby reducing their resistance, in
turn augmenting blood flow.
Augmentation of blood flow does not occur in
nonischemic limbs because the venoarterial reflex that
serves to constrict arterioles in the dependent position
remains functional
64. Arterial Grafts
Because radius is so important in determining both
viscous and inertial energy losses, the graft selected
should be large enough to carry all the flow required at
rest without causing a drop in pressure
it should also be large enough to accommodate any
increased flow likely to be required during exercise
without an appreciable drop in pressure.
65.
66. Clearly, a graft with an inside diameter of less than 3
mm would be of marginal value at flow rates normally
observed at rest (60 to 150 mL/min) and would be
completely unsatisfactory during exercise (300 to 500
mL/min).
Therefore, under most physiologic conditions, an
aortofemoral graft with 7-mm limbs should suffice,
with flow restricted only during strenuous exercise.
However, 6-mm grafts might begin to show some
restriction of flow even with mild to moderate
exercise.
67. PSEUDOINTIMA
prosthetic grafts develop a thin layer (0.5 to 1.0 mm) of
pseudointima.
Therefore, after implantation, a 6-mm prosthetic graft
might have an internal diameter of only 4 to 5 mm, and a 7-
mm graft might have a luminal diameter of 5 to 6 mm.
For this reason, when one is performing a femoropopliteal
bypass with a prosthetic graft, it is appropriate to select a
graft with an original diameter of at least 6 mm.
Similarly, the original diameter of an aortofemoral graft
limb should be at least 7 mm.
68. the diameter of a graft must not be too much larger
than that of the recipient arteries
thrombus accumulates on the inner walls of grafts with
excessive diameter (much as they do in aneurysms) as
the flow stream tries to approximate the diameter of
the recipient vessels and achieve optimal flow
conditions
69. High flow velocity (high shear) is conducive to the
formation of a thin, tightly adherent pseudointima,
so the diameter of a prosthetic graft should be small
enough to ensure a rapid velocity of flow but large
enough to avoid restriction of arterial inflow
70. SHEAR RATES
Long-term patency of autogenous vein grafts is
compromised by intimal hyperplasia, the development
of which has also been associated with low shear rates.
Low shear rates cause smooth muscle cells to become
secretory and enhance platelet adherence;
high shear rates foster continued patency and lessen
the tendency of the intima to become hyperplastic.
71. Aneurysm rupture and diameter
Tangenital stress and
tensile stress,
Laplace’s law, which
defines tangential
tension (T) as the
product of pressure and
radius
72. Rupture occurs when the tangential stress within the
arterial wall becomes greater than the tensile strength.
The tangential stress (τ) within the wall of a fluid-filled
cylindrical tube can be expressed as:
τ= P r/δ ,
P is the pressure exerted by the fluid,
r is the internal radius,
δ is the thickness of the tube wall.
73. The tendency of larger aneurysms to
rupture is explained by the effect of increased radius
on tangential stress .
The relationship between tangential stress and
blood pressure accounts for the contribution of
hypertension.
74. Posterolateral rupture
55% of ruptured abdominal aortic aneurysms, the site
of rupture is in the posterolateral aspect of the
aneurysm wall.
The posterior wall of the aorta is relatively fixed
against the spine, and repeated flexion in that area
could result in structural fatigue and a localized area
of weakness that might predispose to rupture,.
.
