Hemodynamics & Instrumentation - Video Notes (Vocabulary flashcards)

Hemodynamics: Overview

• Definition: The study of the movements of blood and the forces concerned therein; describes the set of forces or mechanisms involved in regulation of blood flow within the body.

• Energy concepts: Energy is the capacity to do work and overcome resistance; energy cannot be created or destroyed; Energy Conservation – net energy never changes, just changes form.

  • Types of hemodynamic energy:

  • Potential energy (Pressure) – Systolic Pressure

  • Kinetic energy (Velocity) – movement of blood

  • Total Fluid energy = Potential energy + Kinetic energy

  • In the vascular system, most energy is dissipated as heat due to friction.

• Flow basics: Q = V × A (Flow = Velocity × Area). The vascular system attempts to maintain flow.

  • If Velocity increases, Vessel Area has decreased (diameter reduced).

  • If Area increases, Velocity decreases.

  • Flow directions follow pressure gradients: Flow goes from higher pressure to lower pressure.

  • Pressure hierarchy (typical): Left ventricle = highest pressure; Right atrium = lowest pressure.

• Summary concept: Energy conservation with friction leads to dissipation, while the system maintains overall flow through adjustments in velocity and cross-sectional area.

Bernoulli Principle, Continuity, and Flow Relationships

• Bernoulli Principle (applied to vessels): Relationship between energy, velocity, and pressure assuming (approximately) frictionless, constant flow along a streamline.

  • As pressure decreases, velocity increases.

  • As pressure increases, velocity decreases.

  • Interpretations: Pressure is potential energy; Velocity is kinetic energy.

  • Along a narrowing (decreased area): velocity increases and pressure decreases.

  • Along an expanding section: velocity decreases and pressure can rise.

  • Mathematical intuition: Along a streamline, P + rac{1}{2}
    ho v^{2} = ext{constant} (ideal, inviscid flow).

• Continuity Equation (flow conservation):

  • Q = vA or equivalently, velocity and area trade off to conserve flow.

  • Higher velocity implies smaller cross-sectional area; larger area implies lower velocity.

• Practical interpretation: In real vessels, energy is lost to friction/heating, so the equality is an approximation; the net energy is conserved in form, not in magnitude.

Poiseuille’s Law and Flow Determinants

• Poiseuille’s Law (laminar flow in a cylindrical vessel):

  • Q = rac{(P1 - P2)\pi
    ^4}{8\,
    u\,L}

  • Corrected standard form: Q = rac{(P1 - P2)\pi
    ^4}{8\,
    u\,L} where

  • $P1 - P2$ = pressure gradient (gradient driving flow)

  • $
    $ = radius of vessel

  • $
    u$ = dynamic viscosity of blood

  • $L$ = vessel length

• Assumptions (as listed): constant flow rate, constant viscosity, straight rigid vessel.

• Derived relationships:

  • Flow is directly proportional to pressure gradient: larger gradient → more flow.

  • Flow is directly proportional to radius to the fourth power: Q \propto r^4

  • Resistance $R$ in Poiseuille’s context is R = \frac{8\nu L}{\,\pi r^4}, so $R$ is inversely proportional to $r^4$.

• Practical example: Halving the radius results in a 16-fold decrease in flow, illustrating the sensitivity of flow to small radius changes.

Parameter Effects on Flow and Vascular Resistance

• Pressure gradient: directly proportional to flow.

• Velocity: directly proportional to flow.

• Area (cross-sectional area): directly proportional to flow when considering velocity; inversely related to velocity through the Continuity equation.

• Resistance: inversely proportional to flow; in physiological terms, resistance decreases as radius increases.

• Ways to decrease resistance (while keeping other factors constant):

  • Increase radius (dilate vessel)

  • Decrease vessel length (shorter path)

  • Decrease viscosity (e.g., hematocrit changes vary over time; generally, viscosity effects are maintained)

  • Parallel vessels reduce overall resistance vs. series arrangements

• Vessels in series vs parallel:

  • Series arrangement yields higher resistance than parallel arrangements.

  • Overall flow distribution depends on network geometry.

• Summary: Increases in friction/resistance reduce flow; factors that reduce resistance or increase driving gradient promote flow.

Arterial Hemodynamics

• Arterial system characteristics:

  • High-pressure system with mean arterial pressure roughly MAP \approx 70-100 \,mmHg (classic clinical range).

  • Blood flow pattern depends on location, size, and course of vessels.

