What would the oxygen saturation (SpO2) levels be in a patient with an unbalanced atrioventricular (AV) canal and interrupted aortic arch?

SpO2 Saturations in Unbalanced AV Canal with Interrupted Aortic Arch

In neonates with unbalanced atrioventricular canal and interrupted aortic arch, expect SpO2 saturations in the 75-85% range, reflecting significant mixing of oxygenated and deoxygenated blood through the large intracardiac defect combined with ductal-dependent systemic circulation.

Pathophysiological Basis for Expected Saturations

Mixing Physiology from the AV Canal Defect

• The unbalanced complete atrioventricular canal creates a large communication between both atria and ventricles, resulting in complete mixing of systemic venous return (deoxygenated blood) with pulmonary venous return (oxygenated blood) 1

• This mixing physiology means that blood ejected to both the pulmonary and systemic circulations has similar oxygen saturation, typically resulting in systemic saturations between 75-85% 1

• The degree of saturation depends on the ratio of pulmonary to systemic blood flow (Qp:Qs), with higher pulmonary flow producing slightly higher saturations 1

Impact of Interrupted Aortic Arch

• The interrupted aortic arch creates ductal-dependent systemic circulation, where descending aortic flow depends entirely on right-to-left shunting through the patent ductus arteriosus 2, 3, 4

• This anatomic arrangement means that mixed venous blood from the right ventricle (already desaturated from intracardiac mixing) supplies the descending aorta, further compromising systemic oxygen delivery 2, 4

• Pre-ductal saturations (right arm) may be 5-10% higher than post-ductal saturations (lower extremities) due to preferential streaming of relatively better-oxygenated blood to the ascending aorta 2, 3

Clinical Presentation and Monitoring

Expected Saturation Ranges by Location

• Right upper extremity (pre-ductal): 80-90% - receives blood primarily from the left ventricle before complete mixing 2, 3

• Lower extremities (post-ductal): 75-85% - receives completely mixed blood through the ductus arteriosus 2, 3, 4

• The differential between pre- and post-ductal saturations helps confirm the diagnosis and assess ductal patency 2, 3

Critical Monitoring Considerations

• SpO2 alone does not reflect adequate tissue oxygen delivery in these patients, as total oxygen content depends on hemoglobin concentration and cardiac output, not just saturation 5

• A saturation of 80% with adequate hemoglobin (>15 g/dL) and good cardiac output provides better tissue oxygenation than 95% saturation with severe anemia (hemoglobin 8 g/dL) 5

• Clinical signs of perfusion (capillary refill, urine output, lactate levels, mental status) are more reliable than SpO2 for assessing adequacy of oxygen delivery 5

Factors Affecting Saturation Variability

Pulmonary Blood Flow Dynamics

• Excessive pulmonary blood flow (unrestricted by pulmonary stenosis): May produce saturations approaching 85-90% but at the expense of systemic perfusion and congestive heart failure 2, 3

• Balanced pulmonary flow: Typically results in saturations of 75-85% with adequate systemic perfusion 2, 4

• Restricted pulmonary flow (if pulmonary stenosis present): May result in saturations <75% with severe cyanosis 2

Ductal Patency Status

• Patent ductus arteriosus: Maintains systemic perfusion but saturations remain 75-85% due to mixing 2, 3, 4

• Ductal constriction: Causes precipitous clinical deterioration with profound metabolic acidosis, even if saturations appear "acceptable" at 80-85% 2, 4

• Prostaglandin E1 infusion is mandatory to maintain ductal patency and systemic perfusion, regardless of saturation readings 2, 3, 4

Critical Clinical Pitfalls

Do Not Target Normal Saturations

• Attempting to achieve SpO2 >90% with supplemental oxygen is counterproductive and dangerous in these patients 1

• High inspired oxygen concentrations will decrease pulmonary vascular resistance, increase pulmonary blood flow, and "steal" from systemic circulation, causing systemic hypoperfusion despite higher saturations 1

• Target saturations of 75-85% with room air or minimal supplemental oxygen (FiO2 0.21-0.30) to maintain balanced circulation 1

Recognize Limitations of Pulse Oximetry

• SpO2 readings may be falsely reassuring if cardiac output is severely compromised - a saturation of 85% means nothing if tissue perfusion is inadequate 5

• Always correlate SpO2 with clinical perfusion markers: warm extremities, brisk capillary refill (<2 seconds), adequate urine output (>1 mL/kg/hr), normal lactate (<2 mmol/L), and appropriate mental status 5

• In severe anemia (hemoglobin <10 g/dL), SpO2 of 85% provides far less oxygen delivery than the same saturation with normal hemoglobin 5

Immediate Management Priorities

Stabilization Protocol

• Maintain ductal patency with prostaglandin E1 infusion (0.01-0.1 mcg/kg/min) - this is more critical than oxygen saturation 2, 3, 4

• Avoid supplemental oxygen unless saturations fall below 70% - room air is preferred to prevent pulmonary overcirculation 1

• Monitor four-extremity blood pressures and pre/post-ductal saturations to assess ductal patency and systemic perfusion 2, 3

• Obtain arterial blood gas to assess pH, lactate, and base deficit - these reflect tissue perfusion better than SpO2 6, 5

Surgical Considerations

• These complex lesions typically require staged surgical repair, with initial arch reconstruction, atrial septectomy, and pulmonary artery banding performed in the neonatal period 2, 4, 7

• Single-stage complete repair has lower mortality (12% early, 20% late) compared to two-stage procedures (37% early, 26% late) when anatomically feasible 8, 7

• The presence of left ventricular hypoplasia significantly worsens prognosis, with early mortality up to 42% and late mortality 50% 8