CHAPTER 65

Albree Tower-Rader

Transthoracic Echocardiography

I.Introduction. Transthoracic echocardiography is a reliable and versatile tool for the assessment of cardiac structure, function, and hemodynamics. Compared with other cardiovascular imaging modalities, it is relatively inexpensive, does not expose the patient to radiation, is noninvasive, displays live real-time images, and is widely available.

II.Basic Principles of Echocardiography. Sound waves consist of mechanical vibrations that produce alternating compressions and rarefactions of the medium through which they travel. Ultrasound consists of sound waves in the frequency range that is higher than what is audible to humans (>20,000 Hz). All waves can be described by their frequency (f), wavelength (λ), velocity of propagation (v), and amplitude. The velocity of ultrasound in soft tissue (e.g., myocardium and blood) is 1,540 m/s. Frequency is defined by the number of cycles occurring per second (cycles/second or Hz) and wavelength is measured in meters. Velocity, frequency, and wavelength are described by the following relationship:

Velocity = frequency × wavelength or v = f × λ

The typical adult echocardiographic examination uses a transducer with ultrasound frequency between 2.5 and 3.5 million Hertz (MHz). Using the equation above a 3-MHz transducer will have an ultrasound wavelength of approximately 0.50 mm. This has important implications because image resolution cannot be >1 to 2 wavelengths (e.g., 1 mm with a 3-MHz transducer). In addition, the depth of penetration of the ultrasound wave is directly related to the wavelength, with shorter wavelengths penetrating a shorter distance. Therefore, higher frequency transducers result in the use of shorter wavelengths that improve image resolution but at the cost of reduced depth penetration.

Transducers use a piezoelectric crystal to generate and receive ultrasound waves. A piezoelectric substance has the property of changing its size and shape when an electric current is applied to it. An alternating electrical current will result in rapid expansions and compressions of the material and thus produce an ultrasound wave. The piezoelectric crystal also deforms in shape when an ultrasound wave strikes the material, resulting in the production of an electric current. The transducer, and the piezoelectric crystal, thus oscillates between a short burst of transmitting ultrasound waves, with a brief period of no ultrasound transmission when it awaits reception of the reflected signals.

Tissue harmonic imaging has become the standard imaging technique in many laboratories. It utilizes the principle that as ultrasound waves propagate through tissue, the waveform becomes altered by the tissue, with the generation of new waveforms of higher frequency but which are multiples of the baseline fundamental frequency. Setting the transducer to receive only harmonic sound waves that are multiples of the fundamental frequency improves image quality significantly. This image quality improvement is based on the fact that weak signals, which tend to be artifacts, create almost no harmonics. In addition, shallow structures, such as the chest wall, generate weak harmonic signals, whereas at depths of 4 to 8 cm, where the heart is located, maximal harmonic frequencies develop. These phenomena result in fewer near-field artifacts and better endocardial definition. One limitation of harmonic imaging is that valve leaflets appear thicker—an artifact generated during image processing that appears to be related to the rapid motion of the leaflets.

The steps involved in creating a final ultrasound image are transmission and reception of waves, conversion to electrical signals, filtering, and extensive computer processing. The details of image processing, formation of artifacts, advanced physics, and technical aspects of echocardiography are beyond the scope of this chapter, but are briefly discussed in Section VII.

Patient and probe positioning, electrocardiographic lead placement, and transducer selection are the first steps to beginning the echocardiographic examination.

A.Patient and probe positioning. For the parasternal and apical positions, the patient should be in the left lateral decubitus position, with the left arm extended behind the head, because this brings the heart into contact with the chest wall. The subcostal and suprasternal views require the patient to be in the supine position.

B.Electrocardiographic lead placement. The electrocardiogram (ECG) allows identification of arrhythmias and timing of cardiac events during the echocardiographic examination, and it is used as a timing marker for digital recording of images. Typically, digital “clips” are set to record a predefined number of cardiac cycles (usually one but sometimes two), with timing based on the ECG. It is important that irregular beats be identified and excluded from the analysis. For example, a postectopic beat will falsely increase the two-dimensional (2D) assessment of ejection fraction (EF) and the Doppler assessment of transaortic gradient. In general, any Doppler index requires the average of at least three measurements. For patients in atrial fibrillation, 7 to 10 beats should be averaged. For patients with very high heart rates, or with a noisy electrocardiographic signal, the digital clips can be set to record for a predefined period of time (usually 2 seconds).

C.Transducer selection. The adult echocardiographic examination typically begins with a 2.5- to 3.5-MHz phased array transducer. Transducer frequency is important, because at higher frequencies, spatial resolution improves but at the expense of reduced depth penetration. Higher frequency (3.0 to 5.0 MHz) transducers may be used in thin or pediatric patients or intraoperatively for epiaortic scanning. Therefore, for optimal 2D resolution, select the highest frequency transducer that will provide adequate far-field penetration.

With regard to transducer frequency for the Doppler examination, lower frequency transducers can record higher velocities (see Doppler equation Section IIIC.1). The Pedoff probe is a continuous-wave (CW), nonimaging probe (typical frequency being 1.8 MHz) used mainly to detect higher velocity profiles and confirm velocities obtained by other imaging methods.

III.Imaging Modalities in Standard Echocardiograms

A.M-mode. Prior to 2D imaging, the echocardiogram was obtained when the transducer sent an ultrasound wave along a single line and then displayed the amplitude of reflected signal as well as the depth of that signal on an oscilloscope. This was called an A-mode echocardiography. When these line-of-sight ultrasound images were plotted with respect to time, “motion” mode, or M-mode, was produced. Despite the increasing emphasis on 2D imaging, the M-mode display remains a complementary element of the transthoracic examination. Its high sampling rate of up to 2,000 frames/s, compared with 30 frames/s for 2D echocardiography, provides excellent temporal resolution, and thus it is very useful in the timing of subtle cardiac events that can be missed by the naked eye in 2D imaging. Rapidly moving structures such as the aortic valve, mitral valve, and endocardium have characteristic movements in M-mode. Deviations from these, such as diastolic fluttering of the mitral valve in aortic regurgitation (AR) and systolic aortic valve notching in dynamic left ventricular outflow tract (LVOT) obstruction, may be the only way to detect the underlying dysfunction not appreciated in other imaging modalities. M-mode also has a great spatial resolution along the single line and can be used for precise size measurements such as ventricular dimensions in systole and diastole. The M-mode image is displayed like a graph, with time on the x-axis and distance from the transducer on the y-axis, with the structures closest to the transducer at the top of the image. In order to align the line of sight accurately, 2D imaging should be used to position the M-mode cursor through the structures of interest.

B.2D imaging. 2D imaging provides tomographic views of various 2D planes of cardiac structures and acts as guide for the M-mode and Doppler portions of the examination. A 2D echocardiographic image is essentially the scan line from M-mode that, instead of having a fixed line of sight, is swept back and forth across an arc. After complex manipulation of the data received by the transducer from the multiple scan lines, a 2D tomographic image is generated for display.

Depending on the depth of the image, a finite amount of time is needed for each scan line to be sent and received by the transducer. As opposed to M-mode that has only one scan line and can provide up to 2,000 frames/s, 2D echocardiographic imaging can utilize 128 scan lines but at the expense of a lower rate of 30 frames/s. Faster frame rates can be obtained by electronic manipulation using parallel processing on current ultrasound machines. Doppler overlay of the 2D image tends to slow down the frame rate. This reduction in temporal resolution reinforces the need for M-mode to complement 2D imaging in echocardiography, especially for rapidly moving structures and in precise timing of events.

C.Doppler echocardiography. The introduction of Doppler technique to echocardiography not only added new imaging capabilities but also transformed echocardiography into a modality that could provide hemodynamic assessment of the heart. Echocardiography has now become the preferred method, and in some cases the gold standard, over cardiac catheterization for certain hemodynamic assessments.

1.Doppler principles. The Doppler principle states that sound frequency increases as the source moves toward the observer and decreases as the source moves away. The change in frequency between the transmitted sound and the reflected sound is termed the Doppler shift. This phenomenon is appreciated daily when an ambulance’s siren becomes higher pitched, because of the increase in wave frequency, as it approaches the observer and then lower pitched once it has passed. This Doppler frequency shift directly relates to the velocity of the red blood cell by the following Doppler equation:

where v = velocity, f_R = frequency received, f_T = frequency transmitted, c = speed of sound in blood (1,540 m/s), and θ = angle between moving object and ultrasound beam.

