Hemodynamics Notes

Hemodynamics

What is Hemodynamics?

• The study of blood moving through the circulatory system.
• Flow: The volume of blood moving at a particular time.
• Velocity: The speed of a fluid moving from one location to another.
• Three basic forms of blood flow:
  • Pulsatile
  • Phasic
  • Steady

Types of Blood Flow

Pulsatile Flow

• Occurs when blood moves with a variable velocity.
• Blood accelerates and decelerates as a result of cardiac contraction.
• Example: arterial blood flow.

Phasic Flow

• Fluid also moves with a variable velocity as a result of respiration.
• Example: venous blood flow.

Steady Flow

• Fluid moving at a constant speed/velocity.
• Present in venous circulation when individuals stop breathing for a brief moment.
• Example: Water flowing through a garden hose.

Laminar vs. Turbulent Flow

Laminar Flow

• Flow streamlines are aligned and parallel.
• Characterized by layers of blood that travel at individual speeds.
• Commonly found in normal physiologic states.
• Two forms:
  • Plug
  • Parabolic

Plug Flow

• Occurs when all of the layers and blood cells travel at the same velocity.

Parabolic Flow

• Has a bullet-shaped profile.
• Velocity is the highest in the center of the vessel lumen and gradually decreases to its minimum at the vessel wall.
• Both plug and parabolic patterns are associated with normal physiology.

Reynolds Number

• A unitless number indicating laminar or turbulent flow.
  • Less than 1500: Laminar flow
  • Greater than 2000: Turbulent flow
  • Between 1500 and 2000: Unknown

Turbulent Flow

• Chaotic flow in many directions and speeds.
• Flow varies from instant to instant and from location to location.
• Small, hurricane-like, rotational patterns appear (eddy current or vortex).
• Often associated with cardiovascular pathology and elevated blood velocities (e.g., stenoses).
• Converts flow energy into other forms such as sound and vibration.

Sound and Tissue Vibration

• Sound associated with turbulence is called a murmur or bruit.
• Tissue vibration associated with turbulence is called a thrill.
• A thrill is also described as a palpable murmur, which you can feel with your fingertips.

Energy Gradient

• Blood moves from regions of higher energy to lower energy.
• Energy is imparted to blood by the contraction of the heart.

Forms of Energy

Kinetic Energy

• Associated with a moving object.
• Determined by an object’s mass ($m$) and the speed ($v$) at which it moves.
• Heavy, swiftly moving objects have lots of kinetic energy.
• Light, slowly moving objects have little kinetic energy.
• Formula: $KE = <br /> icefrac{1}{2} mv^2$

Pressure Energy

• A form of stored or potential energy.
• It has the ability to perform work.

Gravitational Energy

• Another form of stored or potential energy.
• Associated with any elevated object.
• Identical objects at the same height have the same gravitational energy.

Energy Losses in the Circulation

Viscous Energy Loss

• Viscosity describes the thickness of a fluid.
• More energy is lost with movement of high viscosity fluids.
• Viscous loss is associated with blood overcoming its internal stickiness.
• Viscosity is measured in units of poise.
• Viscous energy loss in blood is determined by hematocrit, which is the percentage of red blood cells in the blood.

Frictional Energy Loss

• Occurs when flow energy is converted to heat as one object rubs against another.
• Example: Blood sliding across vessel walls creates heat.

Inertial Energy Loss

• Occurs during three events:
  • Pulsatile flow (found in arterial circulation)
  • Phasic flow (found in venous circulation)
  • Velocity changes at a stenosis
• Velocity increases as the vessel narrows.
• Maximum velocity exists where the vessel is narrowest.
• Velocity decreases as blood flows out of the stenosis into a vessel segment of normal diameter.

Intertia

• Relates to the tendency of a fluid to resist changes in its velocity.
• Energy is lost when the speed of a fluid changes, regardless of whether the fluid speeds up or slows down.

Stenosis

• A narrowing in the lumen of a vessel.
• Effects of a stenosis:
  • Change in flow direction
  • Increased velocity as vessel narrows
  • Turbulence downstream from the stenosis
  • Loss of pulsatility

Bernoulli’s Principle

• Describes the relationship between velocity and pressure in a moving fluid.
• The sum of kinetic energy and pressure energy remains constant.

Venous Hemodynamics

• During normal function, veins have low pressure, are only partially filled with blood, and are only partially expanded.
• Veins are low resistance vessels.
• Normal venous resistance and arterial resistance are similar.

Hydrostatic Pressure

• Pressure related to the weight of blood pressing on a vessel measured at a height above or below heart level.
• Reported in units of mmHg, the same units used to measure blood pressure.
• When a person is supine, all parts of the body are at the same level as the heart, and the hydrostatic pressure is zero everywhere.