Ann Thorac Surg 1998;66:627-633
© 1998 The Society of Thoracic Surgeons
Supplement
Echocardiography of hypoplastic ventricles
Norman H. Silverman, MDa,
Doff B. McElhinney, MDa
a Division of Pediatric Cardiology, Department of Pediatrics, University of California, San Francisco, California, USA
Address reprint requests to Dr Silverman, University of California, San Francisco, Box 0214, M342A, San Francisco, CA 94143-0214
e-mail: (norman_silverman{at}pedcardgateway.ucsf.edu)
Presented at the Workshop on "One and One-Half Ventricle Repairs," Gubbio, Italy, Dec 6–7, 1996.
Abstract
Background. Evaluation of hypoplastic ventricles with echocardiography requires an appreciation of the ultrasound methods used to assess ventricles of normal size. In this review, we present an overview of the most common techniques used to measure ventricular size, which may be analyzed as long- or short-axis dimension, area, volume, or mass. In addition to methods for evaluation, we review pertinent studies of sonographic evaluation of hypoplastic ventricles in consideration of their suitability for biventricular repair.
Methods and Results. Standard methods of volumetric and functional evaluation of the right and left ventricles are described, with a focus on their suitability for and applicability to the patient with a small ventricle. When applied to the patient with a hypoplastic ventricle, assessment may be more complicated in some respects, and requires consideration of functional characteristics of the ventricle itself, as well as the size and function of the corresponding atrioventricular valve.
Conclusions. Echocardiography allows for excellent evaluation of ventricular size, morphology, and function. This holds true in patients with a hypoplastic ventricle as well, although the task is somewhat more complicated in such patients.
The definition of hypoplasia requires a definition of what constitutes normal size. There are several measurements that may be made to define normally and abnormally sized ventricles. Size can be assessed by defining the dimensions (long-axis or short-axis), area, or volume of the ventricular chambers. Chamber volumes are calculated from area outlines and distance measurements in still frames chosen as representative of end-diastole and end-systole. The points of end-diastole and end-systole may be defined from the QRS complex of the simultaneously recorded electrocardiogram or, when phonocardiography is used, from the points of origin of the first heart sound for end-diastole and the onset of the second heart sound for end-systole. These points may be complemented by defining diastole and systole by the closure of the atrioventricular valve and the opening of the semilunar valves, respectively, or these points alone may be used.
The calculation of volume is based on geometric models that allow estimation of the entire volume from one or a few cross-sectional images (Fig 1). Numerous geometric models have been employed, mainly for calculation of left ventricular volume [1, 2]. In our experience, the most reliable approach is to use the biplane Simpson’s rule (method of discs), which is independent of the geometric shape of the chamber to be measured and can be applied universally to calculate the volume of any regularly or irregularly shaped chamber. This method requires imaging in two orthogonal planes sharing a common long axis, and reconstructs the chamber volume from a number of elliptical slices of equal thickness. Each slice has a volume, which is defined by the two short-axis dimensions in the orthogonal planes and the thickness from its compound of the disc from the common long axis; the sum of the volume of all slices is the estimate of the chamber volume (Fig 2). This approach has been shown to be accurate in adults [3] and children [2], and even in very small ventricles in the fetus [4].
