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Transcript
What Makes the Blood Go
Round?
William Schroedter, BS RVT RPhS FSVU
Quality Vascular Imaging, Inc
Virtual Vein Center
Disclosures
Co-Owner
Quality Vascular Imaging, Inc
Virtual Vein Center
Background
• Not fully understood
• The hemodynamics of the venous system
• How it accomplishes the return of blood to the heart
• Duplex is the Gold Standard for assessment of venous system
function
• Limited in it’s ability / accuracy in identifying and detecting flow
abnormalities in small veins
• Much system’s hemodynamics unavailable for interrogation
Background (continued)
Reflux within a vein segment
• A surrogate for venous SYSTEM dysfunction
• Utilize reflux provoking maneuvers that do not realistically simulate the
physiological patterns of venous blood flow in a vein segment let alone
the system as a whole.
Objectives
• Attempt to elaborate on what we need to
determine to optimally help the patient
• Provide some “food for thought”
• Neuhardt Axiom
• Raise many more questions than I address
Physiology of the Venous Circulation
Simplistic View
• Return blood to the heart
• Effective venous return requires:
• Central pump
• Pressure gradient
• Peripheral venous pump
• Competent venous valves
• An appreciation of the relationship between volume and
pressure is essential to understanding normal and
abnormal function
What makes the blood go round?
Fluid Movement – Energy / Pressure Gradient
Physiology of the Venous Circulation
Dynamic Venous Pressure
≈ 8 mm/Hg
Reference - Right Atrium
≈ 0-3 mm/Hg
** not so fast – to be re-considered later
Hydrostatic Pressure
Hydrostatic
pressure
Height
in cm
+/- cm
from RA
0
178
+44
153
+21
0
134
0
15
113
-21
32 (leg)
46 (arm)
93
-41
59
57
-77
95
10
-124
Normal venous system
• High capacitance / compliance
• Can accommodate a large change in volume in a short period of time with a small
increase in pressure.
• As dynamic pressures increase, capacitance and compliance decrease
Note vessel “collapse”
PRESSURE
From: Katz AI, et.al. Biophysical J 1969;9:1261-79.14
Peripheral Pumps
• Muscular contraction (systole) has a much greater effect on the deep veins
• Strong muscular contraction results in a high velocity ejection of blood
• High velocity / low pressure (Bernoulli) draws blood from superficial to the deep
system
• During subsequent relaxation
• Competent valves prevent retrograde flow (reflux)
• Generates negative pressure
• Draws blood from the superficial to deep system via perforating veins
Venous Valves
• Remain open while supine at rest
• Two Functions:
• Divide the hydrostatic column of pressure
• Ensure cephalad blood flow
• Direct blood flow INWARD and UPWARD
Venous Pathology
SYSTEM overload
Develops when venous return is impaired for any reason
May be:
• deep
• superficial
• mixed
Principle causes:
• venous outflow (obstruction – thrombotic or not)
• venous inflow (reflux, segmental or axial)
• impaired pump function (calf muscle)
• failure of any part of the circulatory system
What allows reflux to occur?
Reservoir
• A proximal reservoir of blood
• Venous segment with incompetent
valves
• A pool for the blood to flow into
• Pressure gradient
Incompetent
Venous Segment
Pool
Venous Side Pressures
Tend to be ignored because they
are very low
Venous System Function
(Reality)
•
•
•
•
•
•
•
Maintain right atrial filling pressure by controlling venous return
Capacitance – blood volume redistribution
Pressure buffer
Orthostatic tolerance
Post capillary pressure – Controls capillary perfusion pressure
Venules – site of leukocyte adhesion and extravasation
Venous end of capillaries –
• angiogenesis begins
• Site of fluid filtration / reabsorption and molecular exchange
• Heat exchange / thermoregulation
Veins and venous flow is unique
• Vessel collapse – important in venous physiology
• Vascular tone and wall stiffness impacted by these
elastic vessels embedded in tissues – governed by “tube
law”
• Wave speed limitations
• Rhythmic (unsteady) flow velocity and volumes
• high / low velocity, transient, stationary
What makes the blood go round?
The Heart ??
• The primary energy that drives blood flow is ELASTIC RECOIL
produced by volume filling of the vessels
• ≈ 25-30% of blood volume “stretches” the vessel wall – referred to
as stressed volume – only this volume percentage is important for
flow but the remaining volume can be recruited when needed
• Venous resistance ultimately determines cardiac output
Magder S. Scharf SM. Venous Return. In: Scharf SM, Pinsky MR, Magder SA, editors, Respiratory – Circulatory
Interactions in Health and Disease. 2nd ed. New York: Marcel Dekker, Inc 2001. pp.92-112
The classical Guyton view that mean systemic pressure, right atrial pressure, and venous resistance govern
venous return is / in not correct. J Appl Physiol 2006, No;101(5):1523-5.
Permutt S, Riley S. Hemodynamics of Collapsible vessels with tone: the vascular waterfall. J Appl Physiol
1963;18(5):924-32.
What makes the blood go round?
The Heart ??
• Consider a bath tub – the faucet is the LV and the drain the RA
• Flow out of the bathtub is determined by:
• the height of the water
• the drain
• If we increase the volume or pressure of the inlet, the height of the
water can result in increased pressure gradient but does not
significantly increase the volume flow through the system
What makes the blood go round?
The Heart ?? NO??
• Permissive – contraction lowers RA pressure which is the
venous outflow pressure for blood return
• The heart can never generate a pressure or flow higher than the
recoil of the veins to atmospheric pressure
• Restorative – returns blood to the veins and venules so the
elastic recoil force is maintained
Are We Oblivious About
Venous Hemodynamics?
I know I am!
How about you?
William Schroedter, BS RVT RPhS FSVU
Quality Vascular Imaging, Inc
Virtual Vein Center