Understanding Blood Flow

• The study of blood flow is essential for medical students due to its relevance to hemodynamics, cardiovascular physiology, and clinical conditions such as hypertension and atherosclerosis
• Biophysics helps to measure, affect and explain how blood moves
• Blood flow characteristics are examined in blood vessels with Doppler ultrasound, rheological properties of liquids and biological materials
• A Doppler ultrasound examination of the carotid arteries measures blood flow velocity curve (time waveforms), mean (V av) and maximum flow velocity (V max), and pulsation (PI) and resistance (RI) indices

Types of Blood Flow

• Blood flow in vessels varies based on vessel diameter, velocity, and pressure
• Laminar refers to normal blood flow where blood moves in parallel layers, with the fastest flow in the center and slowest near the vessel walls due to friction
• Poiseuille's Law relates to laminar flow
• Laminar flow is silent and energy-efficient
• Turbulent flow is abnormal or high-velocity flow where blood moves in multiple directions, creating vortices
• Turbulent flow occurs when flow velocity increases or vessel walls become irregular due to stenosis, aneurysm or arterial plaque
• Turbulent flow produces murmurs in arteries, increases energy loss and stress on vessel walls, and can lead to vascular damage and clot formation
• Reynolds number (Re) determines if flow is turbulent; Re > 2000 indicates turbulence
• Pulsatile flow is unsteady blood flow that pulses due to the beating of the heart and is found in large arteries like the aorta
• Single-file flow happens in capillaries, where red blood cells move one at a time due to their small diameter

Blood Flow Measurement & Biophysical Principles

• Poiseuille's Law explains how blood flow (Q) depends on vessel radius (r), length (L), and blood viscosity (η)
• Flow is directly proportional to the fourth power of the radius, meaning a small increase in radius leads to a large increase in flow
• Flow is inversely proportional to viscosity and vessel length
• Reynolds number (Re) determines if blood flow remains smooth (laminar) or becomes turbulent
• If Re < 2000, flow is laminar
• If Re > 2000, flow is turbulent, indicating a risk of vascular damage
• Bernoulli's principle states that as the velocity of blood increases, pressure decreases
• Narrowed arteries (stenosis) cause blood velocity to increase, but pressure drops, which may lead to ischemia
• Laplace's Law describes the tension (T) on vessel walls
• Large arteries (like the aorta) experience high wall tension, making them more prone to aneurysms
• Veins (low pressure) require valves and muscle contraction to maintain blood flow

Methods to Examine Blood Flow in Medicine

• Doppler Ultrasound uses the Doppler effect to measure blood flow velocity and direction
• Doppler Ultrasound detects narrowed arteries, venous thrombosis, and fetal circulation issues
• Plethysmography measures blood volume changes in limbs to evaluate peripheral arterial disease (PAD)
• Angiography uses X-rays with contrast dye to visualize arteries
• Angiography detects blockages, aneurysms, and vascular malformations

Clinical Applications of Blood Flow Biophysics

• Hypertension (High Blood Pressure) results in increased pressure and more wall stress (Laplace's Law)
• Long-term effects of high blood pressure include heart disease, stroke and kidney failure
• Atherosclerosis (Narrowed Arteries) leads to increased resistance (Poiseuille's Law) and turbulent flow (Reynolds Number), resulting in bruits
• Aneurysms (Weakened Artery Walls) cause increased radius, leading to increased tension (Laplace's Law), resulting in a high risk of rupture if tension exceeds vessel strength
• Deep Vein Thrombosis (DVT) results from slow flow (stasis) promoting clot formation
• The risk of embolism from DVT can lead to pulmonary embolism (PE)

Key Takeaways

• Blood flow is affected by vessel radius, pressure, viscosity, and resistance
• Poiseuille's Law shows, small radius changes greatly affect flow
• Turbulence (high Reynolds Number) occurs in narrowed arteries and aneurysms
• Laplace's Law explains vessel wall stress, important in aneurysms
• Vascular diagnostic tools such as, Doppler ultrasound, angiography and blood pressure cuffs assess blood flow

