Echocardiography

Published on 23/05/2015 by admin

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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.