Unit 18: Hemodynamics

Introduction to Hemodynamics

• Study of blood movement through the circulatory system.
• Key to understanding Doppler Ultrasound.

Key Concepts in Hemodynamics

• Volume Flow Rate:

  • Measures how much blood moves through a point in the circulatory system.
  • Expressed as volume over time, e.g., gallons per second.

• Velocity:

  • Measures how fast blood flows with direction.
  • Expressed as distance over time, e.g., cm/s.

Section 18.1: Flow of Fluids

• Vessels act like pipes with blood flowing through them.
• Heart acts as a pump.
• Blood flow affected by extrinsic physical forces (pressure, resistance, size of vessel).

Key Terms:

• Viscosity:

  • Resistance of a fluid to flow; describes thickness.
  • Expressed in poise.
  • Blood is ~5 times thicker than water. Conditions like anemia and polycythemia affect viscosity.

• Pressure:

  • Driving force behind fluid flow.
  • Measured in force per unit area (e.g., Pascals, psi).
  • Difference in pressure needed for flow (pressure gradient).

• Volumetric Flow Rate (Q):

  • Amount of blood that passes a point over time.
  • Measured in ml/min or ml/s.
  • Related to cardiac output (~5000 ml/min in adults).

Formula for Flow Rate:

• Q = ΔP / R
  • ΔP: Change in pressure across the vessel.
  • R: Resistance (measured in poise).

Resistance Formula:

• R = (8 × Length × Viscosity) / (π × Radius⁴)
  • Resistance affected by length, viscosity, and radius of the vessel.

Section 18.2: Types of Flow

• Laminar Flow:

  • Streamlines are parallel.
  • Includes plug, parabolic, and disturbed flow.

• Turbulent Flow:

  • Chaotic flow with eddies and vortices.
  • Occurs after areas of high velocity, such as stenosis.

• Reynolds Number:

  • Predicts flow type (laminar vs turbulent).
  • <1500 indicates laminar, >2000 indicates turbulent.

Section 18.3: Energy

• Conservation of Energy:
  • Energy can't be created or destroyed, only transformed.

Types of Energy Loss:

• Viscous Loss:

  • Overcoming fluid's own stickiness.

• Frictional Loss:

  • Heat loss due to friction with vessel walls.

• Inertial Loss:

  • Kinetic energy loss due to changes in flow direction.

Effects of Stenosis:

• Changes direction of flow.

• Increases velocity within narrowing.

• Causes turbulent flow distal to stenosis.

• Creates pressure gradient (decrease in pressure).

• Loses pulsatility approaching stenosis.

• Bernoulli's Principle:

  • Explains pressure and velocity changes in stenosis.

Section 18.4: Hydrostatic Pressure

• Describes relationship between blood weight, gravity, and height.
• Height, gravity, and density increase hydrostatic pressure.
• Heart level as baseline (0 mmHg) for measuring pressure.

Effects on Blood Pressure:

• Hydrostatic pressure affects measured pressure.
• Supine position = 0 hydrostatic pressure throughout body.
• Standing affects pressure readings above/below heart level.

Section 18.5: Vessel Considerations

• Anatomy of vessels (arteries vs veins).

• Vasoconstriction/Vasodilation:

  • Affects flow rate by changing vessel diameter.

• Venous Flow:

  • Affected by respiration and muscle pumps.
  • Valves prevent backflow.

Effects of Respiration:

• Inhale: Diaphragm down, decreases thoracic pressure, increases abdominal pressure.
• Exhale: Diaphragm up, increases thoracic pressure, decreases abdominal pressure.

Conclusion

• Hemodynamics essential for understanding blood flow and pathology.
• Study of hemodynamics aids in diagnosing using ultrasound physics.