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Fig 1. Different geometric models used to calculate chamber volumes from two-dimensional echocardiography. (I) The biplane Simpson’s rule based on orthogonal views in an apical two- and four-chamber view. The volume is calculated as the sum of volumes of ellipsoidal cylinders with the major and minor axis a and b and the height L/n, where L is the common long axis and n the number of segments chosen. In the example shown n equals 20. (II, III) The principle of Simpson’s rule can be applied to a different method that calculates the chamber volume from three (II) or four (III) area measurements obtained in the parasternal short axis; the height of the segments is taken from equivalents of the long axis measured from the apical window. The assumption that all slices in the parasternal short axis are equidistant from each other and are perpendicular to the long axis is almost impossible to satisfy in practice. (IV) Using a biplane area-length method, the areas A1 and A2 are traced in apical two- and four-chamber views; the long axis L is taken from either plane. The formula used in this calculation is that for an ellipse, which may be reasonable for the left ventricle, but not for the right ventricle. (V) The hemisphere-cylinder (or bullet) model uses a cross-sectional area of the left ventricle in a parasternal short axis at the level of the tips of the papillary muscles and a length taken from an apical view. The formula considers the chamber volume as the sum of a hemisphere and a cylinder, which is also not valid for the right ventricle. (VI) Biplane ellipsoidal method using the length L taken from an apical plane and the diameters D1 (anteroposterior) and D2 (lateral) taken from a parasternal short axis at the level of the tips of the papillary muscles. This model can also be used only for left ventricular volume calculation. (VII) The single area-length method is similar to the biplane area-length method (IV), but assumes both orthogonal areas to be equal; either the apical two- or four-chamber view may be used with this method. (Reproduced with permission from Silverman NH, Snider AR. Two-dimensional echocardiography in congenital heart disease. Norwalk: Appleton-Century-Crofts, 1982.)
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Fig 2. Subcostal coronal (top) and sagittal images (bottom) in diastole (left) and systole (right) indicating the area outlines for the right ventricle (RV) using the Simpson’s rule method. The common long axis for the right ventricle is shared by the pulmonary valve superiorly and the diaphragmatic surface of the left ventricle (LV) inferiorly. The area outlines indicate the changes in the normal right ventricle in systole and diastole (AO = aorta; PA = pulmonary artery; RA = right atrium.)
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In defining the chamber area, it is important to reduce possible sources of error. High gain settings will broaden the endocardial surface; therefore, the lowest gain setting that gives satisfactory resolution should be used. Tracing the area should be performed from a large format of the echocardiographic image, eg, directly from the video monitor. When the endocardium is imaged with axial resolution, it should be traced along the border to the ventricular cavity; when it is imaged with lateral resolution, it should be traced through the middle of the endocardial echoes.
The internal surface of the ventricle, particularly when hypoplastic, may be difficult to outline. Conventional imaging modalities may be enhanced with Doppler color-flow mapping or tissue tagging (Fig 3). Contrast echocardiographic techniques also offer promise, particularly to enhance automated outline detection techniques. These ancillary techniques enhance the echocardiographer’s ability to distinguish ventricular cavity from ventricular wall, particularly when the images are moving.
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Fig 3. (A) (Top) End-systolic frame of the right (left) and left (right) ventricles (top) in a patient with pulmonary atresia. (Bottom) Superimposition of Doppler color-flow maps with the Nyquist limit lowered to identify the blood flow pool more accurately, after which the area outlines of the right (left) and left ventricle (right) can be traced. The right ventricular area is substantially larger than if color flow information had not been added. (B) An example of pulmonary atresia with intact septum and hypoplastic right ventricle. (Left) The arrows in the right atrium (RA) indicate the tricuspid annulus size. (Right) A two-dimensional tissue-tagged image facilitates characterization of right ventricular size, which has been traced manually. (LA = left atrium; LV = left ventricle; RV = right ventricle.) (Figures reproduced in black and white.)
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Another useful technique for defining chamber size is comparing the outline of both ventricular areas in the apical four-chamber view (Fig 4). We have found this to be of predictive value in determining the requirement for additional pulmonary blood flow in patients with pulmonary atresia undergoing right ventricular outflow tract reconstruction. When the area as measured by planimetry of the right ventricle was greater than 45% of the left ventricular area, we found that patients did not require an additional source of pulmonary blood flow [5]. We also noted that as the right ventricles of these patients were decompressed, the ventricular volume and consequently the ejection fraction diminished, with preoperative diastolic volumes recovering only by the fourteenth postoperative day.
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Fig 4. (Top) Apical four-chamber view in a patient with pulmonary atresia and intact septum. The right (RA) and left (LA) atria and right (RV) and left (LV) ventricles are shown. The area ratio between the right and left ventricles is 0.59. (Bottom) Area outlines drawn over these particular images from which the ratios were established.
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Area outlines from the apical four-chamber view have also been employed to predict unfavorable outcome in Ebstein’s malformation. The condition was considered severe when the right atrial area measured by planimetry was equal to that of the other three chambers combined [6].
