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Vascular Ultrasound

Vascular Ultrasound. Fluid Hemodynamics. Fluid Hemodynamics. Blood flow influenced by Cardiac function Elasticity of the vessel Tone of vascular smooth muscle Dimension, pattern and interconnection of branching vessels. Fluid Hemodynamics.

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Vascular Ultrasound

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  1. Vascular Ultrasound Fluid Hemodynamics

  2. Fluid Hemodynamics Blood flow influenced by • Cardiac function • Elasticity of the vessel • Tone of vascular smooth muscle • Dimension, pattern and interconnection of branching vessels.

  3. Fluid Hemodynamics • In order for blood to flow between two there has to be a difference in the energy level between these two points. • This difference in energy level is translated in a difference of pressure.

  4. Circulatory System Arterial Reservoir: - High energy - High pressure Venous Pool - Low energy - Low pressure

  5. Energy Level The large volume of blood entering the arterial reservoir is responsible for the high energy level.

  6. The Heart as a Pump The function of the heart and blood vessels is normally regulated to maintain volume and pressure in the arteries within the limits required for smooth function.

  7. The Heart as a Pump To achieve this there must be a balance between the amount of blood that enters and the amount of blood that leaves the arterial reservoir

  8. Arterial Reservoir • Amount of blood entering – depends on the CO • Amount of blood leaving – depends on the AP and the TPR

  9. Total Peripheral Resistance • Controlled by the amount of vasoconstriction in the microcirculation.

  10. Determinant of Blood Flow Flow to all the body tissues is adjusted according to the tissues’ particular need at that instance. • High need – Vasodilatation • Low need - Vasoconstriction

  11. Energy in Flowing Blood Main form is Potential Energy • Created by the pumping action of the heart. • Pressure distending the vessel.

  12. Kinetic Energy • Small compared with PE • Proportional to its density and square of its velocity.

  13. Kinetic Energy Increases • High flow states (exercise) • Stenotic lesion

  14. Energy/Pressure Differences Hydrostatic Pressure • Pressure that depends on the weight of column of blood resting on the blood at that level. • Increases in the more dependent part of the body.

  15. Hydrostatic Pressure As the hydrostatic pressure increases: • The transmural pressure increases • The vessel become more distended.

  16. Gravitational Potential Energy • Gravitational PE is reduced in the dependent part of the body by the same amount of the increase, resulting from the hydrostatic pressure.

  17. Fluid Hemodynamics • Therefore differences in the level of the body parts usually do not lead to changes in the driving pressure along the vascular tree unless the column of blood is interrupted.

  18. Laminar Flow Flow in which blood moves in concentric layers.

  19. Parabolic Velocity Profile • Laminar flow • Highest velocity is in the middle of the vessel • Gradual decrease in velocity as you move towards the walls of the vessel.

  20. Parabolic Velocity Profile • The rate of change of velocity is great near the vessel wall and decreases towards the center • Mean velocity is half the maximum velocity.

  21. Frictional Losses Loss of energy during blood flow is: • Due to friction • Is determined by the vessel dimension.

  22. Poiseuille’s Law and Equation Describes flow in a cylindrical tube model. The mean linear velocity of laminar flow is • Proportional to the pressure difference between the end of the tube.

  23. Poiseuille’s Law and Equation • Proportional to the radius squared. • Inversely proportional to the length of the tube. • Inversely proportional to the viscosity of the fluid

  24. Poiseuille’s Law and Equation V α(P1- P2) x r2 L h V = Mean linear velocity Q = V x CSA = V x pr2

  25. Poiseuille’s Law and Equation Q a (P1- P2) x r2 x pr2 = p (P1- P2) x r4 L h 8Lh 8Lh = P1- P2 pr4 Q

  26. Poiseuille’s Law and Equation R = P1- P2 Q Q = P1- P2 R

  27. Resistance Vessel Interconnection Resistance to flow is influenced by the presence of numerous interconnected vessels.

  28. Resistance Vessels in series: RT = R1 + R2 + R 3 + …………….. Rn

  29. Resistance Vessels in parallel: 1 = 1 + 1 + 1 + ………. 1 RT R1 R2 R3 Rn

  30. Resistance Thus the contribution of any single vessel to the total resistance of a vascular bed, or the effect of a change in the dimension of a vessel, depends on the presence and relative size of the other vessels linked in series or parallel.

  31. Non Laminar Flow Occurs due to • Changes in flow velocity during the cardiac cycle. • Alteration in lines of flow whenever vessel changes dimensions. • Distortion of line of flow at curves, bifurcation and branches

  32. Turbulence The factors that affect the development of turbulence are expressed by the dimensionless Reynolds number (Re): Re = vq2r h h = viscosity q = density r = radius of the tube v = velocity

  33. Turbulence • In the tube model turbulence occurs at Re > 2000 • In the circulatory system, turbulence occur at Re < 2000 due to: • Body movement • Pulsatile nature of the blood • Changes in vessel dimension • Roughness of the endothelial surface.

  34. Pressure Wave • Ventricular contraction leads to - stroke volume. • Stroke volume leads to - pressure wave.

  35. Pressure Wave • The pressure waves changes as it traverses the arterial system. • There are changes in its: • Shape • Speed • Amplitude

  36. Ventricular Contraction • 2 Phases: • Rapid phase • Late Phase

  37. Ventricular Contraction Rapid Phase: • Outflow through the peripheral resistant vessel is less than volume ejected by the heart. • Increased volume in the arterial end. • Increased pressure to systolic peak.

  38. Ventricular Contraction Late Phase • Decrease in cardiac ejection • Outflow through the peripheral resistant vessel is greater than volume ejected by the heart. • Decrease in pressure.

  39. Ventricular Contraction Cardiac contraction leads to: • Forward flow • Distension of the arteries

  40. Ventricular Contraction • The distention of the arteries serve as a reservoir for storing blood volume and energy. • This is responsible for the continuous flow to tissue during diastole.

  41. Pressure Wave The shape and amplitude are affected by • Stroke volume • Time course of ventricular ejection • Peripheral resistance • Stiffness of the arterial wall

  42. Pressure Wave An increase in any of these factors will lead to an increase in the pulse amplitude and the systolic pressure

  43. Propagation Speed Depends on • Stiffness of the arterial wall • Ratio of the wall thickness to diameter.

  44. Propagation Speed • In circulation, the arteries become progressively stiffer from aorta toward the periphery. • Therefore the speed of propagation increases as it moves peripherally.

  45. Fluid Hemodynamics

  46. Pressure Wave • Changes in pressure as wave travels from aorta to small limb arteries. • Slight decrease in MAP • Minor changes in DAP • The amplitude and SAP increases (Systolic amplification).

  47. Pressure Wave Systolic Amplification is due to • Increase stiffness • Presence of reflected waves: - changes in vessel diameter - dividing branches

  48. Pulsatility Changes Pulsatility changes Small and Medium arteries • Vasoconstriction leads to increased pulsatilty. • Vasodilatation leads to decreased pulsatility.

  49. Pulsatility Changes Minute arteries, arterioles and capillaries • Vasoconstriction leads to decreased pulsatilty. • Vasodilatation leads to increased pulsatility.

  50. Flow Reversal Reversal of flow due to • Pressure gradient • Arterial branches supplying both high and low resistance vascular area.

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