Right ventricular mid-cavity membrane with bidirectional ostium secondum shunt in an adult- Blurred line between isolated right ventricular hypoplasia and double chambered right ventricle physiology. (QF_SA_129)

A 26 year old female with easy fatigability, exercise intolerance, clubbing of fingers, SpO2: 83%.ECG: incomplete right bundle branch block. Normal auscultation. No family history of congenital cardiac diseases.

Diagnostic Techniques and Their Most Important Findings:

2D echo: Large ostium secondum ASD(18x16 mm).On Power Doppler: Bidirectional flow. A thin membrane like structure in right ventricular mid cavity.

Cardiac MRI plain: Mildly dilated right atrium. Normal tricuspid and pulmonary valve morphology with a small TR jet. RV length from tricuspid valve to apex: 5.2 cm. LV length from mitral valve to apex: 7.9cm.RV to LV length ratio: 0.65.A thin elongated membrane like structure in RV mid cavity with fenestrations attached to few trabeculae carneae, extending from mid ventricular septum to RV free wall and reaching upto base of RVOT, dividing RV cavity into a small inlet chamber and a mildly dilated apical or trabecular chamber both of which communicate with RVOT. Aneurysmal bulging of lateral wall of the trabecular chamber just below the attachment of the membrane. No stenotic jet in RVOT. Interventricular septum: Intact, with left ward bowing exaggerated in diastole. Relatively small pulmonary artery. No RV hypertrophy. Large ostium secondum ASD with bidirectional flow. LV ejection fraction: 57%.RV ejection fraction: 54%.Qp:Qs ratio: 0.62(net right to left shunt).EDV and ESV of inlet chamber:18 ml and 17 ml.EDV and ESV of distal RV chamber: 42 ml and 13 ml, implying that the inlet chamber had no effective contraction.(Figure 1 and 2)

Cardiac catheterisation with chamber pressures and O2 saturation study: Thin horizontal filling defect in mid RV cavity with dominant A wave in the right atrial pressure waveform.(Table 1)

Learning Points from this Case:

The mid RV cavity partial obstruction and effective small size of right ventricle can be considered a variant of isolated right ventricular hypoplasia as well as double chambered right ventricle. This condition led to a unique hemodynamic situation: 
1.Large ostium ASD with a net right to left shunt but no RV dilatation, no RV hypertrophy, no pulmonary hypertension and only mild tricuspid regurgitation: The elevated RV end-diastolic pressures due to small RV capacity, partial RV mid cavity obstruction, impaired RV geometry and reduced RV compliance transmitted to the right atrium in diastole resulting in a dominant right to left shunt across the ASD. During mid to late systole, left atrial pressure exceeded the right atrial pressure resulting in a left to right shunt, accompanied by right atrial enlargement.
2.RV mid-cavitatory obstruction but no hypertrophied muscle bundle or trabeculae causing obstruction to RVOT, no ventricular septal defect(which is usually associated with the double chambered RV and implied in its etiopathogenesis): The obstructing structure could be a variant of moderator band with high origin, however the fenestrations and thin, membrane like morphology could also imply a true anomalous band.
Why CMRI?: a) Unrestricted visualisation of right ventricular chamber morphology, dimensions and function, which was central to explaining the pathophysiology in this case b)excellent characterisation of mid-cavity membrane, its complete extent, presence of fenestrations. c) Assessing ASD hemodynamic severity through indirect shunt quantification(Qp:Qs), net flow (Qs-Qp),direct en face PC-MRI of the ASD(shunt ratio), estimating defect eccentricity and defect area through planimetry, thus creating a road-map prior to cardiac catheterisation

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