A fenestration is a controlled opening or window created in a body structure, often to relieve pressure or allow temporary fluid movement. In cardiac surgery, this opening is a small shunt between two circulatory systems within the heart or associated vessels. A “double fenestration” specifically indicates the presence of two such openings, which significantly alter the path and mixing of blood flow. These openings are almost exclusively encountered in patients who have undergone complex, multi-stage heart defect repair procedures, representing either two intentional shunts or one intentional shunt combined with an unplanned residual opening.
Defining Double Fenestration
A single fenestration is a surgically created communication, typically a small shunt, connecting the systemic venous circulation to the pulmonary venous atrium. This opening is usually an intentional feature of the Fontan procedure, the final reconstructive surgery for patients with a functional single-ventricle heart. The fenestration, often 3 to 6 millimeters in diameter, is created in the baffle or conduit directing deoxygenated blood to the lungs, allowing a small amount of blood to bypass the lungs and re-enter the systemic circulation.
The concept of a double fenestration arises when two separate, patent openings exist within the Fontan circuit, both allowing a right-to-left shunt. These two shunts may be intentionally created to provide greater pressure relief in a high-risk patient, or they might represent one intentional fenestration alongside a secondary, unintended opening. The second opening could be a residual defect, such as a patent foramen ovale, or a leak that develops over time. Regardless of origin, the two openings create distinct pathways for deoxygenated blood to mix with oxygenated blood before being pumped to the body.
Context of Fenestration in Cardiac Surgery
Fenestrations are most commonly associated with the Fontan procedure, the final stage of palliative surgery for individuals born with a single functional ventricle (e.g., hypoplastic left heart syndrome). The Fontan circulation connects systemic veins directly to the pulmonary arteries, allowing deoxygenated blood to flow passively to the lungs without a dedicated ventricle. This passive flow mechanism is highly sensitive to resistance and pressure changes.
A fenestration is created during Fontan surgery to act as a pressure relief valve for the systemic venous system, preventing excessive pressure buildup post-operation. This temporary shunt allows blood to flow from the high-pressure venous conduit into the lower-pressure systemic ventricle, helping maintain cardiac output and reducing the risk of complications like pleural effusions. The presence of a double fenestration suggests a need for greater decompression or the persistence of an additional connection that was expected to close.
Impact on Blood Circulation
The physiological consequence of a double fenestration is an increased right-to-left shunt, causing greater mixing of deoxygenated and oxygenated blood in the systemic circulation. Since blood bypassing the lungs is not re-oxygenated, its presence in the systemic circulation results in systemic desaturation, meaning the blood pumped to the body has a lower-than-normal oxygen content. This desaturation is the most noticeable clinical sign of a patent fenestration.
The two shunts effectively lower systemic venous pressure, aiding post-operative recovery and reducing complications like liver congestion. However, this benefit comes at the expense of arterial oxygen saturation. Increased shunting can drop saturation significantly, sometimes limiting a patient’s exercise tolerance. The degree of desaturation is proportional to the total flow across the fenestrations, influenced by the openings’ size and the pressure gradient. This trade-off—lower venous pressure versus reduced oxygen saturation—is the central dilemma in managing fenestrated Fontan patients.
The long-term presence of a double fenestration also carries an increased risk of systemic thromboembolic events. Blood clots forming in the systemic veins may pass directly through the shunts into the systemic circulation, bypassing the lungs’ filtering action and potentially causing a stroke. Therefore, the benefits of pressure relief must be weighed against the risks of chronic desaturation and potential clot migration.
Identifying and Managing the Condition
Diagnosis
Identifying a double fenestration begins with clinical observation of persistent cyanosis and reduced oxygen saturation in a Fontan patient. Non-invasive imaging, particularly a transthoracic echocardiogram (TTE), is the first step, using color Doppler to visualize blood flow and estimate the shunts’ size and location. A transesophageal echocardiogram provides clearer images of the fenestration anatomy, especially in the extracardiac Fontan conduit.
The most definitive diagnostic tool is cardiac catheterization, which allows physicians to directly measure pressures and oxygen saturation levels throughout the heart and vessels. During the procedure, a test occlusion is performed by temporarily blocking the fenestration with a balloon to simulate closure. This test determines if the circulatory system can tolerate the subsequent rise in systemic venous pressure and if cardiac output remains stable without the shunts’ pressure relief.
Management
Management focuses on closure, especially when the double fenestration causes significant cyanosis or long-term risk. The most common interventional approach is catheter-based closure, where a specialized occlusion device, such as an Amplatzer device, is delivered through a catheter to plug the openings. Successful closure results in an immediate increase in arterial oxygen saturation, often rising by 9% or more. However, if the test occlusion shows that closure leads to a dangerous rise in systemic venous pressure or a drop in cardiac output, the fenestrations may be left open. In these cases, the patient’s condition is managed with medication to support heart function and blood flow.