What Is the Anatomical Name and Description of a Bidirectional Glenn Procedure? Why Is It Described as “Bidirectional?”

First introduced in 1958 by William Glenn, superior cavopulmonary anastomosis (SCPA) is a procedure in which the superior vena cava (SVC) is disconnected from the atrium and anastomosed to the right pulmonary artery (Figure 71.1). The main pulmonary artery, if not atretic, is disconnected from the heart, and other surgically created sources of pulmonary blood flow, such as an existing systemic to pulmonary shunt, are removed. It is referred to as “bidirectional” because blood from the SVC flows to the right pulmonary artery as well as across to the left pulmonary artery, thereby supplying both lungs.

Figure 71.1 A schematic of hypoplastic left heart syndrome following superior cavopulmonary anastomosis (SCPA) or the Glenn procedure highlighting connection of the SVC and pulmonary artery (PA).

Printed with permission from Texas Children’s Hospital

What Type of Cardiac Lesions Require SCPA?

SCPA is typically the second stage in the palliation of single ventricle cardiac lesions. Patients may require single ventricle palliation for a variety of pathologies including hypoplastic right heart lesions (tricuspid atresia, pulmonary atresia with intact ventricular septum, severe forms of Ebstein’s anomaly with tricuspid dysplasia), hypoplastic left heart lesions (aortic atresia or stenosis, mitral atresia, or stenosis), or lesions which do not permit surgical septation of the heart into two ventricle physiology (unbalanced atrioventricular canal defects, double inlet ventricle, some forms of heterotaxy).

The Flow of Blood to the Heart in the Glenn Patient

All blood flow to the pulmonary circulation is passive and supplied from the upper body via the SVC anastomosis to the pulmonary arteries. Because patients have had a complete atrial septectomy as part of their first stage procedure, when oxygenated blood returns to the heart from the pulmonary veins, it mixes with deoxygenated blood returning from the lower body via the inferior vena cava (IVC) and then proceeds to the single ventricle.

What Are Typical Hemoglobin–Oxygen Saturations for a Patient with SCPA Physiology?

Typical hemoglobin–oxygen saturations for a patient with Glenn physiology are between 75 and 85%.

What Is the Typical Hematocrit in a Patient with SCPA Physiology?

Due to the cyanotic nature of the physiology and the need to maximize oxygen delivery, the hematocrit of SCPA patients is generally expected to be maintained between 40% and 45%.

At What Age Is SCPA Generally Performed? Why Is It Not Performed on Newborns?

The SCPA/Glenn procedure is usually performed at two to six months of age, as pulmonary vascular resistance (PVR) continues to fall after birth and passive pulmonary blood flow becomes possible. It cannot be performed earlier due to high PVR in newborns that would preclude successful passive pulmonary blood flow.

What Physiologic Advantage Does SCPA Offer Over Native Circulation or a Systemic-to-Pulmonary Arterial Shunt?

Prior to SCPA, blood exits the single ventricle via an outflow tract that then provides flow to both the systemic and pulmonary vascular circulations, with their relative resistances determining the amount of flow to each. This arrangement, known as parallel circulation, places a volume burden on the ventricle. SCPA places the two vascular beds in series by making pulmonary blood flow a diversion of systemic venous return. This effectively reduces the volume load on the ventricle, which in turn allows ventricular remodeling to occur, improving ventricular function and lowering end diastolic pressure. In addition, the transition to a series circulation also eliminates reliance on the balance between systemic and pulmonary vascular resistance to provide adequate flow to each system.

In comparison to a modified Blalock–Taussig (BT) shunt, which is made of synthetic Gore-Tex, the native tissue of the SVC provides a more stable source of pulmonary blood flow that is able to grow with the patient and is less likely to be compromised by thrombosis.

What Is the Final Stage in Single Ventricle Palliation?

At two to five years of age, the patient will undergo total cavopulmonary anastomosis (TCPA), also known as the Fontan procedure. This surgery results in all systemic venous return being diverted passively to the pulmonary system by connecting the inferior vena cava (IVC) to the pulmonary artery, most often via an extracardiac conduit. After undergoing the Fontan procedure most single ventricle patients will have hemoglobin-oxygen saturation in the high 90s.

Why Is SCPA Considered More Stable for Noncardiac Surgery than TCPA?

In comparison to TCPA, cardiac output after SCPA is not entirely dependent on pulmonary blood flow because the IVC remains directly connected to the heart, providing venous return. Therefore, should pulmonary blood flow be compromised by an increase in PVR, such as during bronchospasm or light anesthesia, cardiac output can be maintained via IVC flow. After TCPA, an abrupt increase in PVR could result in a precipitous decrease in pulmonary blood flow and preload to the single ventricle, unless a fenestration was created at the time of surgery, which allows IVC blood to potentially divert from the extracardiac conduit to the atrium. In TCPA patients with a fenestration, blood can shunt from the conduit to the single ventricle, as the resultant decrease in hemoglobin-oxygen saturation is better tolerated than low cardiac output. These patients may have hemoglobin-oxygen saturation in the 85–95% range.