Echocardiography

Published on 23/05/2015 by admin

Filed under Internal Medicine

Last modified 22/04/2025

Print this page

This article have been viewed 1138 times

Chapter 5

Echocardiography

Hisham Dokainish, MD, FACC, FASE and Glenn N. Levine, MD, FACC, FAHA

1. How does echocardiography work?

Echocardiography uses transthoracic and transesophageal probes that emit ultrasound directed at cardiac structures. Returning ultrasound signals are received by the probe, and the computer in the ultrasound machine uses algorithms to reconstruct images of the heart. The time it takes for the ultrasound to return to the probe determines the depth of the structures relative to the probe because the speed of sound in soft tissue is relatively constant (1540 m/sec). The amplitude (intensity) of the returning signal determines the density and size of the structures with which the ultrasound comes in contact.

The probes also perform Doppler ultrasonography, which measures the frequency shift of the returning ultrasound signal to determine the speed and direction of moving blood through heart structures (e.g., through the aortic valve) or in the myocardium itself (tissue Doppler imaging).

Appropriateness criteria for obtaining an echocardiogram are given in Box 5-1.

Box 5-1 APPROPRIATENESS CRITERIA FOR ECHOCARDIOGRAPHY

Appropriate indications include, but are not limited to:

Symptoms possibly related to cardiac etiology, such as dyspnea, shortness of breath, lightheadedness, syncope, cerebrovascular events

Initial evaluation of left-sided ventricular function after acute myocardial infarction

Evaluation of cardiac murmur in suspected valve disease

Sustained ventricular tachycardia or supraventricular tachycardia

Evaluation of suspected pulmonary artery hypertension

Evaluation of acute chest pain with nondiagnostic laboratory markers and electrocardiogram

Evaluation of known native or prosthetic valve disease in a patient with change of clinical status

Uncertain indications for echocardiography:

Cardiovascular source of embolic event in a patient who has normal transthoracic echocardiogram (TTE) and electrocardiogram findings and no history of atrial fibrillation or flutter

Inappropriate indications for echocardiography:

Routine monitoring of known conditions, such as heart failure, mild valvular disease, hypertensive cardiomyopathy, repair of congenital heart disease, or monitoring of an artificial valve, when the patient is clinically stable

Echocardiography is also not the test of choice in the initial evaluation for pulmonary embolus and should not be routinely used to screen asymptomatic hypertensive patients for heart disease.

Appropriate indications for transesophageal echocardiography (TEE) as the initial test instead of TTE:

Evaluation of suspected aortic pathology, including dissection

Guidance during percutaneous cardiac procedures, including ablation and mitral valvuloplasty

To determine the mechanism of regurgitation and suitability of valve repair

Diagnose or manage endocarditis in patients with moderate to high probability of endocarditis

Persistent fever in a patient with an intracardiac device

TEE is not appropriate in evaluation for a left atrial thrombus in the setting of atrial fibrillation when it has already been decided to treat the patient with anticoagulant drugs.

Modified from Douglas PS, Khandheria B, Stainback RF, et al: ACCF/ASE/ACEP/ASNC/SCAI/SCCT/SCMR 2007 appropriateness criteria for transthoracic and transesophageal echocardiography. J Am Coll Cardiol 50:187-204, 2007.

2. What is the difference between echocardiography and Doppler?

Echocardiography usually refers to two-dimensional (2-D) ultrasound interrogation of the heart in which the brightness mode is used to image cardiac structures based on their density and location relative to the chest wall. Two-dimensional echocardiography is particularly useful for identifying cardiac anatomy and morphology, such as identifying a pericardial effusion, left ventricular aneurysm, or cardiac mass.

Doppler refers to interrogation of the movement of blood in and around the heart, based on the shift in frequency (Doppler shift) that ultrasound undergoes when it comes in contact with a moving object (usually red blood cells). Doppler has three modes:

Pulsed Doppler (Fig. 5-1, A), which can localize the site of flow acceleration but is prone to aliasing

Figure 5-1 Doppler assessment used in patients with aortic stenosis. A shows pulsed Doppler in the left ventricular outflow tract in a patient with aortic stenosis. The peak velocity of the spectral tracing (arrow) is 1.2 msec, indicating normal flow velocity proximal to the aortic valve. B shows continuous Doppler across the aortic valve revealing a peak velocity of 4.5 msec (dashed arrow). Therefore, the blood-flow velocity nearly quadrupled across the stenotic aortic valve, consistent with severe aortic stenosis.

Continuous-wave Doppler (Fig. 5-1, B), which cannot localize the level of flow acceleration but can identify very high velocities without aliasing

Color Doppler (Fig. 5-2), which uses different colors (usually red and blue) to identify flow toward and away from the transducer, respectively, and identify flow acceleration qualitatively by showing a mix of color to represent high velocity or aliased flow

Figure 5-2 Mitral regurgitation. Apical four-chamber view with color Doppler revealing severe mitral regurgitation (white arrows). Black arrows point to the mitral valve. Note that in actuality, the regurgitant jet is displayed in color, corresponding to the flow of blood. LA, Left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.

