Palliative Procedures for Single Ventricle Physiology

Initial Palliation

• The American College of Cardiology recommends that the BT shunt (systemic-to-pulmonary artery shunt) is typically the first palliative procedure performed in neonates with restricted pulmonary blood flow to increase pulmonary blood flow and improve oxygen saturation 1, 2
• This initial shunt is usually performed from the ascending aorta to the main or right pulmonary artery, as stated by the American Heart Association 2
• The BT shunt causes volume overload on the single ventricle, which can lead to ventricular dilation and failure over time, according to the American College of Cardiology 3

Second Stage: Glenn Procedure

• The Bidirectional Glenn (BDG) or bidirectional cavopulmonary anastomosis (BDCPA) is most commonly performed in infancy or early childhood as the second stage of palliation, as recommended by the American Heart Association 2, 4
• The Glenn procedure involves an end-to-side anastomosis of the divided superior vena cava to the undivided right pulmonary artery, as described by the American College of Cardiology 5
• This provides a stable source of pulmonary blood flow without volume loading the single ventricle, which is a significant advantage over the BT shunt, according to the American Heart Association 2, 4

Benefits of the Glenn Procedure

• The Glenn shunt provides pulmonary blood flow without volume loading the single ventricle, unlike the BT shunt, as stated by the American College of Cardiology 1, 2
• It results in superior vena cava blood being directed to both right and left pulmonary arteries, according to the American College of Cardiology 5
• The 20-year survival rate after Glenn shunt is approximately 50%, compared to the same survival rate for systemic-PA shunts but with less ventricular strain, as reported by the American College of Cardiology 3

Pre-Glenn Evaluation

• Cardiac catheterization is indicated before Glenn procedure to assess pulmonary artery obstruction and restore maximal continuous, effective systemic venous flow to pulmonary artery segments, as recommended by the American College of Cardiology and the American Heart Association 1, 2
• Assessment and elimination of systemic-to-pulmonary vein collaterals and systemic-to-pulmonary artery connections are necessary, according to the American College of Cardiology and the American Heart Association 1, 2

Special Considerations

• The Glenn procedure serves as an intermediate step toward eventual Fontan completion in single ventricle patients, as stated by the American Heart Association 2, 4
• The Fontan procedure has approximately 90% 10-year survival rate in the absence of risk factors, according to the American College of Cardiology 3
• Late complications after Fontan include atrial arrhythmias, thrombus formation, protein-losing enteropathy, progressive systemic ventricular failure, and progressive AV valve regurgitation, as reported by the American College of Cardiology 3

Clinical Pitfalls to Avoid

• Inadequate assessment of pulmonary artery anatomy before Glenn procedure can result in suboptimal outcomes, as warned by the American College of Cardiology and the American Heart Association 1, 2
• Failure to identify and address collateral vessels can compromise Glenn shunt function, according to the American College of Cardiology and the American Heart Association 1, 2

Bidirectional Glenn Procedure

Anatomical Basis

• The bidirectional Glenn procedure directs superior vena cava blood flow to both the right and left pulmonary arteries simultaneously, unlike the classic Glenn which only perfuses the right lung, according to the American College of Cardiology 6
• The procedure involves an end-to-side anastomosis of the divided superior vena cava to the undivided right pulmonary artery, creating a pathway where blood flows in two directions across the pulmonary arterial system, as described by the American College of Cardiology 6
• Superior vena cava blood is directed to both right and left pulmonary arteries through the intact pulmonary artery confluence, establishing bilateral pulmonary perfusion from a single venous source, as noted by the American College of Cardiology 6
• This contrasts sharply with the classic Glenn shunt, where the superior vena cava connects to the distal end of the divided right pulmonary artery, resulting in superior vena cava blood reaching only the right lung, according to the American College of Cardiology 6

Clinical Significance

• Bilateral pulmonary perfusion prevents the development of pulmonary arteriovenous malformations, which commonly occur with the classic Glenn due to absence of hepatic factors to the right lung, as stated by the American College of Cardiology 6
• The bidirectional Glenn procedure is also called bidirectional cavopulmonary anastomosis (BDCPA) or bidirectional cavopulmonary shunt, all referring to the same procedure with bilateral pulmonary perfusion, according to the American College of Cardiology 6
• The key distinguishing feature remains the undivided pulmonary artery that allows bidirectional flow distribution, as described by the American College of Cardiology 6

Mixed Venous Saturation Measurement in Bidirectional Glenn Physiology

Physiologic Basis

• In patients who have undergone a bidirectional Glenn shunt, blood returning from the superior vena cava (SVC) bypasses the heart and enters the pulmonary arteries directly, so it does not represent true mixed venous blood. Only inferior vena cava (IVC) return mixes with pulmonary venous blood in the single functional ventricle. 7
• Consequently, the IVC is the sole source of venous blood that truly mixes with oxygenated blood before systemic ejection in Glenn physiology. 7

Sampling Recommendations

• Sample blood from the IVC (or the right atrium where IVC flow enters) to obtain the true mixed venous oxygen saturation in patients with a bidirectional Glenn connection. This approach replaces the conventional pulmonary artery sampling used in two‑ventricle circulation. 7
• When calculating transpulmonary gradient and pulmonary vascular resistance index (PVRI) prior to Fontan completion, use the IVC saturation as the mixed venous value because accurate hemodynamic assessment is critical for selecting candidates for cavopulmonary surgery. 7

Technical Pitfalls to Avoid

• Do not obtain saturation samples from the SVC, as this blood is already partially oxygenated after passing through the lungs and therefore overestimates mixed venous oxygen content. 7
• Traditional mixed‑venous formulas that average SVC and IVC saturations are inappropriate in Glenn physiology because they assume both cavae drain into a right atrium, which is not the case. 7
• Systemic‑to‑pulmonary collaterals and venous collaterals or arteriovenous malformations can alter measured saturations and create a volume load on the ventricle; these must be accounted for when interpreting IVC values. 8

Clinical Monitoring Applications

• For continuous central venous oxygen saturation (ScvO₂) monitoring during or after a Glenn procedure, position the catheter in the IVC or right atrium rather than the SVC to obtain a reliable mixed venous reading. 7
• In research settings, right‑atrial sampling (which receives IVC blood) has been shown to provide estimates of systemic oxygen delivery that are closer to true mixed venous values than SVC sampling. 7

Practical Algorithm for Measurement

Evidence level: Not explicitly stated in the cited sources; recommendations are based on expert consensus and guideline statements from the cited cardiovascular literature.