Doppler instrumentation and hemodynamics
a misrepresentation of the Doppler shift in a negative direction occurring when the pulse repetition frequency is set too low.
smallest arteries in the circulatory system controlling the needs of organs and tissues.
pressure reduction in a region of high flow speed.
auscultatory sound within an artery produced by turbulent blood flow.
the smallest of the body’s blood vessels connecting the arterioles and venules and allowing the interchange of oxygen or carbon dioxide and nutrients to the tissue cells.
noise in the Doppler signal caused by high-amplitude Doppler shifts.
observed frequency change of the reflected sound resulting from movement relative to the sound source or observer.
frequency shift created between the transmitted frequency and received frequency by an interface moving with velocity at an angle to the sound.
energy difference between two points.
to move in a stream, continually changing position and direction.
electronic device controlling the transmission or reception of a Doppler signal; size of the gate is determined by the beam diameter, receiver gate length, and length of the ultrasound pulse.
science or physical principles concerned with the study of blood circulation.
the perceived color; any one or a combination of primary colors.
the pressure created in a fluid system, such as the circulatory system; when supine, the hydrostatic pressure is 0 mm Hg. When upright, the pressure is negative above the heart and positive below the heart.
the resistance to acceleration.
consists of the arterioles, capillaries, and venules.
the highest frequency in a sampled signal represented unambiguously; equal to one half the pulse repetition frequency.
positioning of multiple pulsed Doppler gates over the area of interest.
maximum velocity at any given time.
speed is constant across the vessel.
predicts volume flow in a cylindrical vessel.
difference in pressure required for flow to occur.
a parameter used to convey the pulsatility of a time-varying waveform.
predicts the onset of turbulent flow.
difference between the maximum and minimum Doppler frequency shifts divided by the maximum Doppler frequency shift; also known as Pourcelot index.
electronic device that controls the region of Doppler flow detection.
degree to which the original color is diluted with white; the paler the color (or the less saturated it is), the faster the flow velocity; the purer the color, the slower the flow velocity.
increase in the range of Doppler shift frequencies displayed resulting in a loss of the spectral window; usually seen with stenosis.
amount of blood moving in a forward direction; blood being ejected.
the average velocity is calculated, with the colors placed side-to-side.
rate of motion with respect to time.
all measured velocities for each gate are averaged, then the colors are arranged up and down.
the smallest veins that receive blood from the capillaries and drain into larger-caliber veins.
the quantity of blood moving through the vessel per unit of time.
Hemodynamics
• A difference in pressure (pressure gradient) is required for flow to occur.
• Pressure difference can be generated by the heart or gravity.
• Blood flows from the higher pressure to the lower pressure.
• Equal pressure at both ends will result in no flow.
• The greater the pressure difference, the greater the volume of blood flow.
Cardiac circulation
• Deoxygenated blood flows from the superior and inferior vena cava into the right atrium.
• From the right atrium, blood courses through the tricuspid valve to the right ventricle.
• Blood flows into the lungs through the pulmonary arteries from the right ventricle.
• Oxygenated blood flows into the left atrium through the pulmonary veins.
• Blood continues to flow through the mitral valve into the left ventricle.
• From the left ventricle, blood is pumped into the aorta and systemic circulation.
• Valves are present in the heart to permit forward flow and to prevent reverse flow.
• Peripheral resistance is a primary regulatory control on cardiac output.
• Vasodilation of the lower extremity arteries decreases resistance, increasing the flow to the limbs.
• Vasoconstriction of the lower extremity arteries increases resistance, decreasing the flow to the limbs.
