Echocardiography

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CHAPTER 6 Echocardiography

DESCRIPTION OF TECHNICAL REQUIREMENTS

Because the quality of an ultrasound study is operator dependent, it is highly recommended that the operator have a solid knowledge of ultrasound physics and the various options available on the ultrasound equipment used.

Moreover, information such as patient identification, blood pressure measurements, any other specific clinical condition during which the study is recorded (e.g., the use of a specific drug, electrocardiogram tracing) are all necessary for a clear understanding of the problem to be studied. Specific details regarding each of the echocardiography techniques are discussed below.

Techniques

M Mode

The M (for motion) mode consists of a graphic in which it is possible to analyze the motion of cardiac structures over time. It is necessary for the correct position of a cursor line along the anatomic cardiac structure to be studied and this cursor line has its origin point at the ultrasound transducer itself. Errors in measurements with M mode are possible because some planes might be in an oblique orientation. This can be corrected with the anatomic M mode, an adjustable cursor line (angle up to 180 degrees), available in actual echocardiograph machines. The M mode is used for cardiac dimensions, subtle cardiac motion abnormalities, and assessment other time-related parameters (Fig. 6-1).

Doppler Echocardiography and Color Flow Imaging

Doppler echocardiography uses ultrasound waves to calculate blood flow velocities based on the way the flowing blood reflects the ultrasound waves. The reflected ultrasound frequency is higher when the blood flows toward the transducer, and the opposite effect is true when it flows away from the transducer. One important aspect of the way Doppler works is the angle of interrogation. The more parallel the reflected waves to the transducer, the higher the reflected sound waves (called Doppler shift) and the peak velocities. Pulsed wave Doppler uses only one crystal to transmit and receive the reflected frequency. This kind of Doppler is used to study lower blood flow velocities such as mitral and tricuspid diastolic flows. The continuous wave Doppler uses one crystal to send and another one to receive the sound frequency from the blood and it can measure highest velocities along the way of its beam, such as the flow through a stenotic aortic valve. The color flow Doppler uses the same principle as the pulsed wave Doppler, with the difference being for the several sampling sites along multiple ultrasound beams, so it can analyze the blood flow in different velocities, directions, and extent of turbulence. The flow is coded in multiple colors: blue for the flow going away from the transducer and red if it is coming toward the transducer. Properly configured, the green color can also show turbulence and its extent.

Important hemodynamic information can be obtained by using 2D and Doppler techniques but none of them, not even the invasive approach, is perfect, being influenced by several other hemodynamic factors. Flow velocities can be converted to pressure gradients by using the modified Bernoulli equation1:

image

where v is peak velocity. It is modified because several other elements can be ignored such as the velocity proximal to a fixed orifice and also flow acceleration and viscous friction. The pressure gradient obtained by this equation represents the instantaneous gradient, which is different from the peak-to-peak pressure measurement in the catheterization (cath) lab because those two peaks are not simultaneous. Several pressure measurements can be made using this equation such as: aortic stenosis gradient, right ventricle systolic pressure, pulmonary end-diastolic and mean arterial pressure, left atrial systolic pressure, and left ventricular end-diastolic pressure.

With the use of Doppler, it is also possible to obtain the stroke volume and cardiac output based on the equation that uses the hydraulic orifice formula, which is1:

image

The flow velocity is given by the sum of all instantaneous velocities of the curve by Doppler tracings, after tracing the area enclosed by the baseline and the Doppler spectrum. The CSA is given by the assumption that the orifice is a perfect circle so the diameter is measured and applied in the equation:

image

in which “D” means diameter. To calculate the systolic volume, the left ventricular outflow tract (LVOT) area is used in the first equation. And then, to get the cardiac output, the systolic volume is multiplied by the heart rate. Any flow across an orifice in the heart can be calculated using this method. Hence, it is possible to get the right ventricular outflow tract (RVOT) flow and, together with the LVOT flow, to get the ratio between pulmonary and systemic flows—important in some quantification of severity in congenital abnormalities. This method is also useful to obtain the regurgitant volume across a heart valve, such as mitral or aortic regurgitation (Fig. 6-3).

Transesophageal Echocardiography (TEE)

Since its first use, TEE has been considered not only an accessory imaging technique but in some circumstances, it is the main imaging modality for cardiac diagnoses, such as mitral valve disease, aortic dissection, endocarditis and its complications, atrial septum defects, cardiac tumors, and electrophysiology studies monitoring. In patients with atrial fibrillation, TEE information can be decisive in the choice of treatment.

The TEE uses a gastroesophageal endoscopy probe modified with a 7 to 10 MHz transducer on the tip. The study technique consists of inserting the probe through the esophagus, making it possible to get high resolution images of the heart. The study is performed in a special room, with emergency-trained personnel, suction and oxygen devices, and all necessary resuscitation equipment. The probe then can be anteflexed and retroflexed, moved from side to side, rotated clockwise and counterclockwise manually and the planes can be moved from 0 to 180 degrees wide with the touch of one button (Fig. 6-4).