The cos θ in the Doppler equation makes the calculation of velocity depend on the angle between the beam and the moving structure (red blood cell). Echocardiography machines do not typically incorporate the angle for calculating the resultant velocity, and thus the goal is to have the angle between the ultrasound beam and the blood flow jet of interest to be as close to zero as possible (cos 0 = 1). When this is not possible, the angle should be <20°, so that the true flow velocity is underestimated by <6% (cos 20 = 0.94). Adhering to this requirement sometimes mandates off-axis or unusual 2D images to align the Doppler ultrasound signal with desired target.

2.Spectral analysis is the term used to describe the way in which pulsed-wave (PW) Doppler and CW Doppler are displayed. By convention, the horizontal axis reflects the time and is placed in the middle of the screen with upward deflections representing frequency shifts toward the transducer and downward deflections for frequency shifts away from the transducer. The vertical axis represents the blood flow velocity (or frequency shifts), with the density of pixels on a gray scale reflecting the amplitude of the signal. The final result is that at each time point, the spectral analysis shows blood flow direction, velocity/frequency shift, and signal amplitude.

a.PW Doppler. The purpose of PW Doppler mode is to measure the Doppler shift, and thus velocity, at a specific location of interest within a small sample volume (e.g., mitral inflow velocity at the mitral valve leaflet tips, systolic velocity at the LVOT, and blood flow within the pulmonary veins). In this mode, a single crystal sends short bursts of ultrasound waves at a specific pulse repetition frequency (PRF) to a specific location, which are reflected from moving blood cells at this location and received by the same crystal. The maximal velocity that can be measured is limited by the time required to transmit and receive the ultrasound wave. This is called the Nyquist limit (one-half of the PRF). If a velocity greater than the Nyquist limit is measured, the signal appears as a wrap around the baseline, known as signal aliasing. Hence, the peak velocity is limited by the depth of the area of interest and also by the transducer frequency (inverse relationship according to the Doppler equation; see previous text). PW Doppler has excellent spatial/depth resolution, but it has limited capacity to measure high velocities because of the Nyquist limit. It is, therefore, used primarily to measure low-velocity flow (<2 m/s) at specific sites in the heart.

b.CW Doppler. CW Doppler employs two crystals, one continuously sending ultrasound waves and the other continuously receiving the waves. It measures Doppler shift along the entire beam, rather than at a specific location. Unlike PW Doppler, CW Doppler measures the maximal velocity along the entire ultrasound beam but it does not localize the precise position of that peak velocity. However, this is often apparent anatomically or can be deduced using PW Doppler or color flow Doppler. In general, CW Doppler is used to assess high-velocity flow and PW Doppler is used to measure low-velocity flow in specific areas. Clinical applications of PW versus CW Doppler are listed in Table 65.1.

TABLE 65.1 Differences and Uses of PW Doppler and CW Doppler
Factor	PW	CW
Transducer crystal	Same transmitting and receiving	Different transmitting and receiving
Spatial resolution	Excellent—localizes to precise point	Poor—may be anywhere along the entire beam
Ability to measure high velocity (>2 m/s)	No (limited by Nyquist)	Excellent
Uses	Mitral inflow	Gradients in aortic stenosis
	Pulmonary venous flow and LVOT flow	Gradient and pressure half-time in mitral stenosis
	Hepatic vein flow	Peak velocity in mitral regurgitation and measurement of dp/dt
	Tricuspid inflow	TR velocity—estimate RV systolic pressure

CW, continuous wave; LVOT, left ventricular outflow tract; PW, pulsed wave; RV, right ventricular; TR, tricuspid regurgitation.

c.Color flow imaging. Although spectral (PW and CW) Doppler imaging is superior for accurate measurement of specific intracardiac blood flow velocities, the best way to visualize the overall pattern of intracardiac blood flow is with color flow imaging. Color flow Doppler is based on the principle of PW Doppler, with multiple sampling volumes at varying depths along a single scan line. A full-color flow map is generated by combining multiple scan lines along the areas of interest. To accurately estimate the velocity along a given scan line, the instrument compares the Doppler shift changes from several successive pulses (typically eight), and this is known as the burst length. Where Doppler shifts are detected, color pixels are displayed at that location, with the different colors representing the different degrees of Doppler shift based on a predetermined color spectrum. Tradition has set blood velocity toward the transducer as shades of red and blood flow away as shades of blue (Blue Away).

Because this modality uses properties based on PW Doppler technology, color flow Doppler has limitations similar to those of PW Doppler for velocity determination. When the flow velocity is higher than the Nyquist limit (indicated on the color map), color aliasing occurs (depicted as color reversal, red to blue or blue to red transition). The fact that color aliasing occurs can actually provide important hemodynamic information, such as identification of flow acceleration or calculation of the proximal isovelocity surface area (PISA), which is discussed in Section VI.C.6.

d.Tissue Doppler imaging (TDI). TDI is based on adjusting standard Doppler to focus primarily on the low-velocity, high-amplitude motion of the myocardium (usually <20 cm/s) instead of the high-velocity, low-amplitude motion of red blood cells. Decreasing the filters (which normally eliminates low-velocity signals) and the Doppler transmit gain (which excludes the low-amplitude blood signals) results in the Doppler focusing primarily on myocardial motion. TDI can be displayed as either PW Doppler, typically at one aspect of the mitral annulus (usually septal or lateral), or color flow TDI mapping of the entire myocardial area of interest (Fig. 65.1). TDI has primarily been used as an adjunct for the evaluation of left ventricular (LV) diastolic function, where the mitral annular TDI pattern shows a systolic (S) wave toward the transducer and two diastolic waves away from the transducer (corresponding to early relaxation and late atrial diastolic myocardial motion, labeled as E′ and A′) (Fig. 65.2 and Table 65.2). With worsening diastolic function, E′ velocity decreases and is directly proportional to the rate of relaxation. TDI annular velocities decrease with age and may be affected by a myocardial infarction in the region adjacent to the annulus or surgery of the mitral valve. Therefore, TDI can be used to help differentiate a normal mitral inflow pattern (normal E′) from a pseudonormal filling pattern (reduced E′) (Fig. 65.2 and Table 65.3). TDI can also be used to assess LV filling pressures, myocardial deformation, and ventricular dyssynchrony.

FIGURE 65.1 Tissue Doppler imaging (TDI) for diastolic function recorded from the apical four-chamber window using a 2-mm sample volume positioned in the lateral wall 1 cm from the mitral annulus. The TDI signal is toward the transducer in systole (S) as the myocardium moves toward the apex. In diastole, the myocardial velocity is directed away from the transducer first with early diastolic filling (E′) and then with atrial contraction (A′).

FIGURE 65.2 Diastolic function/dysfunction staging. A, atrial kick mitral inflow velocity; A_m, atrial annular velocity (A′); D, diastolic; E, early mitral inflow velocity; E_m, early diastolic annular velocity (E′); Nl, normal; PV, pulmonary vein; S, systolic; S_m, systolic annular velocity; Tissue Doppler, mitral annular velocity by Tissue Doppler; V_p, velocity of propagation.

TABLE 65.2 Diastolic Function/Dysfunction Staging
	Normal Young	Normal Adult	Normal Elderly	Delayed Relaxation	Pseudonormal Filling	Restrictive Filling
Stage	Normal	Normal	Normal	I	II	III
E/A ratio	>1 (often >2)	>1	<1	<1	1–2	>2
DT (ms)	<220	<220	>220	>220	150–200	<150
S/D ratio	<1	≥1	>1	>1	<1	<1
Ar (cm/s)	<35	<35	<35	<35	>35	>25
Vp (cm/s)	>55	>55	<55	<55	<45	<45
E′ (cm/s) (E annulus)	>10	>8	<8	<8	<8	<8

Ar, atrial reversal; DT, deceleration time.; Vp, propagation velocity from color M-mode; S/D ratio, systolic flow/diastolic flow from

pulmonary vein tracing; E’, mitral annular early diastolic velocity; E, peak mitral inflow early diastolic flow velocity; A, peak mitral inflow atrial contraction velocity

D.Color M-mode (CMM). This technique, whereby color flow Doppler is imposed on an M-mode image, permits excellent spatiotemporal distribution of velocity (color) data, although it is limited to the defined scan line. It is a valuable adjunct in the timing of cardiac events, which may not be readily appreciated by 2D and color flow imaging alone. Its primary use has been in evaluating diastolic filling pattern where the LV inflow CMM pattern typically has two appreciable waves, the first demonstrating the early passive filling wave and the second later wave resulting from atrial contraction (Fig. 65.2). The slope of the early filling wave (velocity of propagation, V_p) is primarily dependent on the rate of relaxation and is reduced with delayed relaxation. It is useful for differentiating a normal mitral inflow pattern (normal V_p) from a pseudonormal filling pattern (where impaired relaxation results in delayed flow propagation into the left ventricle, slower V_p) (Fig. 65.2 and Table 65.2).

