Quantitative and Semiquantitative Echocardiography

Dimensions and Flows

Manish Bansal, Jagat Narula and Partho P. Sengupta

Assessing cardiac chamber dimensions and hemodynamics is an integral component of any echocardiographic examination. Estimating chamber size not only helps in evaluating the contractile performance of a particular chamber but also provides valuable information regarding the hemodynamic significance of concomitant lesions such as valve regurgitation and intracardiac shunts. A more detailed assessment of cardiac hemodynamics can be accomplished by directly measuring intracardiac flows and pressures. Altogether, such information has immense diagnostic, prognostic, and therapeutic significance. It allows for judicious administration of medical therapy, especially intravenous fluids, diuretics, vasopressors, and vasodilators. In addition, detailed assessment of cardiac hemodynamics forms the framework required for making crucial decisions such as whether surgical intervention is required for specific cardiac lesions and is also helpful for evaluating the hemodynamic significance of the residual lesions following intervention.

Since its advent in the 1970s, transesophageal echocardiography (TEE) has rapidly evolved into a useful diagnostic modality for assessing cardiac structures and hemodynamics.1–3 The close proximity of the ultrasound probe to the heart allows better visualization of cardiac structures and permits the use of higher ultrasound frequencies, which significantly improves spatial resolution. More importantly, its relatively noninvasive nature, safety, easy applicability, portability, ability to image the heart in virtually all patients, instant results, and lack of interference with the operating field render TEE the ideal diagnostic tool for cardiac evaluation during the perioperative period.^4–11

Basic Principles

Although the basic principles of assessing cardiac chamber size and intracardiac flows using TEE are the same as those for transthoracic echocardiography (TTE), there are certain notable differences. Most of the experience regarding quantitative echocardiography has been obtained via transthoracic imaging, so the optimal views for quantifying cardiac chamber dimensions have been standardized only for TTE. ¹² It is often quite challenging to reproduce the same standard views during TEE. Similarly, aligning the Doppler beam with the direction of blood flow to obtain velocity measurements across certain cardiac chambers such as the right ventricular outflow tract (RVOT) may also prove to be difficult in TEE. Finally, general anesthesia often produces marked alterations in intracardiac pressures, volumes, and chambers sizes that have to be accounted for while interpreting the information derived from TEE performed in the perioperative setting.

Table 11-1 summarizes the recommended views for performing various dimension and flow measurements during TEE.

TABLE 11-1

Measurement of Cardiac Chamber Dimensions

Qualitative assessment of cardiac chamber dimensions begins the moment the first grayscale image appears on the display screen of the echocardiography machine. The visual impression formed from these initial images permits recognition of the “target” structures and guides the subsequent analysis to conduct a meaningful echocardiographic study. However, the formal quantification of cardiac chamber dimensions is invariably required to yield a correct diagnosis and ensure accurate decision making.

Left Ventricle

Left ventricular systolic function is a key determinant of clinical outcomes in almost every illness affecting the heart. Consequently, assessing left ventricular size and thereby its systolic function is one of the most important goals of any echocardiographic examination. No echocardiogram can be considered complete without an assessment of left ventricular size and systolic function. ¹²

Several measurements can be obtained during echocardiography to provide an estimate of left ventricular size and geometry. The most commonly performed measurements include the linear dimensions (minor axis dimension [or internal diameter] and major axis dimension [or length]), left ventricular wall thickness (usually interventricular septum and inferolateral wall), relative wall thickness, left ventricular end-diastolic and end-systolic volumes, and left ventricular mass.

Left Ventricular Linear Dimensions

Measuring the internal dimensions forms the basis of left ventricular size quantification. During TTE, these measurements can be obtained from the M-mode recordings or the two-dimensional (2D) images. M-mode offers better temporal resolution owing to its much higher frame rate but is often limited by the inability to orient the M-mode cursor along the desired imaging plane. This problem is particularly common with TEE, which itself is constrained by the limited imaging planes. Therefore, most of the dimensions in TEE are measured using 2D imaging.

The left ventricular minor axis dimension is the internal diameter of the left ventricular cavity measured at the level of the mitral leaflet tips perpendicular to the long axis of the left ventricular cavity. Because the oblique orientation of the measurement plane will lead to an overestimation of cavity size, great care must be taken to ensure that the measurement plane is perpendicular to the left ventricular long axis.

