What is intra-atrial shunting?

Intra-Atrial Shunting

Intra-atrial shunting is the abnormal flow of blood between the right and left atria through a communication in the atrial septum, which can occur in either direction depending on pressure gradients between the chambers.

Types of Atrial Communications

• Secundum atrial septal defect (ASD) is the most common type (75% of cases), located in the region of the fossa ovalis 1
• Primum ASD (15-20% of cases) is positioned inferiorly near the crux of the heart and is often associated with a cleft in the anterior mitral valve leaflet 1
• Sinus venosus ASD (5-10% of cases) is located superiorly near the superior vena caval entry or inferiorly near the inferior vena caval entry, often associated with partial anomalous pulmonary venous drainage 1, 2
• Coronary sinus septal defect (less than 1% of cases) causes shunting through the ostium of the coronary sinus 1, 2
• Patent foramen ovale (PFO) is a flaplike communication where the septum primum overlaps the superior limbic band of the septum secundum 1

Direction of Shunting

Left-to-Right Shunting

• Most common direction in uncomplicated atrial communications 1
• Results in right ventricular volume overload and pulmonary overcirculation 1
• Can lead to symptoms including frequent pulmonary infections, fatigue, exercise intolerance, and palpitations 1
• Long-term consequences include atrial arrhythmias (atrial flutter, atrial fibrillation, sick sinus syndrome) and flow-related pulmonary arterial hypertension 1

Right-to-Left Shunting

• Occurs when right atrial pressure exceeds left atrial pressure 3
• Common causes include:
  • Eisenmenger syndrome (most common cause) - long-standing left-to-right shunts leading to pulmonary vascular disease with shunt reversal 3
  • Severe pulmonary hypertension when pulmonary arterial pressure exceeds systemic pressure 3
  • Subpulmonary chamber outflow obstruction increasing right atrial pressure 3
  • Abnormalities of subpulmonary chamber compliance 3
• Clinical consequences include hypoxemia unresponsive to supplemental oxygen and risk of paradoxical embolism 3, 4

Diagnosis of Intra-Atrial Shunting

• Imaging techniques should demonstrate the shunt and evidence of right ventricular volume overload 1
• Transthoracic echocardiography is the initial diagnostic tool 1
• Transesophageal echocardiography (TEE) provides superior visualization of atrial septum and can identify sinus venosus defects that are visible by transthoracic imaging in only 25% of cases 1
• Saline contrast echocardiography is more sensitive than Doppler for detection of low-velocity right-to-left shunts 1
  • Well-agitated saline provides dense opacification of the right side of the heart 1
  • It does not pass through normal pulmonary circulation, thus identifying right-to-left shunts 1
  • Sensitivity is increased by performing a Valsalva maneuver, which transiently increases right atrial pressure 1
• Cardiac MRI or CT may be used for further assessment of anatomy and associated lesions 1

Clinical Significance

• Left-to-right shunting can lead to right heart enlargement, pulmonary hypertension, and atrial arrhythmias 1
• Right-to-left shunting can cause hypoxemia and paradoxical embolism 3, 5
• Paradoxical embolism is a risk for all defects regardless of size 1
• Platypnea-orthodeoxia (dyspnea and hypoxemia worsened in upright position) can occur with right-to-left shunting 6
• Interatrial shunting may occur after thoracic surgery through a previously asymptomatic PFO or ASD 6

Therapeutic Applications of Intra-Atrial Shunting

• Atrial septostomy (creation of an intra-atrial right-to-left shunt) can be used as a palliative treatment in patients with severe pulmonary arterial hypertension 1
  • It decompresses the failing right ventricle and increases cardiac index 1
  • Indicated in severely symptomatic patients unresponsive to medical treatment 1
  • Improvements in cardiac index range from 15-58% immediately following the procedure 1
• Interatrial shunting has emerged as a therapy for decompressing the left atrium in patients with acute and chronic left heart failure 7, 8
  • Can be achieved through implantation of specialized devices 8
  • Early studies show improvements in functional status and quality of life 8

Management of Pathological Intra-Atrial Shunting

• Closure of ASD is indicated for right atrial and right ventricular enlargement with or without symptoms 1
• Closure can be performed percutaneously for secundum ASDs or surgically for other types 1
• Closure is contraindicated in patients with right-to-left shunting due to pulmonary hypertension as it could worsen hemodynamic status 3
• Specific contraindications include PA systolic pressure greater than two-thirds systemic and pulmonary vascular resistance greater than two-thirds systemic 3