Blood flow

  • hemodynamics = the dynamics of blood flow
  • blood flow refers to the movement of blood through the vessels (arteries → arterioles → capillaries → venules → veins)
  • blood flow is slowest in the capillaries, which allows time for the exchange of gases and nutrients
  • flow (Q, ml/s) directly depends on the pressure difference between the two ends of the tube and indirectly on the tube resistance:  Q= ΔP/ R
  • pressure (P)
    • pressure is a force that blood exerts against the vessel walls as it moves through the vessels
    • consists of three components:
      • the dynamic component is determined by the cardiac output and peripheral flow resistance
      • the hydrostatic component is determined by the gravity
      • a static component (mean filling pressure) depends on the filling volume and the systemic volume capacity
    • blood flows from a high pressure area to a region with lower pressure
    • very little pressure remains by the time blood leaves the capillaries and enters the venules. Blood flow through the veins is not the direct result of ventricular contraction. Instead, venous return depends on skeletal muscle action, respiratory movements, and constriction of smooth muscle in venous walls
  • resistance (R)
    • a force that opposes the flow of a fluid
    • flow resistance R= 8 – ν – l / r ⁴ . π  directly depends on blood viscosity and artery length; it is also inversely proportional to the fourth power of the artery radius (cross-sectional area) – as vessel diameter decreases, the resistance increases and blood flow decreases
Formula for calculating the flow (Poiseuille law)
  • pulse = the rhythmic expansion of an artery caused by the ejection of blood from the ventricle. Pulse can be palpated on arteries that are close to the surface
  • blood pressure = arterial blood pressure, the pressure in the aorta and its branches
    • systolic blood pressure (SBP) is caused by ventricular contraction
    • diastolic blood pressure (DBP) occurs during cardiac relaxation
    • pulse pressure is the difference between systolic and diastolic blood pressure
    • blood pressure is measured with a sphygmomanometer and is recorded as the systolic pressure over the diastolic pressure
    • 4 significant factors affect blood pressure:
      • cardiac output
      • blood volume
      • peripheral resistance
      • viscosity
    • blood pressure is maintained within normal ranges by changes in cardiac output and peripheral resistance. Baroreceptors in the walls of the large arteries in the neck are important for short-term blood pressure regulation

Character of the flow

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Flow separation

  • flow separation occurs in the areas of arterial branching, aneurysms, atherosclerotic plaques or stenoses or at the site of vessel enlargement
    • in the carotid bulb, the turbulent flow is physiological – there is an increased Reynolds number (> 2000)
  • these zones predispose to early remodeling of the vessel wall and atherosclerotic changes (altered shear stress) [Cunningham, 2005]
Flow separation and shear stress

Waveform

  • the character of the flow depends on several factors:
    • the ejection fraction (EF) of the heart
    • the flow volume and pressure
    • the resistance and geometry of the vascular system
    • the elasticity of the vascular wall
    • blood viscosity
  • the Doppler waveform refers to the morphology of pulsatile blood flow on spectral Doppler ultrasound
    • monophasic – systolic forward flow continuing into diastole, lacking reverse diastolic flow (ICA, MCA, etc.)
    • triphasic (typically in the subclavian artery)  Triphasic flow in subclavian artery
      • systolic forward flow
      • early diastolic flow reversal
      • late diastolic forward flow (slower than in systole)
  • in the Doppler waveform, assess the peak systolic flow velocity (PSV), the peak end-diastolic flow velocity (EDV), and calculate the mean flow velocity (Vmean)
    • acceleration time (AT) / systolic acceleration indicates cardiac contractile force and is influenced by, e.g., proximal stenosis
    • end-diastolic flow represents the magnitude of peripheral resistance (including distal stenosis)
  • resistance and pulsatility index can be calculated from the above-stated values
  • vessels are classified according to the peripheral resistance
    • high resistance limb type (typically ECA)
    • low-resistance parenchymal type (typically ICA, VA)
    • the CCA waveform has an intermediate profile under physiological circumstances, as it depends on peripheral resistance in both intracranial (brain) and extracranial regions (skin, muscles)
Doppler waveform
Normal doppler waveform in extracranial vessels

Resistance and pulsatility index

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Resistance index (RI) and pulsatility index (PI)
Automatic calculation of PI and RI on TCCD

Hemodynamic changes in vascular pathologies and anatomical variants

Variants of normal artery course

  • tortuosity, kinking and coiling
  • maximum flow in the bending is shifted towards the outer wall
  • turbulences may occur → risk of false detection of stenosis
Kinking and coiling in color flow mode

Stenosis

  • the hemodynamic consequences of stenosis:
    • primary (accelerated, turbulent flow)   Stenosis with aliasing and turbulent flow
    • secondary (reduced flow in the pre- and poststenotic segments) Blunted flow in MCA caused by ICA oclusion  V4 segment stenosis of the left vertebral artery with an increased resistance in V2 segment
    • tertiary (collateral circulation)   The right ICA occlusion with collateral supply via ACoA (TCCD)

Steal phenomenon

  • an altered (usually retrograde) blood flow in the vertebral artery (VA) or the internal thoracic artery due to a proximal stenosis/occlusion of the subclavian artery (SA) or brachiocephalic trunk (BCT)
Subclavian steal ultrasound grades
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