What happens to oxygenation in right to left and left to right shunts?

Oxygenation Changes in Cardiac Shunts

Right-to-Left Shunts

Right-to-left shunting causes systemic arterial hypoxemia because deoxygenated blood bypasses the lungs and enters the systemic circulation directly, resulting in cyanosis and decreased tissue oxygenation. 1

Pathophysiology of Hypoxemia

• Deoxygenated venous blood enters the systemic arterial circulation without passing through the pulmonary capillaries for gas exchange, leading to mixing of oxygen-poor blood with oxygenated blood and reducing overall arterial oxygen saturation 1

• The degree of hypoxemia depends on the magnitude of the shunt—larger right-to-left shunts produce more severe arterial desaturation and cyanosis 1

• Arterial oxygen tension (PaO2) remains low even with 100% oxygen administration because the shunted blood never reaches the alveoli for oxygenation 1, 2

Compensatory Mechanisms and Complications

• The kidneys release erythropoietin in response to tissue hypoxia, stimulating red blood cell production (erythrocytosis) to increase oxygen-carrying capacity 1

• Chronic erythrocytosis leads to polycythemia and hyperviscosity, which paradoxically impairs tissue oxygen delivery by reducing blood flow through small capillaries 1

• Iron deficiency commonly develops as iron requirements increase with accelerated red cell production, resulting in microcytic hypochromic red cells that are rigid and less deformable in the microcirculation 1

• Bleeding risk increases due to thrombocytopenia, platelet dysfunction, and consumption of coagulation factors (FV, FVIII, fibrinogen) associated with polycythemia and hyperviscosity 1

Clinical Detection

• Differential cyanosis may occur with patent ductus arteriosus and right-to-left shunting—oxygen saturation should be measured in both hands and both feet to detect this finding 3

• Right-to-left shunts can be detected by lung perfusion scanning using intravenous technetium-labeled macroaggregates, with appearance of radiotracer in brain and splanchnic organs indicating shunting 1

• Radionuclide angiography shows early visualization of left heart chambers or aorta as the tracer bolus traverses the heart 1

Left-to-Right Shunts

Left-to-right shunting causes pulmonary overcirculation without systemic hypoxemia because oxygenated blood recirculates through the lungs, leading to volume overload of the left heart and eventual pulmonary hypertension. 1

Hemodynamic Consequences

• Oxygenated blood from the left side of the heart (higher pressure) flows to the right side (lower pressure), increasing pulmonary blood flow (Qp) relative to systemic blood flow (Qs) 1

• The Qp/Qs ratio exceeds 1.0 in left-to-right shunts—higher ratios indicate larger shunts with greater hemodynamic significance 1, 4

• Systemic arterial oxygenation remains normal or near-normal because blood reaching the systemic circulation has passed through the lungs for gas exchange 1

• Increased pulmonary blood flow causes left atrial and left ventricular enlargement, congestive heart failure, and failure to thrive in infants 1

Progression to Pulmonary Vascular Disease

• Chronic pulmonary overcirculation leads to elevated pulmonary vascular resistance and pulmonary hypertension 1

• Eisenmenger syndrome develops when pulmonary vascular resistance exceeds systemic vascular resistance, causing shunt reversal from left-to-right to bidirectional or right-to-left 1

• Once Eisenmenger physiology develops with fixed pulmonary hypertension and right-to-left shunting, systemic hypoxemia and cyanosis appear 1

• Vasodilator therapy is contraindicated in Eisenmenger syndrome with fixed pulmonary vascular disease because systemic vasodilation increases right-to-left shunting and worsens hypoxemia 1

Diagnostic Findings

• First-pass radionuclide angiography demonstrates persistent high radiotracer activity in the lungs or right ventricle due to early recirculation 1

• Time-activity curves can distinguish intracardiac shunts (atrial septal defect, ventricular septal defect) from extracardiac shunts (patent ductus arteriosus) 1

• Cardiac magnetic resonance with phase-contrast flow quantification is the non-invasive reference standard for measuring Qp/Qs, avoiding the error propagation and invasiveness of catheter-based oximetry 1

Special Considerations in Single Ventricle Physiology

• After bidirectional Glenn or Fontan procedures, changes in pulmonary blood flow affect end-tidal CO2 (PETCO2) measurements—increased right-to-left shunting decreases PETCO2 by reducing the proportion of blood flowing through the lungs 1

• Hypercarbia achieved by hypoventilation may increase oxygenation and cardiac output in patients with Glenn or Fontan physiology by reducing pulmonary vascular resistance 1

• Aortopulmonary collaterals create left-to-right shunts that increase volume load on the single ventricle, potentially causing ventricular dilation, increased filling pressures, and prolonged intensive care stays 1