Doppler is particularly useful for assessing the hemodynamic significance of cardiac structural disease, such as the severity of aortic stenosis (see Fig. 5-1), degree of mitral regurgitation (see Fig. 5-2), flow velocity across a ventricular septal defect, or severity of pulmonary hypertension. The great majority of echocardiograms are ordered as echocardiography with Doppler to answer cardiac morphologic and hemodynamic questions in one study (e.g., a mitral stenosis murmur); 2-D echo to identify the restricted, thickened, and calcified mitral valve (Fig. 5-3); and Doppler to analyze its severity based on transvalvular flow velocities and gradients.

Figure 5-3 Parasternal long-axis view showing typical hockey stick appearance of the mitral valve (arrow) in rheumatic mitral stenosis. Ao, Aortic valve; LA, left atrium; LV, left ventricle; RV, right ventricle.

3. How is systolic function assessed using echocardiography?

The most commonly used measurement of left ventricular (LV) systolic function is left ventricular ejection fraction (LVEF), which is defined by:

The Simpson method (method of discs) in which the LV endocardial border of multiple “slices” of the left ventricle is traced in systole and diastole, and the end-diastolic and end-systolic volumes are computed from these tracings, is one of the most common methods of calculating LVEF.

The Teicholz method, in which the shortening fraction:

is multiplied by 1.7, can also be used to estimate LVEF (although this method is inaccurate in patients with regional wall motion abnormalities).

Visual estimation of LVEF by expert echocardiography readers is also commonly used.

Increasingly, state-of-the-art full volume acquisition using 3-dimensional (3-D) echocardiography can be used to provide accurate LVEF.

Systolic dysfunction in the presence of preserved LVEF (more than 50%-55%)—such as is found in patients with hypertrophic hearts, ischemic heart disease, or infiltrative cardiomyopathies—can be identified by depressed systolic tissue Doppler velocities.

4. What is an echocardiographic diastolic assessment? What information can it provide?

A diastolic assessment does two things: identifies LV relaxation and estimates LV filling pressures. LV relaxation is described as the time it takes for the LV to relax in diastole to accept blood from the left atrium (LA) through an open mitral valve. A normal heart is very elastic (lusitropic) and readily accepts blood during LV filling. When relaxation is impaired, the LV cannot easily accept increased volume, and this increased LV preload results in increases in LA pressure, which in turn results in pulmonary edema.

LV relaxation is usually best determined using tissue Doppler imaging, which assesses early diastolic filling velocity (Ea) of the LV myocardium. Normal hearts have Ea of 10 cm/sec or greater; impaired relaxation is present when Ea is less than 10 cm/sec.

An indicator of LV preload is peak transmitral early diastolic filling velocity (E), which measures the velocity of blood flow across the mitral valve. An estimate of the LV filling pressure can be made using the ratio of blood flow velocity across the mitral valve (E) to the velocity of myocardial tissue during early diastole (Ea). A high ratio (e.g., E/Ea ≥ 15) indicates elevated LV filling pressure (LA pressure ≥ 15 mm Hg); a lower ratio (e.g., E/Ea ≤ 10) indicates normal LV filling pressure (LA pressure < 15 mm Hg).

5. How can echocardiography with Doppler be used to answer cardiac hemodynamic questions?

Stroke volume and cardiac output can be obtained with measurements of the LV outflow tract and time-velocity integral (TVI) of blood through the LV outflow tract.

Doppler evaluation of right ventricular outflow tract diameter and TVI similarly allow measurement of right ventricular output.

Tricuspid regurgitation peak gradient can be added to estimate of right atrial pressure to in turn estimate pulmonary artery systolic pressure.

Mitral inflow velocities, deceleration time, pulmonary venous parameters, and tissue Doppler imaging of the mitral annulus can give accurate assessment of LV diastolic function, including LV filling pressures.

Measurement of TVI and valve annular diameters can be used to assess intracardiac shunts (Qp/Qs) and regurgitant flow volumes, where Qp is flow out the right ventricular outflow tract and Qs is flow out the left ventricular outflow tract.

Pressure gradients across native and prosthetic valves and across cardiac shunts can be used to assess hemodynamic severity of valve stenosis, regurgitation, or shunt severity, respectively.

Respiratory variation in valvular flow can aid in the diagnosis of cardiac tamponade or constrictive pericarditis.

6. How is echocardiography used to evaluate valvular disease?

Two-dimensional echocardiography can provide accurate visualization of valve structure to assess morphologic abnormalities (calcification, prolapse, flail, rheumatic disease, endocarditis). Figure 5-3 demonstrates the restricted movement of the mitral valve in a patient with mitral stenosis.

Color Doppler can provide semiquantitative assessment of the degree of valve regurgitation (mild, moderate, severe) in any position (aortic, mitral, pulmonic, tricuspid).

Pulsed Doppler can help pinpoint the location of a valvular abnormality (e.g., subaortic vs aortic vs supraaortic stenosis). Pulsed Doppler can also be used to quantitate regurgitant volumes and fractions using the continuity equation.

Continuous-wave Doppler is useful for determining the hemodynamic severity of stenotic lesions, such as aortic or mitral stenosis.

7. How can echocardiography help diagnose and manage patients with suspected pericardial disease?

Echocardiography can diagnose pericardial effusions (Fig. 5-4) because fluid in the pericardial space readily transmits ultrasound (appears black on echo).