Intracardiac Echocardiogaphy (ICE)

By using small diameter probes, it is possible to get intracardiac images. These probes are inserted until they reach the right atrium, from which all the image planes are made, and because those probes are multifrequency they have capabilities of complete 2D and Doppler study. The great advantage of ICE is for interventional studies such as transcatheter closure device placement for atrial septum defects, including a more detailed study of the defect, its size, and associated congenital defects such as pulmonary vein anatomy, and the margins of the defect—all this information being useful to predict the success of the closure. Ablation procedures, such as pulmonary vein isolation for atrial flutter, with better visualization of the tissue for radiofrequency energy application, avoids complications such as pulmonary valve stenosis owing to inadvertent ablation.

Indications

In the following sections, the most important clinical situations for the use of echocardiography will be discussed, including transthoracic and transesophageal echocardiography, as well as other techniques.

Systolic Function and Quantification of Cardiac Chambers

The 2D echocardiography is the main tool for the evaluation of systolic function and chamber quantification, even considering the newer techniques such as 3D echocardiography, tissue Doppler, strain and strain rate imaging. With 2D images of the LV it is possible to analyze the thickening of ventricular walls and provide not only a subjective evaluation of LV function but also the quantitative assessment, which can be done by calculating changes in size and volume of the chamber. The systolic function is crucial for guiding the patient’s treatment and prognosis; the evaluation of regional contractility is important for patients with suspected or diagnosed coronary heart disease.

Left ventricle. The LV size can be obtained by the 2D image of parasternal short axis view and the M mode for that plane at the level of the papillary muscles or from the long axis view. This method can be used for normal hearts or with no regional wall motion abnormalities and the mass also can be calculated from the measurements of the interventricular septum and posterior wall thickness. LV function is calculated from the diastolic and systolic diameters and also by the Simpson method. From the diameter values, it is possible to calculate the fractional shortening, the percentage change in LV dimensions, by the following formula:

image

in which LVEDD means left ventricular end-diastolic diameter and LVESD means left ventricular end-systolic diameter. The Simpson method can be used to obtain LV volumes and then the calculated ejection fraction (EF) by the following formula2:

image

Regional wall motion analysis is the most important tool for the evaluation and diagnosis of patients with suspected or confirmed coronary heart disease. For this purpose, the LV is divided into 16 segments. First into three levels: basal, mid, and apical. The basal and mid levels have six segments: antero-septum, anterior, lateral, posterior, inferior, infero-septum; and the apex is then divided into four segments being the anterior, lateral, inferior, and septum. There was only 1 change for an additional “apical cap” segment by the Association Writing Group on Myocardial Segmentation for Cardiac Imaging with the total of 17 segments.3 Each segment is scored according to its contractility: 1-normal; 2-hypokinesis; 3-akinesis, 4-dyskinesis, and 5-aneurysmal. Each value of each segment is summed and the total is divided by the number of segments analyzed for the wall motion score index (WMSI), being normal if equal to 1.

Right ventricle (RV). The RV size is best measured from the apical four-chamber view and its thickness from the subcostal view at the peak R on the ECG. RV size is useful to detect volume and/or pressure overload and can be accompanied by RV wall thickness higher than 5 mm, the abnormal value.2

Left atrium (LA). The left atrium size is measured from the parasternal long axis view (PLAX) at end systole. This view can provide some underestimation on the chamber size because the LA can enlarge longitudinally. Because of this aspect of LA diameter, the best method is to calculate its volume from two orthogonal apical views using one of the four methods available (Fig. 6-5).2