Other uses of CMM are the accurate measurement of AR jet diameter in the LVOT in the parasternal views and, with its superior temporal resolution, detection of diastolic mitral regurgitation (MR), which may be seen in certain conditions (severe acute AR, advanced diastolic dysfunction, and complete heart block).

IV.Tomographic Views and Cardiac Anatomy. Most echocardiography laboratories have similar protocols for acquisition of a complete echocardiogram. Each echocardiographic view is labeled first by the transducer position (parasternal, apical, subcostal, and suprasternal) followed by the tomographic view of the heart (long axis, short axis, four chamber, and two chamber). To acquire these different views, the transducer is placed on different parts of the body and adjusted with rotation and angulation to optimize the final image. Standard imaging planes are illustrated in Figures 65.3 to 65.8; see Table 65.4 for normal echo dimensions, Table 65.3 for standard examination protocol, and Table 65.5 for useful 2D examination tips.

TABLE 65.3 Standard Transthoracic Examination

1. PLAX

•Position transducer in third or fourth left intercostal space parasternally (ridge toward right shoulder)

• Color Doppler—mitral and aortic valve flow and interventricular septum (in cases of ventricular septal defect)

•M-mode—three levels (below mitral leaflets, midmitral leaflets, and aortic valve)

•Move up an intercostal space to get better view of ascending aorta

• Tilt transducer inferomedially to assess RV inflow. Color Doppler to assess tricuspid regurgitation and CW Doppler to estimate RVSP

2. PSAX

•Rotate transducer 90° clockwise from PLAX and tilt transducer from superior to inferior (LV apex view, mid-LV view, mitral valve view, and aortic valve level)

•Color Doppler at mitral valve level (localize mitral regurgitation if present)

•Color Doppler at aortic valve level (localize aortic regurgitation and assess flow in RVOT/pulmonic valve and tricuspid valve)

•PW Doppler—RVOT (level of pulmonic valve annulus)

•CW Doppler—RVOT/pulmonic valve and tricuspid valve (estimate RVSP)

3. A4C

•Transducer at apex—ridge toward left (move laterally and inferiorly if necessary to get true apex)

•Color Doppler—mitral flow and tricuspid flow

•Measure PISA if mitral regurgitation (zoom, decrease Nyquist, and measure radius)

•PW Doppler—mitral inflow—position at level of mitral leaflet tips (gate 1–2 mm)

•PW Doppler—PV (usually right upper PV)—1–2 cm into vein (gate 3–4 mm)

•CW Doppler—across mitral valve (stenosis and/or regurgitation and to calculate PISA)

•CW Doppler—tricuspid flow (estimate RV systolic pressure)

• Tilt transducer anteriorly to obtain “five-chamber view,” that is, open up aortic valve/LVOT

•Color Doppler—LVOT/aortic valve

• PW Doppler—LVOT—at the level of the aortic annulus

•CW Doppler—LVOT/aortic valve

•Tilting transducer posteriorly will bring the coronary sinus in view (along AV junction, emptying into right atrium)

4. A2C

• Rotate transducer approximately 60°–90° anticlockwise

•Tilt posteriorly and rotate clockwise to open out descending aorta

5. Apical long axis (apical three chamber)

•Rotate transducer further 30°–45° anticlockwise

•Color Doppler—LVOT/aortic valve

•Recheck CW Doppler across aortic valve if evaluating for aortic stenosis

6. Subcostal view—patient supine and legs bent at knees

•Subxiphoid, midline, tilt anteriorly under sternum with groove toward patients left for four-chamber view

•Color Doppler across interatrial septum to check for a PFO or ASD

•Rotate transducer 90° from four-chamber view until groove is pointing anterosuperiorly

•Same views as PSAX except rotated 90° clockwise

•Sweep from left to right to get apical, midventricular, and aortic valve levels

•IVC should be visualized when scan plane is directed toward the right midclavicular region with some counterclockwise rotation to open out long axis of IVC

•Color Doppler—IVC flow

•PW of hepatic veins (may need to angle posteriorly)

7. Suprasternal view—patient supine and head tilted backward

•Transducer in suprasternal notch, with groove toward left (rotate to about 1 o’clock), parallel to trachea

•Color Doppler in arch and upper descending aortas (especially if suspected coarctation)

•PW Doppler in upper descending aorta (if assessing AR severity)

8. Pedoff probe

•Especially for checking maximal aortic valve gradient in aortic stenosis

•Apical position

•Right upper sternal border (aortic stenosis)

A2C, apical two chamber; A4C, apical four chamber; AR, aortic regurgitation; ASD, atrial septal defect; AV, atrioventricular; CW, continuous wave; IVC, inferior vena cava; LV, left ventricular; LVOT, left ventricular outflow tract; PFO, patent foramen ovale; PISA, proximal isovelocity surface area; PLAX, parasternal long axis; PSAX, parasternal short axis; PV, pulmonary vein; PW, pulsed wave; RV, right ventricular; RVOT, right ventricular outflow tract; RVSP, right ventricular systolic pressure.

A.Parasternal. The parasternal position is typically obtained by placing the transducer at the left of the sternal border in the third or fourth intercostal space. The optimal position for the patient is usually the left lateral decubitus position, but a hybrid between the steep left lateral and supine position may be required to optimize the view. This position allows imaging of the long axis as well as the short axis of the heart.

FIGURE 65.3 Schematic diagram of the parasternal long-axis view in diastole. AMVL, anterior mitral valve leaflet; Ao, aorta; CS, coronary sinus; DA, descending aorta; LA, left atrium; LV, left ventricle; NCC, noncoronary cusp; PMVL, posterior mitral valve leaflet; post. wall, posterior wall; RCC, right coronary cusp; RPA, right pulmonary artery; RVOT, right ventricular outflow tract; STJ, sinotubular junction. (Reprinted from Otto CM, Pearlman AS. Otto and Pearlman’s Textbook of Clinical Echocardiography. Philadelphia, PA: WB Saunders; 1995:21–64. Copyright © 1995 Elsevier. With permission.)

FIGURE 65.4 Schematic diagram of the parasternal short-axis view at the level of papillary muscles. Ant., anterior wall; Ant. septum, anterior septum; Inf., inferior wall; Inf. septum, inferior septum; Lat., lateral wall; Post., posterior wall. (Reprinted from Otto CM, Pearlman AS. Otto and Pearlman’s Textbook of Clinical Echocardiography. Philadelphia, PA: WB Saunders; 1995:21–64. Copyright © 1995 Elsevier. With permission.)

FIGURE 65.5 Schematic diagram of the parasternal short-axis view at the aortic valve level. LA, left atrium; LCC, left coronary cusp; LMCA, left main coronary artery; LPA, left pulmonary artery; NCC, noncoronary cusp; PA, pulmonary artery; RA, right atrium; RCA, right coronary artery; RCC, right coronary cusp; RPA, right pulmonary artery; RVOT, right ventricular outflow tract; SVC, superior vena cava. (Reprinted from Otto CM, Pearlman AS. Otto and Pearlman’s Textbook of Clinical Echocardiography. Philadelphia, PA: WB Saunders; 1995:21–64. Copyright © 1995 Elsevier. With permission.)

FIGURE 65.6 Schematic diagram of the apical four-chamber view. AMVL, anterior mitral valve leaflet; ATVL, anterior tricuspid valve leaflet; DA, descending aorta; LA, left atrium; LV, left ventricle; MB, moderator band; PMVL, posterior mitral valve leaflet; RA, right atrium; RSPV, right superior pulmonary vein; RV, right ventricle; STVL, septal tricuspid valve leaflet; VAS, ventriculoatrial septum (where communication from LV to RA may occur). (Reprinted from Otto CM, Pearlman AS. Otto and Pearlman’s Textbook of Clinical Echocardiography. Philadelphia, PA: WB Saunders; 1995:21–64. Copyright © 1995 Elsevier. With permission.)

FIGURE 65.7 Schematic diagram of the four-chamber view from the subcostal approach. IVC, inferior vena cava; LA, left atrium; LV, left ventricle; PV, pulmonary vein; RA, right atrium; RV, right ventricle. (Reprinted from Otto CM, Pearlman AS. Otto and Pearlman’s Textbook of Clinical Echocardiography. Philadelphia, PA: WB Saunders; 1995:21–64. Copyright © 1995 Elsevier. With permission.)

