Such reconstruction, although not routinely performed in current pediatric echocardiographic protocols, is becoming more commonly used and in time will become standard practice.44,45 Doppler Interrogation The complete evaluation of a ventricular septal defect includes not only an assessment of the size, site, and number of defects but also an estimate of the hemodynamic consequences By using continuous-wave Doppler ultrasound, it is possible to measure the velocity of flow across any ventricular septal defect Then, by invoking the principles of the Bernoulli equation, it is possible to calculate the instantaneous peak systolic pressure drop between the ventricles In the absence of left ventricular outflow tract or aortic obstruction, it can be assumed that the left ventricular peak systolic pressure is the same as the systolic blood pressure, the right ventricular systolic pressure can then be estimated In the absence of any obstruction within the right ventricular outflow tract, this can be presumed to be equal to the pulmonary arterial systolic pressure Infants and children with congenital cardiac defects, including those with a ventricular septal defect, frequently have mild tricuspid insufficiency In this situation it is possible to estimate the pressure drop across the tricuspid valve and therefore to estimate the right ventricular and pulmonary arterial systolic pressures Care must be taken to interrogate the tricuspid regurgitation jet in such a way as to avoid contamination from flow across the ventricular septal defect Another potential source of error arises when there is a shunt from left ventricle to right atrium through the defect via a deficiency in the septal leaflet of the tricuspid valve In this situation, the regurgitant jet reflects the pressure drop between the left ventricle and right atrium Color-flow mapping has greatly facilitated the echocardiographic diagnosis of ventricular septal defect.46 Its most important uses include accurate alignment of the Doppler beam with the flow of blood, thus enhancing accurate quantification of velocity, the detection of multiple ventricular septal defects, the directionality of shunting, and the demonstration of shunting from left ventricle to right atrium Color-flow mapping also plays a role in distinguishing innocent murmurs from those caused by very small ventricular septal defects Transesophageal Echocardiography Transthoracic cross-sectional echocardiography detects the vast majority of even small ventricular septal defects In a small number of patients—especially older children, adolescents and adults—transesophageal echocardiography may be required for complete assessment This technique may also be of particular value during the perioperative period in the operating theater and intensive care unit, especially to evaluate residual shunts An important further use of transesophageal echocardiography is to distinguish between a ruptured sinus of Valsalva and a perimembranous ventricular septal defect associated with aortic insufficiency The technique is also indispensable in attempting closure of ventricular septal defects by interventional catheterization because it allows precise positioning of the retention discs on either side of the defect Real-time three-dimensional echocardiography, at present, represents a growing area and is now being introduced as a routine clinical tool Reconstruction achieved in this fashion can provide additional information about the morphologic features of a ventricular septal defect The use of three-dimensional transesophageal imaging, however, is currently limited to adult-sized patients.44,47 Magnetic Resonance Imaging As may be anticipated, magnetic resonance imaging clearly shows the location and structure of defects Hardly ever is there a clinical indication for preoperative morphologic investigation of the defect unless echocardiography fails to visualize pertinent anatomic features of the defect Phase-contrast velocity mapping, an accurate tool for the assessment of blood flow, can be used to quantify pulmonary and systemic flows and to calculate the left to right shunt In addition, magnetic resonance imaging can be used for postoperative assessment of the ventricular volumes and function if this information cannot be obtained by echocardiography Computed Tomography Computed tomography is rarely necessary for diagnosis of the isolated ventricular septal defect With improved technology, nonetheless, and the ability to obtain exquisite detail at a reduced radiation exposure of no more than 1 mSv, computed tomographic assessment is increasingly used when any question remains following echocardiographic assessment (Figs 32.32 and 32.33).48 Computed tomography is indicated when the ventricular septal defect is complicated by, or associated with, abnormalities of the airway or lungs, such as bronchial compression, atelectasis, or pneumonia Contrast-enhanced computed tomography can also be useful when obstruction of the aortic arch is suspected but the echocardiographic findings are inconclusive Additionally, the utility of three-dimensional printed models from both computed tomographic and cardiac magnetic resonance datasets have been proven useful in select complex cases, including those with multiple ventricular septal defects (Fig 32.34).49,50 FIG 32.32 (A) Three-dimensional computed tomographic reconstruction viewed in a right anterior oblique projection with the free wall of the right ventricle removed, demonstrating an apical muscular defect crossed by a septoparietal trabeculation (B) The same heart viewed in a four-chamber projection demonstrates the solitary apical muscular defect