Earlier guidelines recommended that the measurements be performed using the leading edge–to–leading edge technique. ¹³ This technique was required because the spatial resolution of the grayscale images at that time was insufficient for accurate delineation of the tissue/blood interfaces. Improvements in echocardiographic instrumentation have now made it possible to accurately identify actual tissue/blood interfaces, so most dimension measurements are currently performed using the inner edge–to–inner edge technique, which allows for measurements of the true cavity size. ¹²

During TEE, the left ventricular minor axis dimension can be obtained either from the midesophageal (ME) two-chamber view or from the transgastric (TG) two-chamber view ( Figs. 11-1 and 11-2, Videos 11-1 and 11-2 ). To obtain the ME two-chamber view, a proper ME four-chamber view (0 degrees) first needs to be obtained. From this position, advancement of the omniplane angle forward to 80 to 100 degrees while keeping the probe tip still and the mitral valve in the center will result in the ME two-chamber view. A slight retroflexion of the tip may be required to direct the imaging plane through the long axis of the left ventricle (LV) (see Video 11-1 ) . The left ventricular diameter is then measured as the distance between the inferior and anterior walls at the level of the mitral leaflet tips.

Figure 11-1 Measurement of left ventricular (LV) length and minor axis diameter from midesophageal two-chamber view. L, Length; LVD, left ventricular diameter. (Modified with permission from Lang RM, Bierig M, Devereux RB, 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.)

Figure 11-2 Measurement of left ventricular minor axis diameter from transgastric two-chamber view. LVD, Left ventricular diameter. (Modified with permission from Lang RM, Bierig M, Devereux RB, 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.)

The TG two-chamber view can be obtained from the TG mid–short-axis view (0 degrees) by advancing the omniplane angle forward to 80 to 90 degrees. A slight anteflexion of the transducer tip may be required to make the LV horizontal (see Video 11-2 ). The measurement is then performed in the same manner as described previously.

The left ventricular length or major axis dimension is the distance between the midpoint on the mitral annular plane and the left ventricular apex. During TTE, this dimension is usually obtained from the apical four-chamber view. However, its analogous view during TEE—the ME four-chamber view—often results in foreshortening of the LV, so the ME two-chamber view is preferred for measuring left ventricular length during TEE (see Fig. 11-1). ¹² It is usually not possible to obtain the same measurement from the TG two-chamber view, because the left ventricular apex is almost invariably excluded from the image sector.

All such measurements should be obtained at ventricular end-diastole and end-systole. An electrocardiogram (ECG) should not be used to time these events, since ECG events do not necessarily coincide with the true left ventricular end-diastole and end-systole because of the inherent electromechanical delay during contraction and relaxation. This is particularly problematic in patients with intraventricular conduction defects. Therefore, in echocardiography, end-diastole and end-systole should preferably be identified by direct visualization of the 2D images. The frame showing the maximum left ventricular cavity size, which is usually the frame just preceding the closing of the mitral valve, should be regarded as representative of end-diastole. Similarly, the frame with the smallest cavity size should be taken to represent end-systole. This usually corresponds to the frame immediately prior to the opening of the mitral valve leaflets.

Reference ranges and partition values for left ventricular linear dimensions are presented in Table 11-2. For the reasons mentioned, these values have been derived only from transthoracic imaging; therefore, no such data exist for TEE. However, previous studies have shown that, when carefully performed, measurements obtained by TEE closely match those obtained from TTE.^14,15 Thus, the American Society of Echocardiography recommends using the same reference ranges for left ventricular dimensions in TEE as well. ¹²

TABLE 11-2

Reference Ranges and Partition Values for Left Ventricle Size

BSA, Body surface area; LVEDV, left ventricular end-diastolic volume; LVIDd, left ventricular internal dimension (diastolic).

Data from Lang RM, Bierig M, Devereux RB, 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.