• Malfunctioning valves can restrict forward flow (stenosis) or allow reverse flow by not closing completely (insufficiency or regurgitation).
| CONTRIBUTING FACTORS | DESCRIPTION |
| Density | Mass per unit volume |
| Fluids | Substances that flow and conform to the shape of their containersGases and liquid |
| Kinetic energy | Proportional to its density and velocity squared |
| Mass | Measure of an object’s resistance to accelerationDirectly related to the inertia and force to accelerate |
| Pressure | Force per unit areaDriving force behind blood flowDirectly related to the blood flow volumeWith each cardiac contraction, the blood is pressure-waved into the arteriole system and microcirculationEqually distributed throughout a static fluid and is forced in all directions |
| Pressure gradient | Pressure difference required for flow to occurProportional to the flow rate |
| Resistance | The resistance of the arterioles accounts for about one half of the total resistance in the systemic systemThe muscular walls of the arterioles can constrict or relax, producing dramatic changes in flow resistanceDirectly related to the length of the vessel and fluid viscosityInversely related to the vessel radius |
| Velocity | Speed at which red blood cells (RBCs) travel in a vesselNot constant or uniform across a vesselDependent on the left-ventricular output, resistance of the arterioles, cross-sectional area, and course of the vessel |
| Viscosity | A fluid’s ability to resist a change in shape or flowResistance to flow offered by a fluid in motionDirectly related to the number of RBCsBlood is 4 times more viscous than waterUnits—Poise or kg/m × s |
Volumetric flow rate
• Volume of blood passing a point per unit time.
• Adult cardiac flows at a rate of 5000 mL/min.
• Determined by the pressure difference and the resistance to flow.
• Depends on the pressure difference, length and diameter of the tube, and viscosity of the fluid.
• Cardiac Output = stroke volume × heart rate.
• Stroke Volume (mL) = end diastolic volume minus end systolic volume.
Continuity rule
• Volumetric flow rate must be constant, because blood is neither created nor destroyed as it flows through a vessel.
• The average flow speed in a stenosis must be greater than that proximal and distal to it so that the volumetric flow rate is constant throughout the vessel.
 | |
| DEFINITION | RELATIONSHIP |
| Predicts flow volume in a long, straight cylindrical vessel | Directly related to the pressure difference and the size or radius of the vesselInversely related to the vessel length, resistance, and fluid viscosityRelates to a steady flow in a long unobstructed tube |
| DEFINITION | RELATIONSHIP |
| Region of decreased pressure in an area of high flow speed | If flow speed increases, pressure energy decreases |
| Pressure decreases before a stenosis to allow the fluid to accelerate into the stenosis and decelerate out of it | Relates to short obstructed vessel |
Types of blood flow
• Blood flow is typically nonuniform through a specific vessel or throughout the body.
• The muscular walls of the arterioles can constrict or relax, controlling blood flow to specific tissues and organs according to their needs.
• Low-resistance waveforms demonstrate a slow upstroke in systole and a large amount of diastolic flow (i.e., internal carotid artery).
• High-resistance waveforms demonstrate a sharp upstroke in systole and very little diastolic flow (i.e., external carotid artery).
| TYPE | DESCRIPTION |
| Laminar | Flow where layers of fluid slide over each otherMaximum flow velocity located in the center of the arteryMinimum flow velocity located near the arterial wallFound in smaller arteries |
| Parabolic flow | Type of laminar flowAverage flow velocity is equal to one half the maximum flow speed at the center |
| Plug | Constant velocity across the vesselFound in large arteries (i.e., aorta) |
| Pulsatile | Steady flow with acceleration and deceleration over the cardiac cycleIncludes added forward flow and/or flow reversal throughout the cardiac cycle in some locations in the circulatory systemArterial diastolic flow shows the state of downstream arterioles |
| Disturbed | Altered or interrupted forward flowFound at bifurcations and mild obstructionsForm of laminar flow |
| Turbulent | Random and chaotic flow patternCharacterized by eddies and multiple flow velocitiesMaintains a net forward flowOnset predicted by a Reynolds number greater than 2000Caused by a curve in a vessel’s course or a decrease in vessel diameter |
Venous hemodynamics
• Veins offer little resistance to flow.
• Venous system demonstrates low-pressure, nonpulsatile flow.
• Pressure is lowest when the patient is lying flat.
• Greatest portion of the circulating blood is located in the venous system.
• Veins accommodate larger changes in blood volume with little change in pressure.
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