FIGURE 65.8 Schematic diagram of the aorta and right pulmonary artery from the suprasternal notch window. Ao, aorta; AV, aortic valve; LA, left atrium; RPA, right pulmonary artery; SVC, superior vena cava. (Reprinted from Otto CM, Pearlman AS. Otto and Pearlman’s Textbook of Clinical Echocardiography. Philadelphia, PA: WB Saunders; 1995:21–64. Copyright © 1995 Elsevier. With permission.)

TABLE 65.4 Normal Echo Dimensions in Adults
Factor	Reference Range
	Male	Female
Left Ventricle
LV end-diastolic diameter (cm)	4.2–5.8	3.8–5.2
LV end-systolic diameter (cm)	2.5–4.0	2.2–3.5
Septal thickness (ED) (cm)	0.6–1.0	0.6–0.9
Posterior wall thickness (ED) (cm)	0.6–1.0	0.6–0.9
LV end-diastolic volume (biplane) (cm3)	62–150	46–106
LV end-systolic volume (biplane) (cm3)	21–61	14–42
Ejection fraction (%)	52–72	54–74
Left Atrium
Left atrium (ES) (cm)	3.0–4.0	2.7–3.8
LA volume index (biplane-AL) (cm3/m2)	16–34	16–34
Aorta
Aortic annulus (cm)	2.3–2.9	2.1–2.5
Sinuses of Valsalva (cm)	3.1–3.7	2.7–3.3
Sinotubular junction (cm)	2.6–3.2	2.3–2.9
Proximal ascending aorta (cm)	2.6–3.4	2.3–3.1

AL, area length; ED, end diastole; ES, end systole; LA, left atrial; LV, left ventricular.

TABLE 65.5 Tips for Transthoracic 2D Examination

1.Optimize patient position (left lateral with left hand above head) and environment (darkened room)

2.Ensure imaging in harmonics mode

3.Consider contrast to improve endocardial delineation for technically difficult studies

4.When parasternal and apical images are limited (body habitus, surgical drains, dressings, etc.), the subcostal window may be the only accessible window

5.Consider off-axis views to enhance visualization of specific structures

6.Obtain images at end of expiration, as the heart is closer to the transducer

7.Avoid foreshortening LV apex (especially in A2C)—true apex may be more inferior and lateral than expected

8.If there is a concern of LV thrombus, zoom and check with low-velocity color Doppler to ensure flow throughout. Consider use of endocardial contrast if unclear

9.If an object is suspicious for an artifact, reassess in other imaging planes

10.M-mode can be useful especially for the accurate timing of cardiac events (especially RV or RA collapse in the setting of possible tamponade or assessment of possible systolic anterior motion of the mitral valve)

11.Adjust transducer frequency to maximum that permits adequate far-field penetration/depth

12.Set time gain compensation in the midrange with lower gain in the near field and higher settings in the far field to compensate for attenuation of the beam with increasing distance from transducer

13.Use the least amount of depth that adequately shows the entire area of interest

14.Adjust the transmit gain/output to optimize image brightness/quality—too low, everything appears black; too high results in a “white-out.” Initially set it to high and then adjust downward

15.Adjust the “compress”/dynamic range. Decrease if image quality is poor to produce high-quality contrast images. Increasing it will “soften” images. Typically, as compress is increased, the transmit gain should be decreased to maximize the spectrum of the gray scale

16.Adjust the focus (focal zone) to include the area of interest, because the beam is narrowest (improves image resolution) within this area, especially when imaging near-field structures (e.g., looking for an apical LV thrombus from the apical windows)

17.Set the persistence to low

A2C, apical two chamber; LV, left ventricular; RA, right atrial; RV, right ventricular.

1.Parasternal long axis (PLAX). The PLAX tomographic view is traditionally the first view of a standard transthoracic echocardiogram. The ultrasound beam is lined up between the patient’s right shoulder and the left flank. The right ventricular outflow tract (RVOT) is located at the top of the image, the aorta to the right, the inferolateral (or posterior) wall on the bottom, and the cardiac apex on the left. The anteroseptum is visualized between the RVOT and the LV cavity. Tilting the transducer’s tail toward the left shoulder with slight clockwise rotation aims the ultrasound beam inferomedially and brings the right ventricular (RV) inflow into view. This is good for obtaining the tricuspid regurgitation (TR) velocity as well as viewing the tricuspid valve, RV apex, and the right atrium.

2.Parasternal short axis (PSAX). While the transducer is in the parasternal position, rotating the transducer clockwise by approximately 90° displays the heart in the short axis. The ultrasound beam in this case is roughly from the left shoulder to the right flank. Using different degrees of transducer tilting, and occasionally moving up or down an intercostal space, results in four traditional views of the heart. On tilting from superior to inferior, the views obtained are aortic valve–RV outflow, mitral valve level, mid-ventricle at the papillary muscles, and the LV apex. The images appear as if viewing the heart from the apex and looking through to the base; therefore, the septum is on the left, lateral wall on the right, the anterior and anteroseptal walls at the top, and the posterior–inferior walls on the bottom of the screen.

B.Apical. The apical position is obtained with the patient in the left lateral position and the probe placed at the apical impulse. This position obtains images of the long axis of the heart.

1.Apical four chamber (A4C). The A4C view is obtained with the ultrasound beam transecting the thorax in a superior–inferior fashion. Most institutions orient the transducer to place the left ventricle on the right side of the screen and the right ventricle on the left side. The apex is at the top of the image and the atria at the bottom regardless of the orientation. The inferoseptal and anterolateral walls as well as the apex of the left ventricle can be assessed in this view.

2.Apical five chamber (A5C). A slight rotation of the transducer introduces a fifth “chamber,” the proximal aorta, along with the aortic valve and LVOT. This view allows for qualitative assessment of aortic valve pathology and hemodynamic assessment of the LVOT and aortic valve.

3.Apical two chamber (A2C). Further rotation, 90° counterclockwise from the A4C view, obtains the A2C view. In addition to the left atrium, the LV anterior wall, inferior wall, apex, and mitral valve are also well visualized.

4.Apical three chamber (A3C). A slightly more counterclockwise rotation (approximately 30°) brings the aorta back into view, resulting in the A3C, or apical long-axis, view. This essentially has the same anatomical structures as those in the PLAX view with a different orientation. The apex is better visualized and the RVOT usually drops out of the image. Additional information on mitral and aortic valve hemodynamics can be obtained in this view, which is not ideally obtained in the PLAX view.

C.Subcostal. The subcostal view provides additional views of the ventricles, atria, and atrial septum to those acquired in earlier portions of the examination. In some patients, the subcostal view may be the only way to obtain images of the heart because the parasternal and apical locations may have poor windows (e.g., hyperinflated lungs).

The subcostal position is obtained with the patient in the supine position and the probe located caudal to the xiphoid process. The transducer is placed in the midline nearly parallel to the long axis of the patient’s body so that the ultrasound beam slices toward the spine. This shows the right ventricle at the top right, the left ventricle at the bottom right, and their respective atria on the left. Clockwise rotation along with inferior tilting brings the inferior vena cava (IVC) and hepatic veins into view for right-sided hemodynamic assessments.

D.Suprasternal. Placing the transducer in the suprasternal notch and pointing inferiorly can assess the ascending aorta, aortic arch, and descending aorta. Hemodynamics from this position can better characterize AR as well as the presence of coarctation.

V.Advanced Echocardiographic Techniques

A.Contrast echocardiography. Contrast echocardiography is performed by injecting either agitated saline or one of the commercially available contrast agents into an arm vein. Both are microbubbles that reflect ultrasound waves and opacify intracardiac chambers. The size of the microbubbles relative to the pulmonary capillary diameter determines whether they cross to the left side of the heart or get trapped in the pulmonary circulation. The choice of agitated saline versus commercial contrast agents depends on whether the goal is to visualize the right atrium and ventricle versus the left ventricle and myocardium.

Agitated saline is sterile saline (preferably mixed with some blood), combined with a small quantity of air, which has been exchanged rapidly using a three-way stopcock between two syringes to create small bubbles. These relatively large (and unstable) bubbles are caught in the lung and do not routinely appear in the left side of the heart unless a shunt is present. The appearance of bubbles in the left atrium within three beats of the cardiac cycle after they are seen within the right atrium suggests a right-to-left intracardiac shunt—typically from a small patent foramen ovale. If bubbles appear in the left atrium more than four beats after they are seen in the right atrium, this more likely signifies an intrapulmonary shunt. There is a small risk of embolic complications in the use of agitated saline. Care should be taken to avoid injecting larger air bubbles by inspecting the syringe closely prior to injection and ensuring that the bubbles are very small.