Doppler Ultrasound in Biophysics

• Understanding blood flow, viscosity, and mechanical properties of biological materials is crucial in diagnosing circulatory disorders, understanding tissue mechanics, and designing biomedical applications
• Doppler ultrasound is a non-invasive imaging technique that uses sound waves to measure blood flow velocity and detect abnormalities in vessels
• The Doppler effect occurs when sound waves are reflected off moving objects (e.g., red blood cells) and this causes a frequency shift
• d is Doppler shift (change in frequency), v is blood flow velocity, f0 is transmitted ultrasound frequency, θ is the angle between ultrasound beam and blood flow and c is the speed of sound in blood (~1540 m/s)
• If blood moves toward the probe, frequency increases (positive shift)
• If blood moves away from the probe, frequency decreases (negative shift)

Types of Doppler Ultrasound

• Continuous Wave Doppler (CWD) uses two transducers (one sends, one receives), measures high-velocity flow (e.g., severe stenosis), but has no depth resolution
• Pulsed Wave Doppler (PWD) sends short pulses and receives echoes, measures velocity at a specific depth
• Pulsed Wave Doppler (PWD) is used in cardiac and vascular studies
• Color Doppler assigns colors to blood flow direction (Red = toward probe, Blue = away) and helps detect blockages, turbulence, or reversed flow
• Power Doppler detects low-velocity flow, such as in small capillaries and is useful for organ perfusion studies

Clinical Applications of Doppler Ultrasound

• Doppler Ultrasound detects stenosis (narrowing of arteries)
• Doppler Ultrasound identifies thrombosis (blood clots)
• Doppler Ultrasound assesses fetal blood circulation
• Doppler Ultrasound evaluates valvular heart disease

Rheological Properties of Liquids & Biological Materials

• Rheology is the study of flow and deformation of materials, including blood, mucus, and synovial fluid

Blood as a Non-Newtonian Fluid

• Blood is shear thinning, meaning viscosity decreases at high shear rates (fast flow in arteries)

Factors Affecting Blood Viscosity

• Hematocrit (Hct): Higher hematocrit increases viscosity (polycythemia) and lower hematocrit decreases viscosity (anemia)
• Plasma Proteins: Fibrinogen increases viscosity contributing to clot formation
• Temperature: Lower temperatures increase viscosity, which can affect circulation in hypothermia
• Shear Rate: Higher shear rate lowers viscosity (flowing blood in arteries) and lower shear rate increases viscosity (slow-moving blood in veins)

Clinical Importance of Blood Rheology

• Thrombosis (Clots): High viscosity increases clot risk
• Atherosclerosis: Stiff arteries alter blood flow dynamics
• Shock States: Blood viscosity decreases in severe blood loss

Rheology of Other Biological Materials

• Synovial Fluid (Joint Lubricant) is Non-Newtonian, becoming less viscous with movement and provides joint lubrication and shock absorption
• Osteoarthritis reduces synovial fluid quality, increasing friction
• Mucus (Respiratory & Digestive System) is Viscoelastic
• Mucus protects airways, but in cystic fibrosis, mucus becomes too thick
• Cerebrospinal Fluid (CSF) has Near-Newtonian properties that maintains brain buoyancy and shock absorption

Performing a Doppler Ultrasound Examination of the Carotid Arteries

• Carotid artery Doppler ultrasound is a non-invasive diagnostic tool to assess blood flow characteristics, detect stenosis, and evaluate vascular health
• Carotid artery Doppler provides real-time information on blood velocity, resistance, and pulsatility using biophysical principles

Doppler Effect in Ultrasound

• Doppler ultrasound measures blood flow velocity based on the Doppler effect
• The Doppler effect states the frequency of sound waves changes when reflected by a moving object, like red blood cells
• fd is Doppler shift (change in frequency), v is blood velocity, f0 is transmitted ultrasound frequency, θ is the angle between ultrasound beam and blood flow, c is speed of sound in blood (~1540 m/s)