The major advantage of linear measurements is that they are simple, easy to perform, provide a quick estimate of the left ventricular size and, most importantly, have the least interobserver variability.16–18 However, because they are performed only in one dimension, linear measurements do not provide a true assessment of left ventricular size in the case of distorted ventricles, which are commonly observed in patients with coronary artery disease. Nevertheless, linear measurements have been shown to be reliable and have proven useful in clinical decision making regarding diseases affecting left ventricular symmetry, such as valvular heart disease, hypertension, and cardiomyopathies. ¹⁹

Left Ventricular Wall Thickness and Relative Wall Thickness

Left ventricular wall thickness is the end-diastolic thickness of the inferolateral wall and the interventricular septum. The recommended view for performing these measurements in TEE is the TG mid–short-axis view ( Fig. 11-3, Video 11-3 ). To obtain this view, the probe should be inserted into the stomach with the omniplane angle kept at 0 degrees. The probe should then be anteflexed and rotated to the right or left to develop the short-axis view of the LV and bring the LV to the center of the image. A slight advancement or withdrawal of the probe may be required to orient the imaging plane through the midportion of the ventricle, which can be recognized by the presence of papillary muscles. The grayscale image gain may have to be adjusted to optimize endocardial definition. From this view, the thicknesses of the inferolateral wall and septum are measured as shown in Figure 11-3.

Figure 11-3 Measurement of left ventricular posterior wall and septal thickness from transgastric mid–short-axis view. PWT, Posterior wall thickness; SWT, septal wall thickness. (Modified with permission from Lang RM, Bierig M, Devereux RB, 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.)

Relative wall thickness is a measure of myocardial thickness relative to the left ventricular cavity size and is a useful parameter for describing left ventricular geometry. It is calculated using the following equation:

where ILWIDd is the end-diastolic thickness of the inferolateral wall of the LV, and LVIDd is the diameter of the left ventricular cavity at end-diastole.

In patients with left ventricular hypertrophy (defined based on increased left ventricular mass as described later), an increased relative wall thickness (≥0.42) signifies concentric hypertrophy, whereas a normal relative wall thickness (<0.42) indicates the presence of eccentric hypertrophy.20,21 Concentric left ventricular hypertrophy is observed in conditions causing a pressure overload of the LV, such as hypertension, aortic stenosis, and coarctation of the aorta. In contrast, eccentric hypertrophy is common in diseases causing volume overload, such as aortic and mitral regurgitation. Eccentric hypertrophy is also observed during the late degenerative stages of diseases otherwise characterized by concentric hypertrophy, such as hypertrophic cardiomyopathy and aortic stenosis. Sometimes relative wall thickness is increased without a concomitant increase in left ventricular mass. This is known as concentric remodeling and generally occurs in response to pressure overload. The presence of concentric remodeling has been shown to be an adverse prognostic marker in certain disease conditions.^22,23

The normal ranges and partition values for left ventricular wall thickness are presented in Table 11-3.

TABLE 11-3

Reference Ranges and Partition Values for Left Ventricular Wall Thickness and Mass

BSA, Body surface area; IVSd, interventricular septal thickness (diastolic); LV, left ventricle; PWd, posterior wall thickness (diastolic).

Left Ventricular Volume

Measuring left ventricular volume not only provides more accurate information regarding overall left ventricular size compared with linear dimension measurements, but it is also essential for estimating left ventricular ejection fraction (LVEF), which is the most validated and most widely employed measure of left ventricular systolic function. There are several methods for estimating left ventricular volume, and those most commonly used in clinical practice are outlined here.

Linear Method or Cubed Method

The cubed method is the simplest method for estimating left ventricular volume. The volume of the LV is calculated by simply taking the cube of the internal dimension of the LV:

where LVID is the left ventricular internal dimension.

Based on estimations of left ventricular volumes at end-diastole and end-systole, the LVEF can be derived using the following equation:

where LVEDV is the left ventricular end-diastolic volume, and LVESV is the left ventricular end-systolic volume.

Given that the cubed method assumes the LV to be spherical, which clearly is not true, it inevitably introduces errors into the estimation of left ventricular volume. Several modifications of this method have been proposed to correct for this error, such as the Teichholz and Quinones methods.24,25 The Teichholz method is given as ²⁵

(Please note that for this equation, the LVID must be measured in centimeters.)

The greatest advantage of the cubed method is that it is simple, easy to apply, and provides a quick estimate of left ventricular size. However, the method has several potential limitations. First, as with the method employing linear dimensions, the cubed method is applicable only to ventricles with uniform geometry. In patients with distorted ventricles, the cubed method will over- or underestimate left ventricular volume and ejection fraction depending on the location of the regional wall motion abnormalities (WMAs). If the site of measurement includes the region of WMAs, the ejection fraction will be underestimated. Conversely, when the linear measurement site is away from the infracted territory, the ejection fraction will be overestimated. The latter scenario is encountered more commonly because WMAs usually involve the left ventricular apex, whereas the measurement is performed at the base of the LV. Second, because a single measurement is cubed to obtain the volume, any error in measuring the linear dimension is greatly magnified once the volume is calculated. Finally, as mentioned earlier, the method requires geometrical assumptions whereby the LV is inappropriately assumed to be spherical, which results in systematic inaccuracies.