Modern commercial contrast agents consist of either an albumin-based shell containing perfluorocarbon gas (Optison) or a synthetic phospholipid shell containing perfluoropropane gas (Definity). These microbubbles are much smaller (the size of a red blood cell) and more stable; therefore, they can cross the pulmonary capillaries and appear on the left side of the heart, where they opacify the LV cavity for improved endocardial definition and clarify the presence/absence of a suspected LV thrombus/mass. For optimal contrast imaging, it is important to reduce the mechanical index (the output of the machine), typically to 0.4 to 0.6, because higher power ultrasound waves destroy microbubbles.

Contrast enhances Doppler signals. Agitated saline can be used to augment the signal from TR to better estimate peak right ventricular systolic function (RVSP), and commercial microbubble products (Optison and Definity) can be used to enhance the Doppler envelope in patients with aortic stenosis.

Contrast echocardiography was the subject of a black box warning from the Food and Drug Administration (FDA) because of concerns of significant adverse events. More recent data suggest that adverse events following contrast injection are no more common than in those in whom it is not used when appropriate adjustment for severity of illness is made. The FDA has recommended patients with pulmonary hypertension or unstable cardiac conditions such as recent myocardial infarction, unstable angina, decompensated heart failure, ventricular arrhythmia, or respiratory failure should be monitored closely for at least 30 minutes after use. It is contraindicated when a fixed or even transient right-to-left shunt is present or with documented allergy to its components.

B.Three-dimensional (3D) echocardiography is obtained using a transducer that transmits and receives data simultaneously in a 3D volume, in the form of either real-time 3D images or simultaneous biplane (orthogonal) 2D images. The 3D data set can then be manipulated using different software packages to assess function and anatomy. It is of particular benefit for the localization of valvular abnormalities (especially for the complex 3D mitral valve structure), accurate LV volume calculation, improved assessment of the right ventricle, guiding surgical interventions (e.g., mitral valve repair), and complex congenital heart disease. 3D color flow imaging allows for a comprehensive assessment of vena contracta and areas of flow convergence (PISA), which can improve the quantification of valvular regurgitation. It has been documented to allow for a more rapid evaluation of mitral valve area (MVA) in mitral stenosis as compared with conventional 2D planimetry.

C.Myocardial mechanics—tissue strain, strain rate imaging, and speckle tracking. Strain rate imaging allows for the estimation of regional myocardial deformation. Tissue strain, a dimensionless entity, is a measure of the relative deformation of tissue. Myocardial deformation in a segment of interest is assessed with reference to the adjacent segment, avoiding errors introduced by translational motion and tethering. Strain rate is the rate of the deformation between two adjacent points of interest along a scan line and is expressed in seconds. A strain rate curve can be derived by analyzing many adjacent segments along a scan line. Doppler techniques for assessing strain are not always ideal because of angle dependence, signal noise, and the need for a high frame rate. Doppler-independent techniques such as speckle tracking use ultrasonic reflectors (speckles) within tissues that can be followed from frame to frame through the cardiac cycle. This method can be used to assess the radial deformation and torsion of the ventricle. Strain rate is a relatively preload-independent measure of regional myocardial function. Clinical applications include assessment of myocardial ischemia, viability, diastolic function, subclinical LV dysfunction in valve disease and in particular in assessing the early effects of cardiotoxic chemotherapeutic agents on LV function, and cardiac involvement in systemic diseases such as diabetes or amyloidosis. For instance, in amyloidosis, strain is often preserved at the apex, but it is diminished in other areas of the heart.

D.Dyssynchrony. Dyssynchrony occurs when different areas of the ventricles contract in an irregular pattern spatially and temporally. It is primarily seen in patients with impaired systolic function and electrophysiologic conduction delays. M-mode, 2D imaging, color Doppler, and tissue Doppler have all been employed to assess the amount of dyssynchrony but no consensus exists on the optimal approach to evaluate ventricular dyssynchrony (see Chapter 56). After the implantation of a resynchronization device, Doppler echocardiography is used for the optimization of programmed timing. Some useful measures include the following:

1.A difference in the time to peak velocity of >65 ms between opposing walls (basal segments in four-chamber, two-chamber, and three-chamber views yielding a total of six segments) using pulsed tissue Doppler

2.A difference of 40 ms in the interval from the QRS complex to the onset of flow in the RVOT versus the LVOT using pulsed tissue Doppler

3.Using M-mode or speckle tracking, a difference of 130 ms in the septal to posterior wall delay

VI.Special topics

A.Systolic function. 2D imaging is currently the primary echocardiographic means of determining systolic function of the heart. The most utilized measurement is the EF. In the past, EF was based on estimation by visual inspection; however, echocardiographic societies now recommend using standardized objective methods to minimize interobserver differences. LV volume is best measured using the modified Simpson’s method (disk summation method). This involves tracing the LV area in two orthogonal views (typically A4C and A2C) and dividing the left ventricle into a number of cylinders of equal height. Total ventricular volume is calculated by adding up all the volumetric cylinders. All modern machines and digital echo reading systems have integrated software to create and combine the volume data after tracing the LV areas in both apical views. Based on the volumes measured in diastole and systole, stroke volume (SV) and EF can be estimated using Simpson’s method:

Stroke volume (SV) = end-diastolic volume (EDV) − end-systolic volume (ESV)

EF = SV/EDV × 100%

EF = EDV – ESV/EDV × 100%

Newer semiautomated methods in 3D echocardiography using full matrix-array transducers give accurate, reproducible assessments of LV volume and EF that are superior to 2D methods when magnetic resonance imaging is used as the gold standard. LV torsion uses speckle tracking to assess LV systolic function by assessing the difference between the clockwise rotation of the base of the heart and the counterclockwise rotation of the apex of the heart (approximately 12°). Global strain also appears to be a robust parameter for assessment of LV function.

B.Diastolic function. Doppler echocardiography remains the primary modality for assessing LV diastolic function and estimating filling pressures. In the most recent iteration of the guidelines, information obtained from PW Doppler of mitral inflow, left atrial (LA) volume index, TDI of the mitral annulus and TR velocity obtained from CW Doppler are used for staging.

Patients with normal LVEF and diastolic dysfunction can be identified if they have >50% of the following:

1.Average E/e′ > 14,

2.Septal e′ velocity < 7 cm/s or lateral e′ velocity < 10 cm/s,

3.TR velocity > 2.8 m/s, or

4.LA volume index > 34 mL/mm².

Those with <50% have normal diastolic function and those with 50% have indeterminate diastolic function.

For patients with abnormal LVEF or normal LVEF and abnormal diastolic function, grading of diastolic dysfunction relies on examination of the E/A ratio and E velocity in addition to the other variables.

Patients with an E/A ≤0.8 and E ≤50 cm/s have grade I diastolic dysfunction with normal left atrial pressure (LAP).

Those with E/A ≥2 have grade III diastolic dysfunction and increased LAP. For those with an intermediate E/A ratio (>0.8 but <2) or E/A ≤ 0.8 with E > 50 cm/s, the following criteria are taken into account:

1.Average E/e′ >14,

2.TR velocity > 2.8 m/s, and

3.LA volume index > 34 mL/mm².

If two or three of the three are negative, the patient has grade I diastolic dysfunction with normal LAP. If two or three of the three are positive, the patient has grade II diastolic dysfunction with increased LAP.

In the circumstance that data regarding only two of these variables are available, if both are negative, the patient has grade I diastolic dysfunction with normal LAP; if both are positive, the patient has grade II diastolic dysfunction with increased LAP; and if only one is positive, the grade of diastolic dysfunction and LAP cannot be determined.

Accurate assessment of diastolic dysfunction can be limited by many factors including inadequate views, rhythm (atrial fibrillation or ventricular pacing), and mitral valvular dysfunction (severe annular calcification, severe regurgitation, prior valve replacement or repair).

C.Hemodynamics

1.Transvalvular pressure gradient. The Bernoulli equation allows measurement of relative pressure differences across valves, shunts, or the LVOT. In its complete form, the Bernoulli equation is too complex for routine clinical use, because it incorporates three main components, namely, convective acceleration, inertial term (flow acceleration), and viscous friction. In many clinical situations, the latter two components can be ignored, leaving the flow gradient across an orifice to be derived from the convective acceleration term alone:

ΔP = 4 × (V₂² – V₁²)

where V₂ is the velocity distal to an obstruction and V₁ is the velocity proximal to an obstruction.

