Hemodynamics 101:
What waveforms tell us
Steve Knight BSc RVT RDCS
Disclosures:
• I will be discussing some physics
Pressure and Flow
• All fluids flow down a pressure gradient
– High pressure to low pressure
Blood Pressure and Pulse Pressure
• Difference between these systolic and diastolic pressures
Pressure pulse = Psystolic - Pdiastolic
Question
• If blood flows from high to low pressure why is the pressure we
measure higher at the ankle than the arm?
• Cue Jeopardy music:
Question
• If blood flows from high to low pressure why is the pressure we
measure higher at the ankle than the arm?
• Cue Jeopardy music:
Pulse pressure
Pulse pressure
Pulse pressure
Mean Arterial Pressure
(MAP)
• Estimated by formula
MAP ≈ DP + 1/3 PP
Elastance varies with location
Wave reflections occur at
level of muscular arteries
From Bortolotto and Safar
Arquivos Brasileiros de Cardiologia - Volume 86,
Nº 3, March 2006
Why does the pulse pressure widen further from
the heart?
• (All) waves reflect when they encounter a change in
impedance (diameter, stiffness, wall thickness)
– Constructive interference: pressure augmentation
• MAP doesn’t differ significantly between heart & ankle
Windkessel effect
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Not to be confused with the Phil Kessel effect (Boston joke)
Otto Frank (German physiologist)
Translation: “Wind tank”
Code name for your mother in law
Water pump in house on well water.
– Provides constant flow of when pump off
• Bag pipes are a Windkessel
Windkessel
From: Introduction to Vascular Ultrasonography, 6th Edition
Pellerito & Polak
Blood Flow Velocity
• Does the opposite of a pressure wave
• Has viscosity and inertia (pressure waves do not)
• Blood slows down due to viscosity (friction) and flow
disturbances
Pressure & Flow with distance from heart
Location
Typical velocity
Aortic valve
140 cm/s
Femoral artery
80-100 cm/s
Popliteal artery
60-80 cm/s
Tibial artery
40-60 cm/s
Strandness and Sumner 1975
Waveform morphology
• Reflected wave pushes back on the oncoming wave of blood
• The waveform we look at in the CFA is influenced by a wave
that has already been to the ankles
Don’t forget the Windkessel
• It’s still supplying energy
• Blood momentarily slowed or reversed by reversal of pressure
gradient
• This processes may be repeated depending on factors until the
Windkessel is empty or next cardiac cycle
• Explains dicrotic notch, bi & multiphasic waveforms
• Controlled by resistance vessels
Name components of waveform affected by
viscosity:
Name components of waveform affected by
viscosity:
Name components of waveform affected by wave
reflection:
Name components of waveform affected by wave
reflection:
Dichrotic notch or
reflected wave
Name components of waveform affected by
Windkessel:
Name the components of waveform affected by
Windkessel:
Forward flow after peak
systole
Continuity Equation
(What goes in must come out Equation)
A1V1=A2V2
Velocity will change with diameter
(e.g. ectasia, graft)
Oversimplified hemodynamics of stenosis
blood is non-Newtonian (doesn’t follow the rules)
• In turbulent flow:
– Flow is chaotic not laminar
– energy is expended to overcome inertia & get blood travelling in the
correct direction.
Peripheral Doppler Waveform Morphology
• How does all this affect what we see in a Duplex exam?
• Each Doppler waveform is an instantaneous editorial on the
past, present and future of the blood flow we are studying in
our sample volume
The Past
• Everything that has happened before the blood reaches your
sample volume
– Cardiac output
– Heart rate
– Proximal stenosis/collateral pathways
– Compliance of proximal arteries
– Aortic insufficiency (aortic valve doesn’t close tightly)
The Present
• Velocities relative to the adjacent segment
• Normal
– no stenosis
• Reduced:
– increase of cross sectional area (aneurysm, ectasia)
– increased compliance (stretchiness)
• Elevated:
– decrease of cross sectional area
– Decreased compliance (stent/graft material, artery wall composition)
The Future
• Distal resistance to flow
• Vascular tone in high resistance beds
• Vasodilation
– Increased demand
– Can be other reasons than ischemia
• Inflammation (e.g. rheumatoid arthritis)
• Infection (e.g. osteomyelitis)
• Sympathetic tone
“Normal”
• Flow to organs is low resistance
– Organs perfused throughout cardiac cycle
• Flow to muscles at rest high resistance (if happy)
Doppler Waveform Morphology
• Can be hybrid of 2 separate beds
Past
What do you expect a
Doppler waveform here
will look like?
Delayed time to peak
Direction of flow
Retrograde flow
The Present
• Velocity dictated by Poiseuille's Law if there is laminar flow
• Turbulence causes energy losses that exceed those predicted
by Poiseuille’s Law
Critical stenosis
• Velocity dictated by Poiseuille's Law if there is laminar flow
• Turbulence causes energy losses that exceed those predicted
by Poiseuille’s Law
Critical stenosis
• results in a significant reduction in maximal flow capacity
• typically in the 60-80% range
The Present
• Velocity (diameter of vessel)
• Direction (collateral pathway)
• Spectral broadening
The Future
• Low resistance implies vasodilation
• In low resistance there is less or no reflected pressure wave
• Low resistance does not = ischemia
– Carotid waveform is low resistance!
• Hyperemia controlled locally
– Metabolic demand
– Infection/inflammation
– Temperature
– Sympathetic tone
Sympathetic Tone
AVF
Arteriovenous fistula
AVF S/P renal biopsy
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