For calculation of left ventricular volume, we use apical four- and two-chamber views, whereas for right ventricular volumes we use subcostal coronal and sagittal views (see Fig 2) [7]. Other views have been used, especially for right ventricular volume estimation, which is problematic both because the right ventricle has an irregular shape fitting no simple geometric model and because it lies very close to the chest, which makes imaging of the entire ventricular outline difficult [8–11].
In children, however, orthogonal views of the right ventricle can be obtained easily from the subcostal view, and the Simpson’s rule method can be applied to these images for volume calculation by using the common long axis from the diaphragmatic surface to the pulmonary artery and the two area outlines of the right ventricle from the two orthogonal subcostal cuts [10]. The best images of the right ventricle may be slightly oblique, rather than true sagittal and coronal images. For the left ventricle, orthogonal views are best obtained by using the coronal and sagittal planes. The common axis is then the anteroposterior short axis. Recently a pseudobiplane method has been developed for calculating left ventricular volume using the so-called bullet formula (see Fig 1, V), which states that the 
where A is the short-axis left ventricular area. An apical view is used for the measurement of long-axis length and short-axis diameter, which is assumed to represent both short-axis dimensions of the left ventricle. Both of these axial dimensions are unidimensional measurements and require less time and sophisticated computer programming to calculate volume. In using this formula, one must make the same geometric assumptions as for M-mode measurements. Nevertheless, the calculations of mass and volume are accurate enough for clinical use. Obviously, these methods cannot be used to calculate volume of the right ventricle or the atria, where only a method using Simpson’s rule will suffice.
The principle used for left ventricular volume calculation can also be applied to the estimation of left ventricular mass. The volume of the left ventricular myocardium is estimated as the difference between the volumes calculated from the outer (epicardial or right ventricular septal border) and inner outlines of the left ventricle; the volume of the left ventricular myocardium multiplied by muscle density (1.055 g/cm3) yields the left ventricular mass (Fig 5) [12].
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Fig 5. Diagram of the technique used to calculate left ventricular (LV) mass. The top three diagrams show the area outline of the left ventricular outer wall echoes and the corresponding area A1, at papillary muscle tip level. A2 is the endocardial area at the level of the tips of the papillary muscles at end-diastole. The papillary muscles are excluded from this area outline. The thickness (t) at this level is calculated by subtracting the two areas Am = A1 - A2. To calculate the cord (b) from the area and the mean thickness (t), the manipulations in the formula are shown. The bottom diagram shows the various cords used in the formula to calculate mass from the truncated ellipse (TE). The dimensions b and t have been defined. The long axis from the widest minor axis radius to the apex is the dimension (a), and (d) is the truncated semimajor axis from widest short-axis diameter to mitral annulus plane as shown. The bottom formula for left ventricular mass is the truncated ellipse and is in current use on our computer system. The top formula is used for the area length (AL) method. (Reproduced with permission from Schiller NB, Shah PM, Crawford M, et al. Recommendations for quantitation of the left ventricle by two-dimensional echocardiography. J Am Soc Echocardiogr 1989;2:358–67.)
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It has been noted that the diameter and area of the valve annulus correlate well with ventricular volume. In addition, the area of the atrioventricular valvar annulus bears a direct relation to the volume of flow that can pass through its orifice. Accordingly, Rhodes and associates [13] developed a formula using the area of the valve, the length of the ventricle in relation to its contralateral fellow, and the area of the left ventricular outflow tract indexed to body surface area to determine whether or not the left ventricle is hypoplastic. They analyzed retrospectively which left ventricles would have been too small for biventricular repair. Two similar groups of data were obtained. The variables assessed were the anteroposterior and anterolateral dimensions of the mitral (and tricuspid) annuli measured in apical four-chamber and parasternal long-axis views. Valvar area was then calculated from the formula of an ellipse from the diameters, D1 and D2, with 
Left ventricular volume was calculated from the bullet formula, and left ventricular mass was calculated by multiplying the difference between epicardial and endocardial ventricular volumes, ie, the volume of the ventricular wall, by 1.04 g/m (which is another estimate of myocardial density). They measured the relative length of the left ventricle by taking a ratio of the left ventricular long-axis length to the long-axis length from the crux to the apex in the four-chamber view. The aortic annulus was measured from the parasternal long-axis view in systole. All linear and area measurements were normalized to body surface area. A critical value was established for these values to give a simplified score with one point allotted for a left ventricular long-axis ratio of 0.8 or less, an indexed aortic root of 3.5 cm2/m2 or less, a mitral valve index of 4.5 cm2/m2 or less, and a left ventricular mass index of less than 35 g/m2. Mortality was 100% in patients receiving an overall score of two or greater and 8% among patients with a score of one or less. In addition, discriminant analysis was used to determine a linear function with a 
. A score of 0.35 or less was associated with death after two ventricle repair.