The flow proximal to a narrowed orifice (V₁) is much lower than the peak flow velocity (V₂) and can be frequently ignored, leaving a simplified Bernoulli equation:

ΔP = 4V²

The simplified Bernoulli equation is unreliable when

a.V₁ is > 1 m/s, which occurs in serial lesions (subvalvular and valvular stenoses) and mixed stenosis with regurgitation.

b.Viscous resistance becomes significant such as in the evaluation of long stenoses (e.g., coarctation or a tunnel-like ventricular septal defect).

c.The inertial term (flow acceleration) is not negligible (flow through normal valves).

It is important to realize that in aortic stenosis, the Bernoulli equation represents the maximal instantaneous gradient across the valve, which is always higher than the customary peak-to-peak gradient measured in the catheterization laboratory because the LV systolic peak and aortic peak pressures do not occur at the same time and therefore are not instantaneous.

The flow within the heart is pulsatile; hence, mean gradients are an important measure and are obtained by integrating the velocity profile over the ejection time. This can be readily obtained with the software available on all modern echocardiography machines by simply tracing the area of the velocity profile. The mean pressure gradient is then derived from the mean velocity data using the Bernoulli equation.

2.Intracardiac pressure measurement

a.Estimated right atrial (RA) pressure can be derived from the size of the IVC and its response to changes in respiration or a sniff (Table 65.6). Using a dilated IVC to assess elevated RA pressures is not accurate in mechanically ventilated patients; however, a small IVC of size <1.2 cm in a mechanically ventilated patient is 100% specific for an RA pressure <10 mm Hg.

TABLE 65.6 Estimation of RA Pressure
IVC Diameter	Change with Respiration/Sniff	Estimated RA Pressure (mm Hg)
Normal (<2.1 cm)	Decrease >50%	0–5
Dilated (>2.1 cm)	Decrease >50%	6–10
Dilated (>2.1 cm)	Decrease <50%	10–15
Dilated (>2.1 cm)	No change	>15

IVC, inferior vena cava; RA, right atrial.

b.Pulmonary artery systolic pressure (PASP) is estimated from the TR peak velocity. Provided that there is no tricuspid valve obstruction, peak TR velocity will depend on the pressure gradient between the right ventricle and the right atrium. Estimated RVSP is equal to this pressure difference, determined from the peak TR velocity, plus the estimated RA pressure. In the absence of pulmonic stenosis, the RVSP is similar to the PASP:

PASP = 4 × (peak TR velocity)2 + estimated RA pressure

c.Pulmonary artery diastolic pressure (PADP). Pulmonary regurgitation represents the pressure difference between the pulmonary artery (PA) and the right ventricle at end systole. Hence, the end pulmonary regurgitation velocity can be utilized to measure the end-diastolic pressure difference between the PA and the right ventricle. The RV end-diastolic pressure should be similar to the RA pressure; therefore, addition of estimated RA pressure to the end-diastolic pressure difference between the PA and the right ventricle will estimate the PADP:

PADP = 4 × (end pulmonary regurgitant velocity)² + estimated RA pressure

d.Estimated LAP or left ventricular end-diastolic pressure (LVEDP). Provided that there is no mitral stenosis, LVEDP and LAP should be the same. This important measure of LV diastolic function can be estimated by several methods.

(1)Deceleration time (DT) of mitral inflow. A DT of <150 ms is strongly suggestive of an elevated LVEDP/LAP. In very young patients, a DT <150 ms may be normal because of rapid equalization of pressures secondary to vigorous early diastolic relaxation.

(2)Difference between pulmonary venous atrial duration and mitral atrial duration (Ar–A). Normally, mitral A-wave duration is greater than pulmonary venous atrial reversal (Ar) duration. When LVEDP is increased, the velocity and duration of the mitral A-wave decrease, whereas pulmonary vein Ar velocity and duration increase. The difference between the duration of the Ar-wave and the mitral A-wave correlates with LVEDP. An Ar–A duration of >50 ms is specific for an elevated LVEDP >20 mm Hg. This is reliable in patients with reduced EF but not in patients with normal EF. The primary limitation with this method is the difficulty in accurately measuring the duration of Ar.

(3)Combined mitral inflow/CMM index (E/V_p ratio). This index has been demonstrated to correlate with LAP/LVEDP, especially when these filling pressures are elevated. A ratio of >2 is suggestive of elevated filling pressures. In patients with normal EFs, especially with small ventricles and hyperdynamic function, the flow propagation velocities are not accurate.

(4)Combined mitral inflow/TDI index (E/E ratio). This index has been shown to be a semiquantitative measure of LVEDP. A ratio of >10 (using the lateral annulus) or >15 (using the septal annulus) correlates with a wedge pressure of >20 mm Hg. A ratio of <8 (using the lateral annulus) correlates well with normal filling pressures. For intermediate values, other information such as LA size should be incorporated to assess whether filling pressures are elevated.

3.dP/dt. This index of LV contractility is the rate of pressure increase during isovolumic contraction and is traditionally obtained using invasive pressure transducers. It can be estimated from the CW Doppler of the MR jet. During isovolumic contraction, there is no change in LAP; therefore, MR velocity changes reflect dP/dt, with more rapid increases in MR velocity being associated with increased contractility. The pressure change between 1 and 3 m/s = 4 (V₂² – V₁²) = 32 mm Hg. The time it takes the ventricle to accelerate an MR jet velocity from 1 to 3 m/s is measured and then the dP/dt is calculated as follows:

dP/dt = 32 mm Hg/time (in seconds)

This has been demonstrated to correlate well with invasively measured dP/dt. It is considered normal when the calculated value is >1,200 mm Hg/s.

4.Continuity equation is an application of the principle of conservation of mass, which states that flow across a conduit of varying diameter is equal at all points. This equation is useful in quantifying a stenotic aortic valve area (AVA) that cannot be accurately measured using planimetry from the transthoracic window. Flow at any point in the heart is the product of the cross-sectional area (CSA) and the flow velocity. As flow velocity varies during ejection in a pulsatile system, individual velocities must be integrated to measure the total volume of flow (velocity time integral [VTI]). This is determined by tracing the spectral Doppler profile, using standard measurement software built into all echocardiography machines.

Flow at any point = CSA × VTI

Based on the continuity equation, flow through the LVOT must be equal to the flow through the aortic valve; therefore, AVA can be calculated following these steps:

Flow across the LVOT = flow across the aortic valve

Area_LVOT × VTI_LVOT = area_{aortic valve} × VTI_{aortic valve}

Area_{aortic valve} = (area_LVOT × VTI_LVOT)/VTI_{aortic valve}

Area_{aortic valve} = [(diameter_LVOT/2)² × π × VTI_LVOT]/VTI_{aortic valve}

Area_{aortic valve = [(diameterLVOT)² × 0.785 × VTILVOT]/VTIaortic valve}

The assumption is that the LVOT cross-section is a circle. The greatest source of error in this equation is in the measurement of the LVOT diameter because the value is squared, resulting in magnification of any initial measurement error. The appropriate place to measure the LVOT can be difficult to define accurately in some calcified valves. The dimensionless index (DI) is the ratio of the VTI of LVOT to the VTI of aortic valve. It is preferable to use this to assess aortic stenosis when accurate measurement of the LVOT diameter is not possible or in those patients with history of previous aortic valve replacement. A DI <0.25 suggests severe aortic stenosis.

DI = VTI_LVOT/VTI_{aortic valve}

Of note, the flow across the LVOT per beat is the SV, which can thus be calculated from the product of the LVOT diameter and flow velocity (VTI_LVOT):

SV = (diameter_LVOT)² × 0.785 × VTI_LVOT

5.Volumetric method to assess regurgitant volume/regurgitant fraction. This is based on the conservation of flow, with total flow across a regurgitant valve being equal to the sum of the forward flow and the regurgitant flow. For example, for MR:

Total transmitral flow volume = forward flow volume + regurgitant volume

(LVOT flow can be assumed to equal the forward flow provided there is no AR)

Regurgitant volume = mitral forward flow – LVOT flow

Regurgitant volume = (diameter_mitral² × 0.785 × VTI_mitral) – (diameter_LVOT² × 0.785 × VTI_LVOT)

Regurgitant fraction = regurgitant volume/total mitral flow

Because of the multiple assumptions and calculations performed, this method is prone to error and is rarely used clinically.

