Causes of Right-to-Left Shunt
Right-to-left shunting occurs when blood flows abnormally from the right side of the heart to the left side, bypassing the lungs and causing hypoxemia. According to the American College of Cardiology/American Heart Association guidelines, several conditions can cause this pathophysiological phenomenon 1.
Primary Causes of Right-to-Left Shunt
Eisenmenger syndrome: The most common cause, characterized by long-standing left-to-right shunts (through septal defects or connections between great arteries) that eventually lead to pulmonary vascular disease with reversal of shunt direction due to pulmonary hypertension 1
Severe pulmonary hypertension: When pulmonary arterial pressure exceeds systemic pressure, particularly when pulmonary vascular resistance is greater than two-thirds systemic resistance, right-to-left shunting can occur through any existing septal defect 1
Subpulmonary chamber outflow obstruction: Right ventricular outflow tract obstruction can cause right-to-left shunting by increasing right atrial pressure above left atrial pressure 1
Abnormalities of subpulmonary chamber compliance: Conditions that reduce right ventricular compliance can elevate right atrial pressure and promote right-to-left shunting 1
Vascular streaming: Abnormal blood flow patterns can cause right-to-left shunting even without pressure gradients 1
Specific Anatomical Defects Associated with Right-to-Left Shunts
Atrial septal defects (ASD): When pulmonary vascular resistance exceeds systemic resistance, blood can shunt right-to-left through an ASD 1
Ventricular septal defects (VSD): Similar to ASDs, when pulmonary pressure exceeds systemic pressure 1
Patent foramen ovale (PFO): Can allow right-to-left shunting when right atrial pressure exceeds left atrial pressure 1
Patent ductus arteriosus (PDA): Can develop right-to-left shunting with severe pulmonary hypertension 1
Ebstein's anomaly: The abnormal tricuspid valve placement and function can lead to right-to-left shunting through an associated atrial communication 1
Secondary Causes and Clinical Scenarios
Pulmonary thromboembolic disease: Can cause acute right heart pressure elevation and right-to-left shunting through a patent foramen ovale 2, 3
Right ventricular dysfunction: Following procedures such as pericardiocentesis, can lead to right-to-left shunting 3
Hepatopulmonary syndrome in cirrhosis: Can cause intrapulmonary right-to-left shunting 2
Intrapulmonary arteriovenous malformations: Direct communications between pulmonary arteries and veins 2
Severe right heart failure: From any cause can increase right atrial pressure and promote right-to-left shunting through any existing septal defect 1
Clinical Significance
Right-to-left shunts lead to hypoxemia that is typically unresponsive to supplemental oxygen 1
They can cause paradoxical embolism, where venous thrombi bypass the lungs and enter the systemic circulation 1, 2
When right-to-left shunting is present in a patient with ASD, closure of the defect is contraindicated as it could worsen the patient's hemodynamic status 1
The presence of a net right-to-left shunt is a contraindication to ASD closure, particularly when PA systolic pressure is greater than two-thirds systemic and pulmonary vascular resistance is greater than two-thirds systemic 1
Diagnostic Considerations
Pulse oximetry at rest and with exercise can identify patients with right-to-left shunting 1
Contrast echocardiography (transthoracic or transesophageal) can detect both intracardiac and intrapulmonary shunts 2
Cardiac catheterization is essential to measure pressures and resistances in the pulmonary and systemic circulations to determine the mechanism and severity of the shunt 1
Accurate diagnosis of the cause of right-to-left shunting is crucial as treatment approaches differ significantly based on etiology 1