Steps in Performing a Carotid Doppler Examination

• Patient Preparation involves the patient lying supine with the neck slightly extended and rotated away from the examined side, followed by the application of gel to the neck for ultrasound transmission
• Probe Placement & Imaging uses a linear array transducer (5–10 MHz) being placed over the carotid artery in longitudinal and transverse planes
• Common Carotid Artery (CCA) lies below the bifurcation
• Internal Carotid Artery (ICA) supplies the brain and has a low resistance flow
• External Carotid Artery (ECA) supplies the face and has a high resistance flow
• Doppler angle should be ≤ 60° to avoid errors during Doppler Measurements with spectral waveform analysis being performed

Blood Flow Velocity Curve (Time Waveform Analysis)

• Blood flow velocity changes over time, forming a characteristic curve on the Doppler waveform

Key Components of a Blood Flow Velocity Curve

• Systolic Peak (Vmax) is the maximum blood velocity during heart contraction (systole)
• Vmax is higher in stenosed ateries
• Diastolic Flow (Vav) is the average flow velocity during the cardiac cycle
• Vav is higher in low-resistance arteries (e.g., ICA)
• Flow Envelope outlines the maximum velocities at each time point and is important in detecting turbulence
• Window & Spectral Broadening has both Clear window = Laminar flow (healthy arteries) qualities and Filled window = Turbulent flow (arterial disease) qualities

Velocity & Indices: Biophysical Analysis

• Doppler ultrasound calculates vascular health using Mean Velocity (Vav), which is the average blood velocity during the cardiac cycle
• Maximum Velocity (Vmax) is the highest peak systolic velocity (PSV)
• High Vmax suggests stenosis
• Pulsatility Index (PI) measures vascular resistance and flow pulsatility and can indicate increased resistance from high or low PI's
• Resistance Index (RI) indicates vascular resistance and can indicate increased or decreased resistance (atherosclerosis, hypertension. )

Biophysical Factors Affecting Doppler Ultrasound Measurements

• Laminar Flow shows a normal, smooth velocity curve
• Turbulent Flow demonstrates an irregular waveform, seen in stenotic arteries
• A narrowed artery ( ↓ r) increases velocity (↑ Vmax) and turbulence

Final Thoughts

• Doppler ultrasound visualizes blood flow in real time, aiding assessment of vascular conditions
• Interpretation is governed by biophysical principles, such as, Poiseuille's Law, Reynolds Number, and the Doppler effect
• PI and RI help to evaluate vascular resistance and detect diseases like atherosclerosis and stenosis

How Many Frequencies Are Used in Medical Ultrasound?

• Medical ultrasound operates in a range of 2 MHz to 15 MHz depending on the application
• Lower frequency provides greater depth but poorer image quality, due to less attenuation
• Higher frequency offers better resolution but shallower penetration, attributed to more attenuation

What Is the Doppler Effect and What Does It Mean?

• The Doppler Effect defines the change in frequency of a wave due to the motion of a source or observer
• In Doppler ultrasound, sound waves reflect off moving red blood cells, resulting in a frequency shift
• Positive Doppler Shift (↑ Frequency) show blood moves toward the probe
• Negative Doppler Shift (↓ Frequency) show blood moves away from the probe
• High Doppler Shift = High Velocity suggests potential stenosis/narrowing of the artery

What Is an Echo?

• An echo is a reflected sound wave returning to the ultrasound probe after bouncing off a tissue or structure
• Different tissues reflect sound waves at different intensities based on their density and Impedance
• High impedance mismatch (bone, gas) results in strong reflection/hyperechoic visuals
• Low impedance mismatch (fluid, soft tissue) results in weak reflection/hypoechoic visuals

Clinical Examples of echos

• Clinical examples are Hyperechoic echos Bone, soft tissue or Anechoic

What Is Pulsatility Index (PI)?

• Pulsatility Index (PI) measures the degree of pulsation in blood flow and helps assess vascular resistance
• High PI happens from increased resistance due to stiff vessel walls or blockages
• Low PI happens from decreased resistance, seen in low-resistance vascular beds (e.g., brain, kidneys)

What Is Resistance Index (RI) in Connection to Biophysics?

• Resistance Index (RI) quantifies vascular resistance and is used to assess blood flow resistance in arteries
• High RI happens from More energy being lost in friction (Poiseuille's Law) and less blood flow in diastole
• Low RI enables continuous blood flow, which is common in organs needing constant perfusion (brain, kidneys)