Subjects

We prospectively enrolled 70 patients fulfilling the following inclusion criteria: (1) age from day of life 1 to 20 years, (2) request for a clinical echocardiographic examination either in the cardiology outpatient clinic or in the inpatient service at Boston Children’s Hospital, and (3) informed consent. Exclusion criteria were (1) inability to attempt the acquisition of a 3D echocardiographic RV data set because of a lack of patient cooperation and (2) absence of an identifiable right ventricle. To ensure a study population representing a wide range of ages and cardiovascular pathology, subjects were recruited by age strata (0–3, 3–8, 8–13, 13–18, and 18–20 years) and by history of cardiac surgery. Subjects were recruited consecutively, but a given age stratum was closed to enrollment when 20 subjects were recruited (10 with histories of cardiac surgery and 10 without). The study protocol was approved by the Boston Children’s Hospital Committee on Clinical Investigations.

Transthoracic 3DE of the Right Ventricle

Echocardiographic studies were performed using iE33 cardiac ultrasound systems (Philips Medical Systems, Andover, MA) according to our published laboratory protocol. When clinically indicated, young (age < 3 years), uncooperative patients were sedated according to our laboratory’s outpatient sedation protocol (oral chloral hydrate at a dose of 80 mg/kg up to a maximum of 1 g). Transducer frequency (X3-1 or X7-2 matrix probe) and acquisition parameters were optimized according to patient size and acoustic windows. Harmonic imaging was often used to optimize signal-to-noise ratio and to enhance visualization of the blood-endocardial boundaries. All data sets were acquired with the transducer in the subxiphoid (SubX) and apical positions, ensuring that the entire right ventricle could be viewed simultaneously in both orthogonal planes. When possible, the patient was asked to suspend respiration and remain motionless during the four-beat acquisition phase. The 3D volume data set images were then evaluated to ensure that the entire chamber was optimally imaged with minimal spatial and temporal artifacts.

After informed consent was obtained and the clinical echocardiographic examination was completed, a sonographer trained in 3DE acquired two full-volume 3D echocardiographic data sets of the right ventricle, one from the SubX acoustic window and another from an apical four-chamber (A4C) view. Once these data sets were acquired, the sonographer left the examination room, and a second sonographer (blinded to the previously acquired 3D echocardiographic data sets) obtained a second pair of 3D echocardiographic RV data sets from the same views. Thus, a total of four data sets of the right ventricle were attempted in each subject. All 11 sonographers who participated in this study underwent prior training in 3DE. The sequence of 3D RV data set acquisition was not scripted and was performed randomly at the discretion of the sonographer. Sonographers were allowed to repeat an acquisition if they were not satisfied with the technical quality of the image data set.

Data Analysis

Volumetric Measurements

Acquisitions were stored in Digital Imaging and Communications in Medicine format and transferred to a dedicated workstation for offline analysis using commercially available software (4-D EchoView; TomTec, Munich, Germany). RV end-diastolic and end-systolic volumes were measured offline from each data set. Each data set was aligned in two orthogonal planes along the long axis of the right ventricle by one investigator (P.R.). The appropriate end-diastolic and end-systolic frames in sequential cross-sectional planes were then selected. The endocardial border in each slice was then manually traced using the method of disk summation ( Figure 1 ). RV ejection fraction was then calculated using the formula 100 × (RV end-diastolic volume − RV end-systolic volume)/RV end-diastolic volume.

Analysis of a real-time 3D echocardiographic image of the RV. (A) Three-dimensional echocardiographic image showing a full-volume 3D image of the right ventricle. (B) Three-dimensional echocardiograhpic image acquisition showing the right ventricle in orthogonal planes. (C) Manual tracing of the RV endocardial borders in long- and short-axis planes.

Statistical Analysis

Feasibility of 3D echocardiographic RV data set acquisition was determined for each of the two echocardiographic views (SubX and A4C) and on a per patient basis and defined as the percentage of subjects in whom data sets were deemed analyzable. To determine intraobserver reproducibility, the same investigator (P.R.) repeated all measurements on 10 randomly selected patients ≥3 weeks after completing the first set of measurements. For interobserver reproducibility, a second investigator (G.R.M.) analyzed the RV data sets from 10 randomly selected patients without knowledge of the results of the primary investigator. Interstudy reproducibility was determined by one investigator (P.R.), who analyzed the second set of 3D echocardiographic RV data (acquired by the second sonographer) on each subject ≥3 weeks after the original analysis. For each type of variability (intraobserver, interobserver, and interstudy), there were 20 sets of measurements, 10 from the SubX acoustic window and 10 from the A4C window. However, the final number of paired measurements was influenced by missing data (i.e., not all patients had analyzable data sets when repeated measurements were obtained). Paired t tests were used to determine if there were significant differences in measurements between the different observers and across the two studies in the same patient. The absolute difference divided by the mean of the repeated observations was determined. Bland-Altman plots were constructed to provide a visual assessment of the levels of intraobserver, interobserver, and interstudy variability. Interstudy variability was also assessed using intraclass correlation coefficients estimated with variance components models.

To identify factors associated with nonanalyzable 3D echocardiographic RV data sets, candidate predictors were compared between groups using Fisher’s exact test for categorical variables and Wilcoxon’s rank-sum test for continuous variables. Univariate and multivariate logistic regression analyses were performed; a P values ≤ .05 were required for retention in the final model. Statistical analysis was performed using SAS version 9.2 (SAS Institute Inc, Cary, NC).

Patient Characteristics

Patient demographics are summarized in Table 1 , and their primary cardiac diagnoses are detailed in Table 2 . As expected from the study design, the spectrum of ages from birth to 20 years and a broad range of body sizes were well represented. Nearly 20% of the patients had CHD-associated RV dilatation, and 44% had prior cardiac surgery. Of the 70 patients, 16 (23%) required sedation for their clinical echocardiographic studies.

Table 2

Cardiac diagnoses ( n = 70)

Diagnosis	n
No structural abnormalities	15
Left ventricular outflow tract obstruction	12
Atrioventricular canal	9
Tetralogy of Fallot	6
Ventricular septal defect	5
Atrial septal defect	4
Coronary artery anomaly	2
Cardiomyopathy	2
Hypoplastic left heart syndrome	4
Other ∗	11

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