To our knowledge, this is the first report on intracardiac blood flow patterns in Fontan patients using echocardiographic PIV. The PIV technique is noninvasive, and its latest developments allow a high degree of accuracy [8–11]. Shear stress and vortex flow dominate the energetic of any flow; the vortex stores a part of the kinetic energy of the inflow into the rotary motion and redirects it toward the outflow tract .
In our study, we were able to achieve reproducible ventricular vortex flow data in patients with Fontan repair and controls by using contrast echocardiography with PIV. The application of PIV to echocardiography seems promising, and results appear to be qualitatively meaningful.
The 8 cases illustrate that structural abnormalities lead to abnormal flow patterns. Adequate vortex formation has been assumed to be energetically important for an optimal cardiac function, because it enables the storing of kinetic energy during diastole which is then subsequently released during systolic ejection. Abnormal geometry and location of the vortex may lead to suboptimal conditions as shown previously in patients with prosthetic mitral valves  or left ventricle dysfunction [3, 13].
In Fontan patients, diastolic vortex was incoherent, persisted at the centre of ventricle during diastole and systole. Also, this vortex did not dissipate even during systolic ejection, potentially accounting for reduced stroke volume output. The vortex from the Fontan group was consistently shorter, wider and rounder than in controls (Additional files 1 and 2). Pulsatility of systemic ventricular field and vortex represented by RS, VRS and VPC were significantly different in patients with Fontan circulation. There was less red encodes and was not at the centre of ventricle which means not strong vortex. The complexity of the geometry combined with the pulsatile character of the flow, the interaction of the jets with the systemic ventricle flexible walls, and the unsteady motion of the leaflets generate intrinsically complicated turbulent flow structures. So, despite having similar sized systemic ventricle, flow was different in Fontan patients. The clinical implications may be are related to energy loss, adverse flow redirection, high resident times, hemostasis, thrombogenicity, and free emboli formation.
Quantitative vortex parameters may provide a novel method to detect early stages of ventricular dysfunction before gross mechanical changes of ventricular geometry and function as in patients with univentricular heart physiology.
Thus far, heart function has been previously evaluated in echocardiography [14, 15] using 2 dimensions measurements (e.g. LVEDd, Simpson EF, etc.), Doppler blood flow patterns, tissue velocity imaging (e.g. E', myocardial acceleration during isovolumic contraction = IVA) and strain analysis. PIV could be an additional method in the future armentarium of the echocardiographist which could be complementary to other methods because it may be helpful to analyze abnormal kinetics in patients with heart failure or those who are prone to heart failure. In addition echocardiographic PIV can contribute to a better understanding of hemodynamic parameters of the heart.
Particularly in Fontan patients, in whom the systemic venous return is re-routed without interference of a subpulmonary ventricle, studies have focused on methods to reduce energy loss in any part of the circulation by optimizing its hemodynamics [16, 17]. In this context, it is interesting that recently, a MRI study by Sundareswaran et al.,  evaluated the value of 3D flow patterns within lateral tunnel and extracardiac Fontan circulations showing large vortices in patients with a lateral tunnel which may also result in increasing power losses. Due to inadequate acoustic windows, imaging of the systemic venous return including vortices was however not possible. MRI and echocardiography may be complementary to each other in these patients
Our data show that location, shape and sphericity of the main vortices differ clearly from controls. We hypothesize that these differences are related to the size and position of the atrioventricular valve, the dimension and morphology of the systemic ventricle, the spatial relationship between ventricular inflow and left ventricular outflow tract (e.g. transposition), or the absence of interference with a functional subpulmonary ventricle. The insights into systemic ventricular vortex flow in Fontan patients may have additional and potentially incremental value over the conventional methods to assess systemic ventricular function. Vortex flow may influence stroke output and efficiency of the systemic ventricular by redirection of intraventricular flow. This has been explored in a preliminary fashion in a previous study  but not yet in Fontan patients. Diastolic systemic ventricular vortex characterization may have implication for diastolic volumetric filling and may provide an index that links diastolic filling to systolic stroke volume .
After the Fontan procedure a progressive change in systemic ventricular diastolic function is described . Several previous studies have shown impaired systemic ventricular relaxation early after the Fontan procedure [21, 22], coincident with the increase in mass: volume ratio and acquired "hypertrophy" of the ventricle after acute preload reduction on transition to the Fontan state .
However, in the current era, most patients go through a bidirectional cavopulmonary anastomosis prior to the Fontan procedure, thereby precluding the acute preload reduction that used to occur with transitioning from a shunt to the Fontan. We know that the Fontan circulation involves abnormal loading conditions, with decreased preload reserve and chronically increased afterload, as well as issues with afterload mismatch. This may very well be the reason in this study, for the abnormal vortex flow patterns associated with the single ventricle.
Additional late after the Fontan procedure one previous study have demonstrated changes in diastolic Doppler indices consistent with reduced compliance of the systemic ventricle and persisting abnormalities of relaxation. Diastolic function in children with congenital heart disease has also been characterized using echocardiographic assessment of blood and tissue Doppler velocities, specifically in patients with atrial septal defects, tetralogy of Fallot, single ventricle physiology, and following cardiac transplantation .
The PIV and vortices may be a valuable method for the evaluation diastolic and systolic function after the Fontan procedure, but further research is warranted to relate PIV to established measures of cardiac function or symptoms.
Accurate mapping of intraventricular flow provides novel opportunities to evaluate the role of vortices in ventricular function. Similar to any fluid dynamics phenomenon involving vortices, these flow structures are expected to play fundamental roles affecting the dynamics and the energetics of the left ventricle as a pump.
It remains to be determined to what extent these abnormal flow patterns impair systolic and diastolic cardiac function and if surgical corrections aiming at normalizing flow patterns may potentially improve the outcome.