Is an Interatrial Left-to-Right Shunt Normal?
No, an interatrial left-to-right shunt is not normal—it represents a pathological communication between the left and right atria that can lead to right heart volume overload, pulmonary hypertension, and heart failure if hemodynamically significant.
Pathophysiology of Interatrial Shunts
An interatrial left-to-right shunt occurs when blood flows abnormally from the left atrium to the right atrium through a defect in the atrial septum. This is fundamentally abnormal anatomy that creates several pathophysiological consequences:
- Right heart volume overload: The shunt causes increased blood flow through the right heart chambers, leading to right atrial and right ventricular enlargement 1
- Pulmonary overcirculation: Excessive blood flow to the lungs can eventually cause pulmonary vascular remodeling and pulmonary hypertension 1
- Paradoxical embolism risk: Some patients may develop right-to-left shunting or experience paradoxical emboli 1
Common Causes of Interatrial Left-to-Right Shunts
The most frequent anatomic defects causing these shunts include:
- Secundum atrial septal defect (ASD): The most common type, occurring in the central atrial septum 1
- Primum ASD: Associated with atrioventricular septal defects 1
- Sinus venosus defect: Often associated with anomalous pulmonary venous connection 1
- Patent foramen ovale (PFO): Can allow left-to-right shunting when left atrial pressure is elevated 2
When Intervention Is Required
The ACC/AHA guidelines provide clear thresholds for when these abnormal shunts require closure:
Class I recommendation: Closure is indicated when there is impaired functional capacity, right heart enlargement, and a hemodynamically significant shunt (Qp:Qs ≥1.5:1), provided pulmonary artery systolic pressure is <50% of systemic pressure and pulmonary vascular resistance is <1/3 systemic resistance 1, 3
Contraindication to closure: ASD closure should not be performed when pulmonary artery systolic pressure exceeds 2/3 systemic pressure, pulmonary vascular resistance exceeds 2/3 systemic, or there is net right-to-left shunting (Eisenmenger syndrome) 1, 3
Clinical Significance of Shunt Magnitude
The hemodynamic impact determines clinical significance:
- Qp:Qs ≥1.5:1: Indicates a shunt large enough to cause physiological sequelae requiring consideration for closure 1, 3
- Right ventricular enlargement: Presence of RV dilation confirms hemodynamic significance 1, 3
- Elevated left atrial pressure: In heart failure patients, turbulent left-to-right shunt flow can indicate high left atrial pressure, though this represents a pathological state rather than normal physiology 2
Important Caveat: Therapeutic Interatrial Shunts
While naturally occurring interatrial left-to-right shunts are pathological, there is emerging research on intentionally created interatrial shunts as therapy for heart failure with preserved ejection fraction (HFpEF):
- These iatrogenic devices aim to decompress elevated left atrial pressure by creating a controlled left-to-right shunt 4, 5, 6
- This therapeutic approach is based on observations from Lutembacher syndrome (combined mitral stenosis and ASD), where the shunt appeared to reduce symptoms compared to isolated mitral stenosis 5, 7
- However, recent meta-analysis shows concerning safety signals, with significantly higher all-cause mortality (RR: 1.49) and cardiovascular death (RR: 1.66) in patients receiving these devices 8
- These therapeutic shunts remain investigational and do not represent normal physiology 4, 9
Diagnostic Evaluation
When an interatrial shunt is suspected:
- Echocardiography: Initial imaging modality to identify the defect and assess right heart size 1
- CMR or CTA: Recommended for complete anatomic delineation, especially for complex anatomy or anomalous pulmonary venous connections 1, 3
- Pulse oximetry: Useful at rest and with exercise to define shunt direction and identify exercise-induced desaturation 1
- Cardiac catheterization: Can be useful for hemodynamic assessment, shunt quantification, and pulmonary vascular resistance measurement 1