6.The PISA method is another application of the principle of conservation of mass. It is based on the phenomenon that flow accelerates proximal to a narrowed orifice. Using color Doppler, as flow accelerates, its velocity may exceed the Nyquist limit which results in color reversal because of aliasing. This is seen as a series of colored (“isovelocity”) hemispheres with color flow imaging, with the velocity of flow at the surface of this hemisphere being the aliasing velocity (Nyquist limit) of color flow in that direction. Decreasing the aliasing velocity will increase the size of the hemisphere, because the velocity at which color changes is reduced. In keeping with conservation of mass, blood flow at the surface of this hemisphere is the same as flow through the regurgitant orifice, and this is the basis of using the PISA method to estimate the regurgitant orifice area (ROA) of a valve. PISA has been most extensively used to estimate the mitral ROA to quantify MR.

Flow at surface of hemisphere = flow through regurgitant orifice

Surface area of hemisphere × velocity at hemisphere = ROA × peak velocity of regurgitation [using CW Doppler through the mitral valve]

2 × π (radius)² × aliasing velocity = ROA × peak MR velocity

ROA = 2 × π (radius)² × aliasing velocity/MR_CW peak velocity

The greatest source of error is in defining the precise location of the ROA, so as to accurately calculate the radius.

This method can also be used to measure MVA in mitral stenosis (where forward flow convergence is seen and measured) and the aortic ROA in AR, although it may be difficult to obtain satisfactory visualization of the aortic PISA for quantification from the apical long-axis view (best view to appropriately line up AR jet with the Doppler). When the jet is eccentric, and a full hemisphere is not visible, an angle correction should be considered. The PISA equation for MR can be simplified if the aliasing velocity is set to 40 cm/s and it is assumed that peak MR velocity will be 5 m/s (equates to a normal LV-to-LA pressure gradient of 100 mm Hg). Using these two constants, the PISA equation is simplified to

ROA = (radius)²/2

Peak MR velocity will increase or decrease depending on the changes in LV systolic pressure and LAP, and it cannot always be assumed to be 5 m/s. However, this method is useful for semiquantification and rapid assessment. Regurgitant volume can be calculated as follows:

Regurgitant volume = ROA × VTI_MRjet

7.Pressure half-time (P½) is used to estimate the MVA, because the time for the pressure to fall by half across a stenotic valve is proportional to the degree of stenosis. It is the time interval for the peak pressure gradient to fall by half. Using the Bernoulli equation to convert pressure to velocity, there is a constant relationship between peak velocity and the velocity at P½.

Pressure at half the peak pressure = 1/2 peak pressure

4 × (V½)² = 0.5 × (4 × V_max)²

In addition, the P½ has a constant relationship with the DT of the early mitral filling wave, and it is usually estimated from the following:

P½ = 0.29 × DT

Hence, P½ can be easily measured by using the DT or by simply measuring the time interval from peak to ½ peak pressure (which is determined from the V_max). Most echocardiographic measurement software packages automatically calculate P½ when the slope of the CW Doppler of the mitral inflow jet is measured. For mitral stenosis, an empirical constant has been validated to correlate P½ and MVA:

MVA = 220/(P½)

This has only been validated for native valves and will overestimate valve areas for prosthetic valves.

The other primary use of P½ is to help quantify AR. The P½ of the AR Doppler velocity jet becomes shorter when the pressures in the aorta and the left ventricle equilibrate more quickly. This can occur with increasing severity, especially in acute AR. A P½ <250 ms suggests severe AR. There are many limitations to this because of the fact that aortic and LV compliance and systemic vascular resistance affect P½.

VII.Technical Aspects and Advanced Image Acquisition

A.Machine settings. To obtain the best images and accurate Doppler information, it is important to optimize the machine settings during different parts of the examination (Tables 66.3 and 66.5).

1.Time gain compensation. These controls differentially amplify the echo signals returning from different depths to compensate for attenuation of the ultrasound beam with increasing distance from the transducer. This function is useful with higher frequency transducers, because they are associated with more attenuation at greater depths.

2.Depth. Start with the greatest depth to get an overview and then decrease the depth to include all of the target structure. A depth of 16 cm is usually adequate for the apical window and 12 cm for parasternal imaging. Increasing the depth decreases the frame rate, reducing temporal resolution.

3.Transmit gain. This adjusts the displayed amplitude (power) of all received signals and, therefore, affects the brightness of echoes displayed. Setting the power too low results in inadequate returning signals and poor image quality, whereas setting it too high results in image white-out.

4.Compress. The compress setting is also known as a dynamic range. It converts the range of returning echo intensities, which may vary a billion-fold in intensity, into 100 to 200 visual shades of brightness or the “gray scale.” Increasing the compress will “soften” the image and allow identification of lower level signals. Decreasing the compress results in the production of high-quality contrast images such that weaker signals are eliminated, noise is reduced, and the strongest echo signals are enhanced. Therefore, the compress/dynamic range is decreased when image quality is poor.

5.Focus (or position). The focal zone of the transducer indicates the region of the image at which the ultrasound beam is narrowest, and hence where spatial resolution is maximal. Therefore, it is important to reposition the focus to the area of greatest attention/importance, especially those in the near field. When adjusted proximally, however, distal structures may appear blurred as the ultrasound beams scatter.

6.Persistence. Persistence is the temporal averaging of the latest frame with the previous frames to produce a smooth or less noisy display. Fast-moving cardiac structures (e.g., valve leaflets) may appear blurred if the persistence is set above low.

B.Imaging artifacts

1.Acoustic shadowing. Highly reflective structures block transmission of ultrasound to distal structures, causing poor imaging of these far-field structures. For instance, a mechanical mitral prosthesis prevents good visualization of the left atrium from the apical window.

2.Reverberation. This occurs when multiple linear echo signals are generated from a back-and-forth reflection between two strong reflectors of the ultrasound signal, before the signal returns to the transducer. These appear as multiple parallel irregular dense lines extending from the structure into the far field (e.g., linear echodensity in the ascending aorta in the PLAX view simulating a dissection flap, which is a reverberation from a more anteriorly lying structure, such as a rib). Reverberation artifacts will be present at a multiple of the distance between the two strong reflectors—usually at twice the distance between the strong reflectors. Careful analysis of the artifact in multiple views and with color Doppler should be performed. Color flow signals will be seen to pass through the artifact.

3.Refraction. Refraction of the ultrasound beam as it passes through a tissue layer can result in a side-by-side double image. This artifact is often seen in PSAX views of the aortic valve where the image appears to show two aortic valves overlapping.

4.Beam width artifact. Ultrasound beams are 3D and are reflected from 3D structures, but they are displayed in a 2D tomographic plane. Strong reflectors at the edge of a central beam, especially outside the narrow proximal focal zone, can be superimposed on a structure in the central zone, with the resulting appearance of a structure within the image, that is, outside the 2D tomographic plane (e.g., an aortic valve in the left atrium in the A4C view).

5.Range ambiguity. Echo signals from earlier pulse cycles reach the transducer on the next receiving cycle because of re-reflection, resulting in deep structures that appear closer to the transducer than their actual location, and are manifested as the appearance of an anatomically unexpected echo. This can be confirmed by the disappearance of the artifact when the depth setting is changed.

6.Side lobe artifacts. In addition to the central beam, transducers produce side lobes 10° to 30° off axis. All echoes returning from structures in these peripheral beams are displayed, as if they arose from targets within the main beam. Therefore, strong reflectors may be imaged by these low-intensity side lobes and displayed in an erroneous position on the screen. This is a major source of “clutter” in cardiac cavities. Harmonic echoes have much lower intensity side lobes, with a resulting reduction in side lobe artifacts in the image.

C.Factors affecting color Doppler image. Many factors affect spectral Doppler and color flow Doppler, and it is important to consider these. They can be broadly divided into three groups: machine settings, imaging factors, and hemodynamic factors (see Table 65.7 for tips to optimize Doppler settings).