image image

image FIGURE 6-5 Ejection fraction (EF) calculation based on the modified Simpson rule, showing left ventricle volumes in diastolic and systolic phases (white arrows). A and B, four-chamber views. C and D, two-chamber views. The EF for the four-chamber views is EF = end diastolic volume − end systolic volume/end diastolic volume = 106 − 37/106 = 65%. Using the same formula, the EF for the two-chamber view is 73%, and the final EF is then 69%. ESV(Mod), end systolic volume by modified Simpson rule; EDV(Mod), end diastolic volume by modified Simpson rule. E, Images of LV segmentation for wall motion analysis. On the top panel, the three apical views are shown: four-chamber, two-chamber and three-chamber view or long axis; the bottom panel shows three short axis views: basal, mid, and apical levels. These levels are displayed in the heart drawing on the left side of this picture. All wall regions are identified in each view. The apical cap can be used for wall motion analysis but is especially considered for studies such as myocardial perfusion or contrast studies. F, same arrangement for images is shown: top panels for three apical views and bottom panels showing three levels of short axis views. In this image, the coronary supply is shown for each wall and region. LV, left ventricle; CX, circumflex artery; LAD, left anterior descending artery; RCA, right coronary artery. G, H, left atrial (LA) volume calculations. The LA is shown in two apical views, four-chamber (G) and two-chamber (H), with measurements displayed on the top left corner (white arrows). Considering the area-length method: Left atrial volume = 8/3π[(A1)(A2)/(L)], in which A1 and A2 correspond to the two area numbers (Ad) and L corresponds to the length (Ld) and the highest number is included in the formula; then, LA = 0.85[(13.9)(19.9)]/(4.9) = 48 mL. Another method of LA volume calculation is using the modified Simpson rule, the same rule used for left ventricle ejection fraction measurement. The final volume is obtained tracing the endocardial border of the left atrial chamber in the final systole and the final volume is the average of the two orthogonal volumes in the two apical views, which in this case is 49.5 mL, the number comparable with the area-length method (48 mL). LA, left atrium; Ld, length; LV, left ventricle; RA, right atrium; RV, right ventricle.

(F, Modified from Lang RM, et al. Recommendations for Chamber Quantification: A Report from the American Society of Echocardiography’s Guidelines and Standards Committee and the Chamber Quantification Writing Group, Developed in Conjunction with the European Association of Echocardiography, a Branch of the European Society of Cardiology. J Am Soc Echocardiogr 2005; 18:1440-1463.)

Diastolic Function

With the increase in aging population and the options of treatment of several cardiac diseases, especially hypertension and coronary heart disease, diastolic heart failure is becoming more prevalent and it is important to correctly evaluate those patients. The majority of those patients have symptoms of heart failure but not necessarily systolic dysfunction and this is the reason myocardial relaxation abnormalities must be completely studied with available Doppler techniques. Not only the study of flow velocities through the mitral and tricuspid valves and central veins, but also the additional information on how myocardial tissue changes, by using tissue Doppler tracings, makes the use of echocardiography a powerful option in this setting. Increment value can be included if 2D findings are associated, such as the increase in LA diameter, LV thickness (e.g., hypertension, hypertrophic cardiomyopathy [HCM], obesity).

Mitral inflow velocities. With mitral inflow tracings obtained by the use of pulsed wave Doppler, it is possible to measure peak velocities of early (E wave) and late (A wave) diastolic filling, the time from peak E wave to baseline (deceleration time—DT) and also the isovolumic relaxation time (IVRT), between the aortic valve closure and before mitral valve opening. Usually the DT and IVRT are prolonged in relaxation abnormalities because it takes a longer time for the LV filling pressure, in those cases, to equilibrate the LA pressure. And both are shortened in normal individuals or in situations of a higher filling pressure in the LV (with higher LA pressure).1

Mitral annulus velocities. Typical tracings of mitral annulus velocities obtained by tissue Doppler include three waves: one systolic (S′) and two diastolic: an early diastolic wave (E′) and a late diastolic wave (A′). For the purpose of evaluating diastolic function, the most important among those three waves is the E′. It is lower in patients with abnormal relaxation and does not increase with exercise, which is an opposite effect in normal subjects. In general E′ velocity remains lower and E wave velocity increases with higher filling pressures. It has been used as a ratio between those waves to estimate diastolic dysfunction.1

Pulmonary vein flow velocities. There are four different waves in tracings of pulmonary vein flow: two systolic, one diastolic velocity and one atrial flow reversal. Among those, the atrial flow reversal component is the most important, especially its size and morphology.1

Left atrium. The importance of LA dimension is that with progressing LV diastolic stiffness, the LA volume increases being considered one of the best parameters of chronic diastolic dysfunction and is related to further cardiac events such as atrial fibrillation, heart failure, stroke, and death (Figs. 6-5GH and 6-6).4

Pulmonary Hypertension

It is part of the routine echocardiography to measure the pulmonary artery systolic pressure (PASP) using the tricuspid regurgitation velocity. Pulmonary hypertension is when the PASP is greater than 25 mm Hg at rest. Not only can the PASP be obtained by the Doppler techniques, but also the mean and the diastolic pulmonary artery pressure, associating the tracings of pulmonary regurgitation, as discussed earlier.

It is important to distinguish between acute and chronic pulmonary hypertension. Usually acute pulmonary hypertension is due to pulmonary embolism. Chronic pulmonary hypertension causes are related to cor pulmonale. Important findings in both cases are enlargement of right chambers with the LV being “compressed” by the pressure overloaded RV, and also the RV decreased systolic function. It is possible to find images compatible with thrombi, more related to pulmonary embolism. In some cases, it can be difficult to differentiate the acute and chronic pulmonary hypertension. The presence of increased RV thickness may be found in patients with chronic pulmonary hypertension (Fig. 6-7).