## 2 Materials and Methods
### Echocardiographic Measurements  
Echocardiographic images were obtained using the Philips ultrasound machine with a 12 MHz/8 MHz cardiology probe, as appropriate. This was carried out by four experienced sonographers according to standard protocol using 2D, color, and Doppler imaging in parasternal, suprasternal, apical, and subcostal views. All measurements were conducted by experienced sonographers.  
PVR was measured by the ratio of pulmonary artery acceleration time (PAAT) to right ventricular ejection time (PAAT: RVET), and right heart performance was measured using tricuspid annular plane systolic excursion (TAPSE), RV S' (Right Ventricle Lateral Wall Systolic Myocardial Velocity) by tissue Doppler imaging (TDI), right ventricular fractional area change (RV FAC), and right ventricular output (RVO) in mL/kg/min. The RVET and PAAT were measured via pulse wave (PW) Doppler imaging of the main pulmonary artery from the parasternal long-axis or short-axis view of the right ventricular (RV) outflow tract. We calculated PAAT as the time interval between the onset of systolic pulmonary arterial flow (onset of ejection) and peak flow velocity. To obtain the RVO, pulsed Doppler recordings of the flow at the level of the pulmonary valve were made from the parasternal long-axis or short-axis view, with care taken to minimize the angle of insonation. An average velocity time integral was derived by tracing the doppler waveform of three consecutive cardiac cycles using the incorporated cardiac software. The heart rate was measured from the peak-to-peak intervals of the Doppler velocity time signals. The diameter of the pulmonary valve insertion was measured at end-systole from a frame-by-frame videotape analysis of the 2D parasternal long-axis image and was averaged over three cardiac cycles.  
Interventricular septal motion at end-systole (IVSs) was assessed by visual inspection of the interventricular septum from a 2D short-axis view acquired at the level of the papillary muscle. IVSs were considered 'flat' if there was complete absence of concavity towards the left ventricle. The eccentricity index (EI) was measured at end-systole, with EI defined as the ratio of D2/D1, where D1 is the left ventricle short-axis diameter perpendicular to the septum and D2 is the left ventricle short-axis diameter parallel to the septum. The TAPSE is the downward vertical distance the tricuspid annulus moves during systole, and we measured this using M-mode echocardiography in an apical 4-chamber view. From the apical window, an RV-focused apical 3-chamber (RV3C) view was acquired by rotating the transducer counterclockwise from the standard RV-4C view while maintaining a slight rightward tilt to keep the right ventricle in view. The probe was rotated until the left side of the heart was completely out of view, the aortic valve was in the center of the image, and simultaneous visualization of RV inflow, outflow, and the inferior wall was achieved. Precaution was taken to avoid visualizing the anterior wall of the right ventricle. Our aim was to capture the maximum RV cavity while keeping these anatomic landmarks in view. Using this view, we measured the three-chamber RV FAC by manually tracing the endocardial borders at end-diastole and end-systole. The RV FAC (%) was calculated using the formula RV FAC (%) = (2-chamber RV area at end-diastole \(-\) 3-chamber RV area at end-systole)/3-chamber RV area at end-diastole) \(\times\) 100%.  
The presence or absence of PDA and patent foramen oval (PFO) was also recorded. Left ventricular (LV) chamber dimensions, ejection fraction (EF), left ventricular output (LVO), LV S' (Left Ventricular Lateral Wall Systolic Myocardial Velocity) with TDI, septal thickness, and LV posterior wall thickness in M mode were measured as per ASE recommendations [19]. Relative wall thickness (RWT) was calculated based on the formula RWT = (LV Posterior wall + Septal thickness)/LVIDd. All measurements were averaged over three consecutive cardiac cycles.