TABLE 65.7 Tips for the Transthoracic Doppler Examination

1.Doppler (all modalities) is very angle-dependent—angle between the ultrasound beam and the blood flow jet of interest should be <20°. In order to achieve this, off-axis views are often required

PW and CW Doppler

2.Shifting the Doppler baseline up or down can double the maximal velocity detected (still <2 m/s) for PW

3.Increasing the depth decreases the Nyquist limit and reduces the maximal velocity that can be measured with PW

4.Recheck high-velocity jets with the Pedoff (CW) probe to confirm peak velocity (include right upper sternal border positions when trying to obtain peak aortic stenosis velocity)

5.Start with high-gain setting and reduce until noise and clutter are adequately suppressed

6.Set wall filter to low to avoid overestimation of low velocities

7.Decreasing the compress enhances the edges of the spectral envelope; increasing it enhances the various velocities displayed within the Doppler envelope

8.Initially set “reject” at low (20%–40%) to allow the display of a wide range of signals, then increase to remove signals that obscure the image (i.e., to reduce noise)

9.Adjust gate width—1–2 mm for mitral inflow and LVOT, 3–4 mm for pulmonary venous flow, and 5–10 for Doppler tissue imaging

Color flow Doppler

10.Narrow the sector and minimize the depth to maximize color resolution (increase frame rate)

11.Spatial resolution is higher axial to the beam than lateral

12.Higher transducer frequencies result in an increased area of flow disturbance (reduces the Nyquist and increases ability to visualize lower velocities)

13.Adjust color gain until just before noise appears in the color

14.Minimize wall filters during analysis of PISA/flow convergence, to avoid overestimating low velocities

15.Decreasing the Nyquist limit increases the size of any regurgitant jet as lower velocities are detected (normally not color coded at higher Nyquist velocities); therefore, set at 50–60 cm/s initially

16.Be careful not to miss or underestimate very eccentric jets of mitral regurgitation or aortic regurgitation

17.Remember that chamber constraint reduces the size of a jet —wall jets tend to underestimate the severity of regurgitation compared to a jet that is not constrained by a wall

CW, continuous wave; LVOT, left ventricular outflow tract; PISA, proximal isovelocity surface area; PW, pulsed wave.

1.Machine settings

a.Nyquist limit. At any given depth, in color Doppler imaging, the Nyquist or aliasing velocity (which is related to the PRF) can be adjusted. Typically, it is set to 50 to 60 cm/s. The lowest velocity that is displayed on the color map is related to the Nyquist (minimal displayed velocity = Nyquist × 2/32). Therefore, decreasing the Nyquist increases the lowest velocity displayed, which has the effect of increasing the size of the jet area.

b.Transducer frequency. In color flow imaging, higher transducer frequency reduces the peak velocity (Nyquist limit) that can be measured (see Doppler equation above). Lower Nyquist results in an increased color flow jet area. Therefore, higher frequency transesophageal echocardiography generally produces larger areas of flow disturbance than transthoracic echocardiography. In spectral Doppler imaging, lower frequency transducers can measure higher velocities.

c.Depth setting. Minimizing the depth setting to encompass only the region of interest maximizes the PRF and frame rate.

d.Gain. Adjust the color gain until just before random noise appears in the color. Increased color gain increases the size of color flow disturbance. 2D gain should be decreased during the color Doppler examination to maximize color flow disturbance because each pixel is assigned to either 2D or color. In PW and CW Doppler, start with a high-gain setting until the desired signal is appreciated. The gain is decreased until noise and clutter are adequately suppressed.

e.Baseline. Used primarily for unwrapping aliased signals. Generally leave it in the middle of the color bar, but it can be adjusted to maximize the velocity that can be displayed with PW or color Doppler. This is also useful for highlighting a specific velocity as in proximal convergence analysis.

f.Wall filter. Excludes low-velocity, high-amplitude signals from myocardial motion. If set too high, it tends to decrease the color flow disturbance. A typical initial setting is 400 Hz. The setting of the wall filter should be minimized during analysis of the proximal flow convergence region to avoid overestimation of low velocities (i.e., set low for PW Doppler and high for CW Doppler).

g.Beam width. Beam width is especially important with PW and CW Doppler. As the ultrasound beam propagates, it spreads out. For example, when sampling pulmonary venous flow with pulse Doppler from the apical view, the sample volume may be at 16-cm depth and the ultrasound beam may be >1 cm in width. This can lead to the detection of aortic flow, which is displayed as if it arose along the beam axis (from the pulmonary vein) leading to beam width artifact.

h.Gate length or sample size. This is the size of the PW Doppler sampling region. It is usually set at 3 to 5 mm. Narrowing the gate focuses the velocity data to a smaller spatial area and can help improve image quality, but it requires very accurate positioning to prevent missing of the appropriate sample area during cardiac motion.

i.Scale. Controls the range of Doppler velocities displayed. As the velocity scale increases, the velocity limits increase and the displayed waveform size decreases.

j.Compress. In spectral (PW and CW) Doppler, the compress setting adjusts the gray scale, which controls image softness. Decrease the compress to enhance the edges of the spectral envelope. Increase the compress to enhance the various velocities displayed within the Doppler spectrum. Set at 30 dB or higher initially.

k.Reject. In spectral Doppler, the reject control removes low-amplitude signals (“noise”) from the spectral display. The reject control is initially set at a low level (20% to 40% maximum) to allow the display of a wide range of signals. The reject is then increased to remove signals that obscure the image.

2.Imaging factors

a.Interrogation angle. Color flow imaging measures only the component of flow that is parallel to the ultrasound beam. This is related to the true flow velocity by the cosine of the angle between the blood flow and the interrogating ultrasound beam. Satisfactory alignment (as parallel to the flow as possible) is vital to record the full and maximal velocity jet with spectral (both PW and CW) Doppler.

b.Attenuation. Loss of signal strength caused by too high a transducer frequency for the required depth results in a reduced area of color flow disturbance.

c.Acoustic shadowing. Loss of signal strength caused by a proximal reflector of ultrasound (e.g., a mechanical prosthetic valve preventing apical imaging of MR jet in the left atrium).

3.Hemodynamic factors

a.Flow volume. Increasing regurgitant volume results in an increased area of color flow disturbance, and this is the basis for the common practice of judging the severity of valvular regurgitation by the size of the color jet. However, as outlined in this chapter, many factors affect the size of the color flow jet area. Therefore, it is important to include other factors in the assessment of regurgitation, such as ventricular and atrial sizes, the morphologic appearance of the valve, the width of the color jet at its narrowest point (vena contracta), and, in particular, more quantitative analysis using the proximal flow convergence region (PISA). Several cardiac cycles should be inspected with minor adjustments in the angle of interrogation to ensure that the largest jet is visualized.

b.Driving pressure. Increased pressure gradient across a regurgitant orifice results in an increased color flow disturbance in the receiving chamber. Color jet size is closely related to jet momentum, given by flow rate multiplied by jet velocity.

c.Chamber constraint in eccentric jets. Impingement of a regurgitant jet against walls of the receiving chamber will decrease the size of the color disturbance. For example, severe but eccentric MR may have a very small area of color flow disturbance because the jet loses momentum to the constraining LA wall and appears narrower in a 2D view as it is splayed out over a larger surface area of the wall.

4.Doppler artifact. Mirror image artifact can be seen occasionally when the Doppler signal is duplicated on the other side of the baseline.

ACKNOWLEDGMENTS: The author thanks Drs. Patrick J. Nash, Steven Lin, Guy Armstrong, Ron Jacob and Kia Afshar for their contributions to earlier editions of this chapter.

Relevant Guidelines

Douglas PS, Garcia MJ, Haines DE, et al. ACCF/ASE/AHA/ASNC/HFSA/HRS/SCAI/SCCM/SCCT/SCMR 2011 Appropriate Use Criteria for Echocardiography. A Report of the American College of Cardiology Foundation Appropriate Use Criteria Task Force, American Society of Echocardiography, American Heart Association, American Society of Nuclear Cardiology, Heart Failure Society of America, Heart Rhythm Society, Society for Cardiovascular Angiography and Interventions, Society of Critical Care Medicine, Society of Cardiovascular Computed Tomography, and Society for Cardiovascular Magnetic Resonance Endorsed by the American College of Chest Physicians. J Am Coll Cardiol. 2011;57(9):1126–1166.

Lang RM, Badano LP, Mor-Avi V, et al. Recommendations for cardiac chamber quantification by echocardiography in adults: an update from the American Society of Echocardiography and the European Association of Cardiovascular Imaging. J Am Soc Echocardiogr. 2015;28(1):1–39.e14.

Relevant Book Chapters

Armstrong WF, Ryan T, Feigenbaum H. Feigenbaum’s Echocardiography. 7th ed. Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2010:9–66, 217–240.

Oh JK, Seward JB, Tajik AJ. The Echo Manual. 3rd ed. Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2007:1–28, 59–108.

Otto CM. Textbook of Clinical Echocardiography. 5th ed. Philadelphia, PA: Saunders; 2013:1–64, 89–104, 131–189.

Weyman A. Principles and Practice of Echocardiography. 2nd ed. Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams & Wilkins; 1994:3–28.