Understanding hemodynamics





Objectives


On completion of this chapter, you should be able to:




  • Understand the fundamental concepts of hemodynamics as applied to ultrasound



  • State basic knowledge of the cardiac cycle, normal intracardiac pressures and volumes, cardiac output, stroke volume, and its mechanisms



  • Describe the principles of blood flow velocity profiles, how they are displayed, and how they are evaluated by Doppler ultrasound



  • Demonstrate knowledge of how to apply Doppler ultrasound to calculate pressures, velocities, and output





What is hemodynamics ? It is the study of the forces involved with blood flow and circulation. This chapter will focus on blood flow and circulation as it moves through the body. Why is it important to understand hemodynamics? The forces involved with blood flow and circulation allow us to answer important questions regarding a patient’s condition such as cardiac output, right ventricular systolic pressure (RVSP), degree of valvular stenosis or regurgitation, and more. Doppler principles can be applied using ultrasound to indirectly measure such information.


Some basic terms that will be used frequently throughout this chapter include pressure, volume, and velocity. Pressure is defined as the exertion of force on a surface by an object or a fluid it is in contact with, or the force per unit area. Pressure will be used in the context of the force of fluid per unit area. Volume is the amount of space occupied by a three-dimensional object as measured in cubic units such as cubic centimeters (cm 3 ) or milliliters (ml). 3 Velocity is the speed with which something moves in a given direction.


Before discussing basic principles of blood flow, one must review how blood moves through the heart. Blood flow enters the heart through the vena cava (superior and inferior) into the right atrium (RA), through the tricuspid valve, and into the right ventricle (RV). From the RV blood travels to the pulmonary circuit through the pulmonary valve and pulmonary artery (PA). The PA bifurcates and blood flows through to the pulmonary capillaries and circuit, eventually back to the pulmonary veins. The pulmonary veins bring blood into the left atrium (LA), through the mitral valve, and into the left ventricle (LV). Blood leaves the heart through the aortic valve and aorta to the systemic circulatory system.




The cardiac cycle


The cardiac cycle can be best described by the Wiggers diagram ( Figure 31-1 ). This diagram shows the interaction of pressure and volume changes with electrical and mechanical systole and diastole. The two major components of the cardiac cycle are systole and diastole. Within these two major components, there are six minor components of the cardiac cycle; systole can be divided into isovolumic contraction and ejection, and diastole consists of isovolumic relaxation, rapid inflow, diastasis, and atrial systole. During systole the volume in the ventricles decreases as the ventricular and aortic pressure rises. Throughout diastole the pressures in the ventricles stabilize while the ventricular volume gradually increases.




FIGURE 31-1


The Wiggers diagram shows the normal relationships between the chamber pressures, volumes, and heart sounds timed with the ECG for the left side of the heart.


Normal intracardiac pressures and volumes


As shown in the Figure 31-1 , the pressures in the cardiac chambers are constantly changing throughout the cardiac cycle. Figure 31-2 shows normal pressures in the various cardiac chambers. It is important to remember that there are ranges of pressures, both normal and abnormal, but these are typical. In both the right and left ventricles, the pressure in early diastole drops to 0 mm Hg (middle number) and then slowly rises. Note that the right and left atrial pressures are a mean or average over the cardiac cycle. Pulmonary capillary wedge (PCW) is a substitute for left atrial pressure. PCW is obtained using a Swan-Ganz catheter primarily in intensive care settings.




FIGURE 31-2


Normal pressures in the cardiac chambers and great vessels. There are ranges for these pressures—see text for details.


The average pressures in each cardiac chamber, which can be measured directly through cardiac catheterization or indirectly through Doppler ultrasound, are as follows:














Right Atrium
1–5 mm Hg
Left Atrium
2–12 mm Hg
Right Ventricle:
15–30 mm Hg in systole
1–7 mm Hg in diastole
Left ventricle:
90–140 in systole
5–12 mm Hg in diastole
Pulmonary Artery:
15–30 mm Hg in systole
4–12 in diastole
Aorta
90–140 mm Hg in systole
60–90 mm Hg in diastole


Cardiac output, stroke volume, and its mechanisms


Some basic aspects of blood flow as it relates to the heart include cardiac output, stroke volume, preload, and afterload. Cardiac output (CO) refers to the amount of blood pumped by each ventricle in 1 minute. , It is the product of stroke volume and heart rate (HR). The average CO for adults is 4 to 8 liters per minute. Cardiac output has a direct effect on blood pressure; as the cardiac output increases, so does the blood pressure. Stroke volume (SV) is the volume of blood ejected by the ventricles with each contraction. Any effect on SV or HR affects the cardiac output. Preload is the degree that the muscle fiber stretches before contraction (end-diastole). It can also be considered the diastolic filling or ventricular end-diastolic volume and pressure. Preload is largely affected by the ventricle’s ability to stretch and relax. Afterload refers to the aortic arterial pressure and vascular resistance the ventricle must overcome to eject blood or any resistance against which the ventricle must pump in order to eject its volume. CO can be measured directly through cardiac catheterization or indirectly through Doppler ultrasound.


Blood flow velocity profiles


Blood flow velocity depends on many factors, including the shape and size of the vessel or chamber it is traveling through, wall characteristics, the timing within the cardiac cycle, flow rate, and the viscosity of the blood. Flow starts uniformly with similar velocity flow profiles giving a “flat,” laminar appearance ( Figure 31-3 , A ). Most of the flow is traveling at the same velocity with laminar flow. As the shape of the surface blood travels through changes, so does the flow velocity profile. As this change in shape or additional forces continue, the flow will vary so that it appears more parabolic with varying flow velocities. The flow velocities will be highest toward the middle of the vessel or area and slowest along the walls or edges of the surface it travels through. Flow can become turbulent when it travels through an area of obstruction or smaller surface area ( Figure 31-3 , B ). The flow velocities vary significantly when turbulent with high velocity and multidirectional flow. These blood flow velocities can be interrogated by Doppler with ultrasound, which will be discussed further within this chapter.




FIGURE 31-3


Examples of normal (laminar) flow in a blood vessel (top) versus abnormal (turbulent) flow (bottom) as might be seen in valvular stenosis or regurgitation.




Doppler basics


Doppler provides clinical information about blood flow, including the direction and velocity as well as the timing during the cardiac cycle. Qualitative (color flow) and quantitative (pulsed and continuous wave) Doppler aids in the evaluation of stenosis, insufficiency, and shunt lesions. It can also provide estimates of cardiac function such as CO.


Doppler effect and frequency shift


The Doppler effect refers to a change in the frequency of waves (through sound, light, etc.) that occurs as the source and observer change in motion relative to each other (away or toward). The Doppler effect will be discussed relative to the frequency of sound waves for the purposes of this chapter. With sound, the frequency will increase as the source and observer move toward each other and decrease as they move apart, as demonstrated in Figure 31-4 .


May 29, 2019 | Posted by in ULTRASONOGRAPHY | Comments Off on Understanding hemodynamics
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