Right ventricular function after TOF repair: new understanding from MR

Shi-Joon Yoo
Professor of Diagnostic Imaging and Paediatrics, University of Toronto
Section Head, Cardiac Imaging, The Hospital for Sick Children
Toronto, Canada

Although surgical repair of tetralogy of Fallot (TOF) generally provides a favorable outcome, surgical augmentation of the narrow right ventricular outflow tract almost always results in chronic pulmonary regurgitation that causes exercise intolerance, ventricular dilatation and dysfunction, arrhythmia, and sudden death. Magnetic resonance (MR) has been established as an accurate diagnostic tool for quantitative assessment of pulmonary regurgitation and ventricular function as well as for anatomical evaluation of the ventricles and pulmonary vessels. As survivors of TOF repair constitute a large and growing population, repaired TOF is the most common indication for cardiovascular MR in most pediatric institutions. Frequent utilization of MR has allowed better understanding of and insight into the right ventricular function in patients with chronic pulmonary regurgitation and/or residual pulmonary stenosis.
Right ventricular systolic function: MR is advantageous over echocardiography in assessment of the right ventricular systolic function in several respects. In MR, ventricular volumes are directly measured without introduction of geometric assumptions for Simpson’s method. As there are no blind spots for imaging, the functioning part of the right ventricle can be accurately separated from the poorly or non-functional right ventricular outflow tract allowing assessment of the volume and contractility of the functioning part of the right ventricle. As MR velocity mapping allows accurate quantification of the forward and regurgitant flow volumes through the pulmonary artery, a corrected right ventricular ejection fraction can be accurately calculated.
Right ventricular diastolic function: Echocardiography with spectral and tissue Doppler interrogation is best suited for assessment of diastolic ventricular function owing to its high temporal resolution. However, blood flow study in conjunction with volume study at MR also provides additional information. The filling patterns of the right and left ventricles can be simultaneously assessed with MR velocity mapping of the tricuspid and mitral valves. This unique function allows direct comparison of the onsets of the E-waves as well as the times-to-peak and peak velocities of the E and A waves. Assessment of the pulmonary arterial blood flow also provides information regarding restrictive diastolic physiology of the right ventricle. A late diastolic forward flow at MR is a more robust evidence of restrictive physiology than that seen at Doppler echocardiography.
Myocardial performance (Tei) index (MPI) is considered a composite index for both systolic and diastolic ventricular function. A high MPI value is usually indicative of reduced global ventricular function in most situations such as pulmonary hypertension and congestive heart failure. In patients with repaired TOF, MPI is poorly predictive of the ventricular performance because the ejection time is commonly prolonged and the isovolumetric relaxation and contraction times are short or does not exist. The majority of the patients with repaired TOF show a paradoxically reduced MRI.