Two-Dimensional Methods

As mentioned, the cubed method, which is based on a single linear measurement of the LV, has significant limitations and cannot be applied in patients with regional WMAs. By incorporating left ventricular dimensions from more than one plane, 2D methods allow for more accurate estimation of left ventricular volume. There are two different 2D methods that can be used for this purpose: the area-length method and the biplane Simpson’s method. ¹²

The area-length method assumes the LV to be bullet-shaped ( Fig. 11-4). The volume is calculated using the following equation:

where area is the cross-sectional area of the left ventricular cavity measured at the mid-ventricle level, and length is the left ventricular long-axis length as described previously.

Figure 11-4 A and B, Measurement of left ventricular volume using area-length method. Left ventricular cavity cross-sectional area (A) is measured from transgastric mid–short-axis view, and length (L) is measured from midesophageal two-chamber view. Note that papillary muscles are considered to be part of left ventricular cavity. Volume in example will be 53.7 mL (⅚ × 10.4 × 6.2).

The left ventricular cavity cross-sectional area is measured by tracing the endocardial border of the left ventricular cavity in the short-axis view at the papillary muscle level. This can be best accomplished in the TG mid–short-axis view. While tracing the endocardial border, the papillary muscles should be included in the ventricular cavity, and the endocardial border should be traced outside the papillary muscles. Because an optimal endocardial definition is fundamental to obtaining an accurate cross-sectional area, the grayscale image should be optimized by appropriately using gain, contrast, and dynamic range settings. However, despite maximum image optimization, it may not be possible to adequately delineate the endocardial border in the lateral wall and septum in many patients owing to the parallel orientation of the tissue/blood interfaces in these segments to the ultrasound beam.

Although the area-length method provides a more accurate estimation of left ventricular volume than the cubed method, it too does not take into consideration the overall shape of the LV and is therefore not accurate in the presence of significant regional WMAs.

The biplane Simpson’s method overcomes many of the geometric assumptions associated with the aforementioned methods. This method divides the entire left ventricular cavity into a stack of elliptical discs (usually 20), and the volume of the left ventricular cavity is calculated by summation of the volumes of each of these discs ( Fig. 11-5). The volume of each disc is calculated as the cross-sectional area of the disc multiplied by its height. The cross-sectional area is computed from the two orthogonal diameters obtained from the four-chamber and two-chamber views, and the height is derived from the length of the LV. All of these calculations are performed automatically using built-in software available in all modern echocardiographic equipment.

Figure 11-5 A and B, Measurement of left ventricular volume using biplane Simpson’s method. Endocardial border is traced in midesophageal four-chamber and two-chamber views at end-diastole and end-systole. 2C, Two-chamber; 4C, four-chamber; BP, biplane; EDV, end-diastolic volume; EF, ejection fraction; ESV, end-systolic volume; LV, left ventricular; SV, stroke volume. (Modified with permission from Feigenbaum H, Armstrong W, Ryan T, eds. Feigenbaum’s Echocardiography. 6th ed. Philadelphia: Lippincott Williams & Wilkins; 2005.)

To obtain left ventricular volume using the biplane Simpson’s method, the left ventricular endocardial border must be traced in the ME four-chamber and two-chamber views. The endocardial border is traced manually, starting from one end of the mitral annulus, coursing along the entire left ventricular cavity, and finishing at the other end of the mitral annulus. The base of the ventricle is formed by a straight line joining the mitral valve insertion points at the two ends. The long axis of the ventricular cavity is automatically recognized by the software but can be manually adjusted if it does not appear to conform to the true long axis. The software then computes the left ventricular volume.

While performing these measurements, great caution must be exercised to ensure that the left ventricular cavity is not foreshortened. Slight retroflexion of the tip of the transducer to bring the true left ventricular apex into view may help avoid foreshortening (Video 11-4 ), but it may still be difficult to obtain a non-foreshortened view of the LV in the ME four-chamber view.