• Flow patterns in arteries:

  • Laminar flow: most common

  • Plug flow: seen in larger arteries at certain phases of systole

  • Turbulent flow: can occur distal to stenosis or in large vessels; contributes to pressure drop after a stenosis

• Flow patterns quick recap:

  • Laminar: parabolic velocity profile; fastest in the center; slower near walls due to friction; boundary layer is the thin, slow-moving layer near intima.

  • Plug: layers accelerate at similar rates in large arteries during systole.

  • Turbulent: swirling flow, some reverse components; more likely distal to stenosis; contributes to large pressure drops.

• Reynolds number (Re) to predict turbulence:

  • Re = \frac{\rho v D}{\mu} where $\rho$ = density, $v$ = velocity, $D$ = diameter, $\mu$ = viscosity.

  • In most physiological conditions, density and viscosity are relatively constant; turbulence mainly from velocity changes and vessel size.

  • Rule of thumb: Re < 2000 → laminar; Re > 2000 → turbulence.

Waveforms and Arterial Flow Characteristics

• Arterial Doppler waveform characteristics depend on distal arteriolar beds, not the name of the artery.

• High-resistance waveforms:

  • Multiphasic with potential flow reversal due to reflections.

  • Common in peripheral vessels.

• Low-resistance waveforms:

  • Monophasic with one predominant direction of flow during the cardiac cycle.

  • Example: Internal carotid artery (proximal cerebral circulation).

• Systole vs diastole differences:

  • Greater difference implies increased pulsatility or resistance distally.

  • Distal beds: High resistance distally; Low resistance distally in certain vascular beds.

• Post-stenotic consequences:

  • Post-stenotic turbulence occurs at the exit and just distal to a significant stenosis; can have elevated velocity.

• Post-stenotic flow patterns: dampened (parvus tardus) waveform distal to severe disease; delayed upstroke with reduced velocity.

• Velocity graphs: example visual cues include upstroke, peak systole, diastolic troughs, and post-stenotic changes.

Venous Hemodynamics and Transmural Pressure

• Venous system: lower pressure (mean ~5-15 \,mmHg); shape determined by transmural pressure (intraluminal minus interstitial).

• Hydrostatic effects:

  • Hydrostatic pressure (HP) = \rho g h; acts on both arteries and veins; increases with standing.

• Transmural pressure:

  • P{tm} = P{intraluminal} - P_{interstitial}

  • High transmural pressure expands the vessel; low transmural pressure collapses the vessel.

• Respiratory influence on venous pressure:

  • Pressure in chest changes with respiration:

  • Inspiration: chest pressure decreases; abdominal pressure increases; diaphragm descent partly collapses the inferior vena cava (IVC), impeding venous return from the legs; pulmonary venous pooling increases.

  • Expiration: chest pressure increases; abdominal pressure decreases; venous return from legs to heart increases; venous return to thorax decreases.

• Calf muscle pump:

  • Contraction of calf muscles propels venous blood toward the heart; considered the “heart of the venous system” in terms of propulsion.

• Venous valves:

  • Valves throughout the venous system prevent backflow due to gravity; create a holding chamber during venous return.

• Venous waveforms and phasicity:

  • Venous waveforms are phasic with respiration; veins near the heart can be pulsatile.

  • Veins distant from the heart show phasic variations with respiration.

  • Abnormal: lack of phasicity can indicate proximal obstruction.

Hemodynamics: Summary of Key Concepts

• Bernoulli principle ties together pressure and velocity; higher velocity associates with lower pressure, and vice versa, under ideal conditions.

• Poiseuille’s Law links flow to pressure gradient, radius, viscosity, and vessel length; flow is extremely sensitive to radius (fourth power).

• Arterial flow patterns reflect downstream vascular beds and are assessed via waveforms and Doppler velocities.

• Reynolds number predicts laminar vs turbulent flow; turbulence contributes to energy losses and is commonly linked to stenosis.

• Venous hemodynamics are governed by low pressures, hydrostatics, and mechanical aids to return (respiration, calf pump, valves).

• Transmural pressure and hydrostatic effects critically influence venous return and vessel caliber.

• Instrumentation and artifacts in Doppler imaging can affect interpretation and require understanding of controls and common artifacts.

Instrumentation and Doppler Principles

• Doppler shift basics:

  • Doppler shift is the change in frequency due to moving reflectors (RBCs) observed by a stationary transducer.

  • Difference between transmitted frequency and received frequency is the Doppler shift.

  • Positive shift: flow toward the transducer; Negative shift: flow away from the transducer.

  • Most accurate when angle = 0°; Doppler shift is zero at 90°; never exceed 60° in practice.

