A vertical fenestration is a small, intentionally created hole in the surgical barrier (called a baffle) placed inside the heart during the Fontan procedure, a major surgery for children born with only one functioning heart ventricle. The hole, typically 4 to 6 millimeters in diameter, acts as a pressure-relief valve. It allows a small amount of oxygen-poor blood to mix with oxygen-rich blood, which sounds counterintuitive but serves a critical purpose: it keeps blood flowing and prevents dangerously high pressure from building up in the veins after surgery.
Why the Fontan Procedure Needs a Safety Valve
In a normal heart, two ventricles share the workload of pumping blood. The Fontan procedure reroutes circulation so that a single ventricle can do the job of two, directing blood from the veins straight to the lungs without a pumping chamber to push it along. This passive flow system works, but it creates an inherent problem: venous pressure rises because there’s no dedicated pump driving blood through the lungs.
For some patients, that pressure buildup becomes dangerous. When pulmonary artery pressure exceeds 18 mmHg, when the ventricle’s filling pressure is above 12 mmHg, or when there are complicating factors like valve leakage, distorted pulmonary arteries, or unusually complex heart anatomy, the Fontan circuit can fail. Fluid accumulates, cardiac output drops, and recovery stalls. The fenestration was introduced in 1990 specifically to address this problem in high-risk patients, and it significantly reduced both complications and death rates after surgery.
How the Fenestration Changes Blood Flow
The fenestration creates a controlled right-to-left shunt at the atrial level. In plain terms, it gives blood an escape route. When pressure on the venous side of the baffle climbs too high, some blood slips through the hole into the low-pressure chamber that feeds the body. This does two things simultaneously: it relieves the backup of pressure in the veins, and it increases the volume of blood reaching the single ventricle, boosting cardiac output.
The tradeoff is oxygen. Because oxygen-poor blood from the veins mixes with oxygen-rich blood heading to the body, overall oxygen saturation drops. Patients with an open fenestration typically have lower oxygen levels than they would without one. After the fenestration is eventually closed, oxygen saturation rises by about 9.4 percentage points on average, with most patients reaching levels around 94%.
Size and Placement
Surgeons create the fenestration as a single opening in the baffle, sized between 4 and 8 millimeters depending on the patient’s needs. Computational models and clinical experience suggest this range balances enough pressure relief against the degree of oxygen desaturation. A 4 mm fenestration provides modest shunting, while an 8 mm opening allows substantially more flow through the hole. The exact size is a judgment call based on how much pressure relief the surgeon anticipates the patient will need.
Monitoring the Fenestration
After surgery, doctors use echocardiography (ultrasound of the heart) to check whether the fenestration is open, how much blood is flowing through it, and in which direction. Color Doppler imaging, with the scale turned down to about 35 to 40 cm/sec, can detect the low-velocity flow typical of a small fenestration. Pulsed and continuous wave Doppler measure the speed, direction, and timing of that flow, giving the care team a clear picture of how the shunt is behaving as the patient recovers.
When and How the Fenestration Is Closed
The fenestration is designed to be temporary. Most centers evaluate patients for closure roughly 1 to 3 years after the Fontan surgery, with a median closure time of about 1.2 years. Some programs move faster, closing at a median of 9 months. The timeline depends entirely on how well the patient’s circulation has adapted.
Closure is not automatic. Each patient undergoes a test closure first, usually in the catheterization lab, where doctors temporarily block the fenestration and watch what happens to venous pressure, cardiac output, and oxygen saturation. If the numbers look stable, the hole is sealed permanently using a catheter-delivered device, avoiding the need for another open-heart surgery.
After closure, the vast majority of patients see their oxygen levels improve significantly. In a study of the first decade of fenestration closures, average oxygen saturation reached about 94% at follow-up, though a small number of patients (8 out of 106 in one series) remained below 90%. Two patients in that cohort experienced a stroke or transient ischemic attack after device closure, a rare but recognized risk tied to the hardware placed inside the heart.
Open Fenestration Over the Long Term
Not every fenestration gets closed. Some remain open for years, either because the patient isn’t a good candidate for closure or because the shunt continues to serve a useful role. A large Korean multicenter study compared patients whose fenestrations remained open at follow-up with those whose had closed. Venous pressure in the open group was slightly higher (median 14 mmHg versus 12.5 mmHg), but the difference was not statistically significant. Ventricular filling pressure was similar between groups as well.
This suggests that a persistently open fenestration doesn’t necessarily cause progressive harm, but it does mean ongoing mild oxygen desaturation and a continued, small risk of paradoxical embolism, where a blood clot crosses from the venous side to the arterial side through the hole. That risk is one reason many teams prefer to close the fenestration once the patient’s circulation can handle it.