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Dr. Alessandro Giardini
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The Glenn procedure is the second of the three staged operations in the palliation of hypoplastic left heart syndrome. It is performed at around four to six months of age, when the baby's pulmonary vascular resistance has fallen sufficiently to allow a fundamental change in how the circulation is organised. For most families, it marks the transition from the intense anxiety of the interstage period to a period of relative cardiovascular stability, during which the child can grow, develop, and prepare for the final stage of palliation.
Understanding what the Glenn procedure does, why the timing is important, and what the circulation looks like afterwards helps families make sense of what they observe in their child and what the follow-up appointments are measuring.
The Glenn procedure, formally known as the superior bidirectional cavopulmonary anastomosis, connects the superior vena cava directly to the pulmonary artery branches. The superior vena cava is the large vein that drains deoxygenated blood from the head, neck, arms, and upper body back to the heart. After the Glenn, instead of flowing into the right atrium and then being pumped to the lungs by the ventricle, this blood flows directly and passively into the pulmonary arteries, driven by the small pressure difference between the systemic venous return and the pulmonary circulation.
At the same time as the Glenn connection is made, the shunt placed at the Norwood procedure is removed. The right ventricle no longer has to pump blood to both the lungs and the body. Its workload is substantially reduced. Only the blood returning from the lower body, through the inferior vena cava, still passes through the heart before reaching the lungs. That is the component that will be addressed in the Fontan operation.
The result is a more stable and efficient circulation than the post-Norwood physiology. The right ventricle is no longer subjected to the volume overload of pumping blood to two vascular beds simultaneously. Oxygen saturations typically improve from the 75 to 85% range seen after the Norwood to the 80s range after the Glenn, reflecting the more efficient delivery of blood to the lungs.
The Glenn procedure cannot be performed safely at birth or in the first months of life because the pulmonary vascular resistance remains too high. In foetal life and in the neonatal period, the blood vessels in the lungs are constricted, with relatively high resistance to flow. As the baby is exposed to postnatal circulation, these vessels gradually relax and the pulmonary vascular resistance falls over the first four to six months. Once it has fallen sufficiently, passive venous blood flow to the lungs becomes feasible.
If the Glenn is performed too early, when pulmonary vascular resistance is still elevated, the superior vena cava blood cannot flow adequately to the lungs. The result is significant cyanosis, raised venous pressure in the head, and a failing Glenn anastomosis. The correct timing is determined by clinical progress and cardiac investigations (echocardiography and cardiac MRI) in the weeks before the planned operation. In some children a cardiac catheterization may be required which directly measures pressures in the pulmonary circulation and confirms that the resistance is at an acceptable level.
Performing the Glenn later than optimal is also a consideration. A baby who has been living with an increasingly volume-loaded right ventricle for many months as pulmonary flow through the shunt increases starts to accumulate the burden of that overload. Timing the Glenn to coincide with pulmonary readiness while the right ventricle is still in good shape is the objective.
The Glenn procedure is performed under general anaesthesia, typically with cardiopulmonary bypass, though some centres perform it off-pump. The operation itself is shorter and less complex than the Norwood procedure, though it remains major cardiac surgery in a small infant.
The superior vena cava is divided from the right atrium and re-routed directly to the right pulmonary artery, creating the bidirectional Glenn anastomosis. Bidirectional refers to the fact that blood flows from the superior vena cava into both the right and left pulmonary artery branches, as opposed to the original unidirectional classic Glenn operation, which is no longer used. The previously placed shunt is identified and ligated.
Additional procedures may be performed simultaneously, including repair of distorted or narrowed pulmonary artery branches, if the shunt has caused any growth issues, and management of any significant tricuspid regurgitation, which is an important risk factor for eventual right ventricular failure.
Most babies recover significantly faster from the Glenn procedure than from the Norwood. The intensive care stay is typically two to four days, and the overall hospital stay is around ten to fourteen days in straightforward cases, substantially shorter than the six weeks commonly required after stage one.
The most common postoperative complication after the Glenn is a pleural effusion, a collection of fluid around the lung. This occurs because the elevated venous pressure in the Glenn circulation leads to increased lymphatic flow, which can exceed the capacity of the lymphatic system to absorb. Effusions are managed with chest drains and, in some cases, dietary fat restriction to reduce the lymphatic load. They typically resolve within one to two weeks, though occasionally they are more persistent.
After discharge, babies generally make rapid developmental and physical progress. The improved cardiac efficiency translates into better feeding, better weight gain, and increased energy for the interactive play that drives developmental milestones. Families frequently describe a noticeable change in their child's vitality in the weeks after the Glenn.
After the Glenn, the child lives with a partially separated circulation. Deoxygenated blood from the upper body flows directly to the lungs. Deoxygenated blood from the lower body still passes through the right atrium and ventricle before being pumped out, which means the right ventricle is still receiving a mixed blood return and the child remains somewhat cyanotic, with saturations in the 80-90% range rather than the normal 95% or above.
This is expected and acceptable. The Glenn circulation sustains the child safely for the one to two years until the Fontan procedure completes the separation. During this time, the child is mobile, growing, and increasingly active. Regular follow-up echocardiography monitors right ventricular function, the function of the tricuspid valve, and the development of the pulmonary arteries, which must be adequate in size to accommodate the Fontan circulation when the time comes.
The Glenn is timed based on a combination of the baby's age, weight, and clinical condition, and the results of cardiac investigations that measures cardiac anatomy and function as well as the integrity of the aortic arch wich was repaired at the time of the first stage operation. Most babies have this assessment at around four months of age, and the operation is scheduled once all criteria are confirmed to be at an acceptable level. The cardiac team will discuss the timing and the catheterisation findings in detail.
Yes, for most children after the Glenn procedure. Post-Glenn saturations are expected to run lower than the normal 95% or above because the inferior vena cava blood, which is still deoxygenated, continues to mix with oxygenated blood returning from the lungs. Saturations in the 78 to 85% range are typical and do not represent a problem with the surgery. The cardiac team will define the expected range for your individual child.
The Glenn anastomosis itself is durable and does not need to be redone. Occasionally, distortion or narrowing of the pulmonary artery branches requires catheter-based or surgical intervention before the Fontan, but this is not routine. The Glenn connection remains as part of the final Fontan circulation: at the Fontan operation, the inferior vena cava is connected to complete the cavopulmonary circuit, not to replace the Glenn.
Author: Dr. Alessandro Giardini, MD, PhD, Consultant Paediatric Cardiologist
Written 22/06/2026