  • The Doppler shift equation (simplified form):\Delta f = \frac{2 f_0 v \cos\theta}{c} where

  • $f_0$ = transmitted frequency,

  • $v$ = blood velocity,

  • $\theta$ = angle between flow direction and ultrasound beam,

  • $c$ = speed of sound in tissue.

• Pulse-wave Doppler (PW):

  • A pulse is transmitted and returns; provides depth-specific information.

  • Pulse Repetition Frequency (PRF) controls the sampling rate; depth selection introduces trade-offs.

  • Common artifact: aliasing when Doppler shift exceeds \frac{PRF}{2} (Nyquist limit).

  • Aliasing elimination strategies: increase PRF, decrease Doppler shift, or adjust baseline.

• Controls and typical settings:

  • PW Doppler: PRF/Scale, Gate and Sample Volume, Spectral Gain, Baseline, Angle Correction.

  • Color Doppler: PRF/Scale, Gate and Sample Volume, Color Gain, Packet Size, Persistence, Priority.

• Color Doppler concepts:

  • Displays velocities with color pixels; colors (blue/red) show direction relative to transducer.

  • Aliasing can occur with color Doppler in stenotic regions.

• Artifacts (common categories):

  • Reverberation (spurious echoes)

  • Shadowing (from bone, calcifications, or acoustic impedance)

  • Partial volume artifact (slice-thickness issues): echoes/doppler signals from part of the slice thickness can be misattributed; explains why some signals appear in or near anechoic structures.

  • Clutter/Wall motion artifacts

  • Motion artifacts

  • Color Bleeding/Blooming (color spill-over)

  • Mirror Image artifacts (phantom duplication across strong reflectors)

Clinical Flow Patterns: Lab vs Clinical Application

• In lab-like analysis (idealized): velocity and pressure may appear constant or simplified; clinical vascular imaging emphasizes the impact of area changes, pulsatility, and complex flow paths.

Post-Stenotic and Waveform Changes with Disease

• Post-stenotic turbulence: turbulence downstream of a stenosis; can have elevated velocities and disturbed flow patterns.

• Dampened (Tardus-Parvus) waveform: monophasic, delayed upstroke, reduced peak velocity distal to a significant stenosis.

Quick Reference: Numerical and Conceptual Notes

• Flow and energy:

  • Q = vA

  • P + \frac{1}{2}\rho v^{2} = \text{constant} (Bernoulli, ideal case)

  • E = P + \frac{1}{2}\rho v^{2} (total fluid energy per unit volume)

  • HP = \rho g h (Hydrostatic pressure)

• Vascular resistance and flow:

  • Q = \dfrac{(P1 - P2)\pi r^{4}}{8\eta L}

  • R = \dfrac{8\eta L}{\pi r^{4}}; radius has a fourth-power effect on flow

  • Q \propto r^{4}; halving $r$ → flow reduces by factor 16

• Reynolds number:

  • Re = \dfrac{\rho v D}{\mu} (or using $v$ and $D$)

  • $Re < 2000$ laminar; $Re > 2000$ turbulence

• Transmural pressure:

  • P{tm} = P{intraluminal} - P_{interstitial}

• Doppler physics:

  • \Delta f = \dfrac{2 f_0 v \cos\theta}{c}

  • Angle considerations: best near 0°, zero at 90°, never > ~60° in practice

  • Aliasing: occurs when Doppler shift exceeds \frac{PRF}{2}; mitigated by higher PRF, lower Doppler shift, or baseline adjustments

Course Logistics (Context from Transcript)

• Course structure: 30% quizzes (based on reading and class lecture; quizzes at the beginning of most classes)

• 60% exams: midterm and final

• 10% presentation assignment

• Instructor: Adam Olsen MS RVT RDMS; Director of Radiology – Capital Health; Course Instructor at TJU since 2017; TJU graduate 2009; SIU graduate 2025

• Student count (as per transcript): 17 students

Summary and Practical Implications

• Understanding the interplay between pressure, velocity, and cross-sectional area is essential to interpret Doppler signals and waveforms.

• Poiseuille’s law emphasizes how small changes in radius dramatically affect flow, making stenosis a critical determinant of downstream hemodynamics.

• Recognition of flow patterns and waveforms helps differentiate normal physiology from pathology (high vs low resistance beds, post-stenotic changes, tardus-parvus, and dampened flows).

• Venous hemodynamics require integrating hydrostatic effects, respiration, calf pump function, and valves to understand venous return.

• Mastery of Doppler physics and artifacts is essential for accurate image interpretation and avoiding misdiagnosis.