Hoffman [14], in a masterly metaanalysis of left ventricular volume data from a variety of sources in the literature, defined the ventricular size as adequate if the left ventricular diastolic dimension is greater than or equal to 20 mL/m2 of body surface area. Smaller ventricles cannot support a systemic circulation. However, as we have recently shown, ventricular inflow conditions are important factors to consider in evaluating ventricular size. Preoperative measurement of actual left ventricular volume in anomalies with reduced left ventricular inflow, such as totally anomalous pulmonary venous return or right-dominant unbalanced atrioventricular septal defect, tends to underestimate left ventricular volume after repair of the lesion [15]. Therefore, preoperative measurement of potential left ventricular volume may be useful for the assessment of suitability for biventricular repair in borderline cases. Moreover, size is not the only determinant of the capacity of the ventricle, as ventricular and atrioventricular valvar function are important as well (Fig 6).
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Fig 6. (A) Parasternal long-axis view in a patient with a variant of the hypoplastic left heart syndrome, which shows some forward flow across the stenotic left ventricular outflow tract, into a diminutive aorta (AO) and left ventricle (LV). The right ventricle (RV) is apex-forming and has a very heavily trabeculated moderator band apparently dividing the right ventricle into two sections. (B) Doppler color-flow image taken in a similar position in systole demonstrates a jet from aortic stenosis (AS), indicating some forward flow across the aortic valve and mitral regurgitation (MR). (C) Apical four-chamber view, again demonstrating the diminutive left ventricle, the large coronary sinus (CS), the hypoplastic left atrium (LA), the large right atrium (RA), and the right ventricle (RV).
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In the right ventricle, recognition of hypoplasia by echocardiography is less clear. Although calculation of right ventricular volume according to the Simpson’s rule (method of discs) has been described [7], echocardiographers have made little use of the simple method of tracing the right ventricular outline. Simply outlining these small ventricles in diastole and systole can provide the necessary volume data on the indexed diastolic volume and stroke volume to provide an estimate of right ventricular size.
It is clear that there is a relationship between right ventricular size (volume) and the diameter of the tricuspid valve [16]. Tricuspid annular size, therefore, can be used as an indirect assessment of chamber size. The data of Rowlatt and colleagues [17] have been used most often for annulus measurements and determination of Z values, as follows: 
. Data of annulus size by Hanseus and coworkers [18] show that the dimensions and the standard deviations for the tricuspid and mitral valve taken in the apical four-chamber view can be interchanged.
It is clear that the data reported by Hanley and associates [19] in the Congenital Heart Surgeon’s Society study on pulmonary atresia with intact ventricular septum are valuable for the assessment of right ventricular repair. The fact that the measurement is so simple and could be performed on all the echocardiograms submitted to the group made the inferences drawn from it very strong. On the other hand, as there is no absolute correlation of which we are aware between tricuspid valve size and ventricular volumes and function, the calculation of volume, stroke volume, and ejection fraction in the study by Hanley and associates might have provided even greater strength to the assessment of which ventricles were truly hypoplastic.
The calculation of chamber size is obviously only one aspect of the decision making process for determining the best approach for patients with hypoplastic ventricles. New advances in surgical treatment guided by ultrasonic assessment, which can be offered postnatally, as well as the potential for prenatal intervention, provide the opportunity to answer the continuing challenges of these conditions.
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