The biplane Simpson’s method is currently the most accurate (and the recommended) 2D echocardiographic method for estimating left ventricular volume because it employs the fewest mathematical assumptions regarding the left ventricular cavity shape and also corrects for distortion of left ventricular geometry. ¹² However, the accuracy of this method is highly dependent on adequate endocardial border delineation. As already described, the echocardiography machine settings should be adjusted to obtain the best possible endocardial delineation. If, despite these adjustments, the endocardial border cannot be visualized adequately in one of the two requisite views, the single-plane Simpson’s method can be used to obtain left ventricular volumes. However, this method will not be accurate enough in the presence of significant regional WMAs. When the endocardial border cannot be adequately visualized in either of the two views, the Simpson’s method should be abandoned and the area-length method used to estimate left ventricular volume.

Table 11-2 describes the normal ranges and partition values of left ventricular volumes derived from 2D methods.

Three-Dimensional Methods

The recent advent of three-dimensional (3D) echocardiography has significantly improved our ability to accurately estimate left ventricular volumes by eliminating the need for mathematical assumptions regarding the shape of the LV. The left ventricular cavity can be directly imaged in a 3D space, and its volume can be accurately calculated. Comparative studies using magnetic resonance imaging (MRI) as the gold standard have shown excellent agreement between the two techniques and have demonstrated 3D echocardiography to be superior to 2D echocardiography for estimating left ventricular volumes.26–30 The availability of live 3D TEE has further improved image quality, and the application of quantitative techniques to these images has the potential to provide an even more accurate estimation of left ventricular volumes than ever before. However, a detailed description of these techniques, including their advantages and potential pitfalls, is beyond the scope of this chapter.

Left Ventricular Mass

Left ventricular hypertrophy is an important prognosis determinant in a number of clinical conditions such as hypertension, aortic stenosis, aortic regurgitation, and cardiomyopathies.21,31–36 Accordingly, the presence and extent of left ventricular hypertrophy forms the basis of many of the key therapeutic decisions in these disease states. Echocardiography, by allowing for estimation of left ventricular mass, is the most clinically relevant tool for detection and characterization of left ventricular hypertrophy.36–38 Assessing left ventricular mass via echocardiography has been extensively used in clinical and research settings for prognostication, guiding treatment, and following the effects of various pharmacologic and nonpharmacologic measures aimed at bringing about regression of left ventricular hypertrophy.^31,36–39

Although there are several echocardiographic methods for estimating left ventricular mass, the basic principle is the same. First, the left ventricular myocardial volume is calculated, and the volume is then converted to a mass by multiplying it by the specific density of the myocardium. The left ventricular myocardial volume itself is obtained by subtracting the left ventricular cavity volume from the total left ventricular volume, which includes both the left ventricular cavity and the surrounding myocardial shell. All the methods used to estimate left ventricular volume can be used to estimate left ventricular mass as well. Measuring left ventricular mass is performed at end-diastole.

Linear Method

Although the linear method is generally not the preferred method for estimating left ventricular volumes, its simplicity and superior reproducibility have made it the most commonly employed method for estimating left ventricular mass. Using this method, the left ventricular cavity volume is estimated as the LVIDd³, and total left ventricular volume is defined as (LVIDd + IVSd + ILWd)³. From these values, left ventricular mass can then be calculated as:

where 1.04 is the myocardial density, and 0.8 is the correction factor. LVIDd, IVSd, and ILWd represent the end-diastolic left ventricular internal diameter, interventricular septal thickness, and inferolateral wall thickness, respectively.

Two-Dimensional Methods

Using 2D echocardiography, left ventricular mass can be calculated using either the area-length method, the truncated ellipsoid method, or Simpson’s method.

For the area-length method ( Fig. 11-6), the TG mid–short-axis view is obtained at the papillary muscle level, and the left ventricular epicardial and endocardial cross-sectional areas (A₁ and A₂) are measured by tracing the epicardium and endocardium, respectively. When tracing the endocardium, the papillary muscles are included in the cavity, not in the myocardium. Left ventricular length (L) is then measured in the ME two-chamber view as previously described. Left ventricular mass is then calculated using the following equation:

where t is the mean wall thickness derived by subtracting the left ventricular cavity radius from the left ventricular radius as described in the equation:

In the example shown in Figure 11-6, B, t would be 1.16 cm [√(27.8/3.14) − √(10.4/3.14)]. Inserting all the values into the equation will yield a left ventricular mass of 122.6 g.