What is the difference between physiological shunt (hypoxemia due to ventilation-perfusion mismatch) and anatomical shunt (right-to-left shunting due to cardiac or pulmonary abnormalities) in lung disease?

From the Guidelines

Physiological shunts are more amenable to treatment with oxygen therapy and management of underlying lung conditions, whereas anatomical shunts often require surgical or percutaneous intervention, as evidenced by the 2019 AHA/ACC guideline for the management of adults with congenital heart disease 1. When considering the management of shunts in lung disease, it's crucial to differentiate between physiological and anatomical shunts due to their distinct pathophysiology and treatment approaches. Physiological shunts occur in conditions where blood passes through ventilated but not perfused areas of the lung, such as pneumonia or pulmonary edema. These can potentially be improved with oxygen therapy and treatment of the underlying condition. In contrast, anatomical shunts involve abnormal connections that allow blood to bypass the lungs entirely, such as in patent ductus arteriosus (PDA) or other congenital heart defects. The management of anatomical shunts, like PDA, involves assessing the significance of the shunt and the presence of pulmonary hypertension, as closing the shunt can prevent further complications like left atrial enlargement, pulmonary arterial hypertension (PAH), and pulmonary hypertension secondary to left heart failure, as noted in the guideline 1. Key considerations include:

• The importance of accurate assessment of oxygen saturation and cyanosis, potentially through ambulatory pulse oximetry, to identify patients with dynamic shunt reversal induced by exercise 1.
• The role of invasive hemodynamic assessment for evaluating pulmonary vasoreactivity and guiding decision-making, especially in complex cases or when considering closure of a PDA 1.
• The potential benefits and risks of closing a PDA, including the prevention of further left atrial or LV enlargement and symptom relief, but also the potential hazards, particularly in adults with calcification and tissue fragility 1.
• The critical distinction that morbidity and mortality are significantly higher when attempting to close a shunt in patients with Eisenmenger physiology and net right-to-left shunting, emphasizing the need for careful patient selection and consultation with specialists 1.

From the Research

Physiological Shunt vs Anatomical Shunt for Lung Disease

• Physiological shunt refers to the passage of venous blood into the arterial blood system bypassing the alveoli-blood gas exchange, which can lead to respiratory failure 2.
• Anatomical shunt, on the other hand, refers to the presence of a systemic-to-pulmonary shunt, which can be a congenital heart defect or a palliative therapy in certain cases 3, 4.
• The management of systemic-to-pulmonary shunts is crucial to prevent the development of irreversible pulmonary vascular disease, and repair of these shunts is often timed to prevent this complication 3.
• In patients with acute respiratory distress syndrome, functional shunt (venous admixture) was found to be poorly correlated to the anatomical shunt compartment, suggesting that functional shunt may not accurately estimate the anatomical shunt compartment 5.
• The use of preoperative targeted therapy in conjunction with fenestrated or partial closure of intracardiac shunts has been associated with improvement in clinical outcomes in patients with systemic-to-pulmonary shunts and elevated pulmonary vascular resistance 3.
• Indications and results of systemic-to-pulmonary shunts have been reported in various studies, with trends showing a decrease in usage for Tetralogy of Fallot and an increase in usage for single ventricle and pulmonary atresia with septal communication 4.

Key Differences

• Physiological shunt is a functional concept, referring to the inefficient gas exchange in the lungs, while anatomical shunt refers to the presence of a structural abnormality in the heart or lungs 2, 4.
• The assessment of physiological shunt can be done using mathematical models, such as estimating the degree of mixing between oxygenated and deoxygenated blood, while anatomical shunt is typically assessed using imaging techniques such as computed tomography scans 2, 5.
• The management of physiological shunt often involves supplemental oxygen therapy and noninvasive ventilation, while anatomical shunt may require surgical intervention 6, 3.