The Fontan Principle



The Fontan Principle





The Fontan principle is applied to patients with single-ventricle congenital heart disease. The ultimate goal is to achieve a circulation in which the systemic venous return is delivered directly to the pulmonary arteries, and the single ventricle is used for systemic blood flow. The original Fontan operation utilized an atriopulmonary connection for patients with tricuspid atresia. Since then, the procedure and its indications have evolved, allowing for a Fontan circulation in many single-ventricle patients.


PATHOPHYSIOLOGY OF A SINGLE VENTRICLE

The patient with a single ventricle may present in a variety of ways depending on the presence or absence of obstruction to pulmonary or systemic flow. Severe obstruction to pulmonary flow results in cyanosis. Obstruction to systemic flow may lead to inadequate systemic perfusion and a low cardiac output state. The flow of blood through a patent ductus arteriosus bypasses the obstruction in either the pulmonary or systemic circulation, maintaining a clinically stable state. However, when the ductus starts to close, clinical deterioration becomes evident. In a small group of patients, there is no or minimal obstruction to systemic or pulmonary blood flow. Initially, these patients may demonstrate well-balanced pulmonary and systemic circulations. However, as the pulmonary vascular resistance diminishes over the first several weeks of life, pulmonary blood flow increases and congestive heart failure develops. If pulmonary venous obstruction is present, the patient may be cyanotic owing to increased pulmonary vascular resistance.

Management of the neonate with a single ventricle is directed at achieving adequate systemic oxygenation while preventing the development of pulmonary vascular disease. Surgical intervention may be required to achieve unimpeded outflow from the single ventricle into the systemic circulation. Adequate mixing of systemic and pulmonary venous blood must be ensured. These hemodynamic parameters allow the patient to become a candidate for a subsequent Fontan procedure.


Surgical Management

Infants younger than 3 months with inadequate pulmonary blood flow require a systemic-pulmonary artery shunt (see Chapter 18). Infants with excessive pulmonary blood flow with no obstruction to systemic outflow require early intervention aimed at reducing the volume load on the systemic ventricle and reducing pulmonary blood flow to prevent pulmonary vascular disease. In the past, pulmonary artery banding has been used to accomplish these goals (see Chapter 16). However, pulmonary artery banding may not limit pulmonary blood flow sufficiently or may result in distortion of the right or both pulmonary arteries. Therefore, many surgeons believe that division and oversewing of the proximal main pulmonary artery with construction of a systemic-pulmonary shunt is the best palliation in these cases. Patients who have both excessive pulmonary blood flow and obstruction to systemic outflow are best treated with a combined Damus-Kaye-Stansel procedure and a shunt (see Chapter 30).


Management after the Neonatal Period

The goal in these patients is to minimize both the pressure and volume load on the single ventricle as soon as possible. All patients should undergo routine cardiac catheterization at 4 to 6 months of age. If signs or symptoms of ventricular dysfunction, atrioventricular valve problems, or increased pulmonary vascular resistance are noted, the study should be performed earlier. These patients are prone to develop aortopulmonary collateral vessels. Therefore, during cardiac catheterization, a search for collateral vessels should be made, and, if present, they should be occluded with coils.

Any aortic arch or subaortic obstruction that has not been dealt with previously must be corrected before proceeding with any other surgical interventions. Subaortic obstruction may require a Damus-Kay-Stansel procedure (see Chapter 30) or enlargement of the bulboventricular foramen. Aortic arch obstruction or discrete coarctation may respond to balloon angioplasty or may require surgical intervention (see Chapters 15 and 29). Any situation that requires the single ventricle to pump blood to both
the systemic and pulmonary circulations puts a so-called volume load on that ventricle. All single-ventricle complexes initially have an extra volume load on the ventricle. This is true whether the pulmonary blood flow is provided through a systemic-pulmonary artery shunt, or through controlled forward flow from the single ventricle, as is seen with pulmonic stenosis or following a pulmonary artery banding procedure. A superior cavopulmonary connection removes some of the volume load from the ventricle because all pulmonary blood flow is directly from the superior vena cava. The ventricle provides forward flow into the systemic circulation only. This procedure can be performed successfully once the elevated pulmonary vascular resistance has fallen, usually after 3 months of age. Early removal of the volume load on the ventricle will potentially improve its long-term function.

By performing the Fontan connection in two stages, the operative risk for the completion Fontan operation has been reduced. When the volume load is acutely removed from the ventricle, the afterload of the single ventricle has been shown to increase. This afterload increasing effect is smaller when only the superior vena cava is connected to the pulmonary artery as compared to the Fontan procedure in which all systemic venous return is diverted into the pulmonary artery. The staged approach lessens the impact of afterload mismatch at each stage. The decrease in diastolic ventricular volume is also less with the superior cavopulmonary connection. By staging the Fontan, the sometimes fatal combination of ventricular hypertrophy and a sudden decrease in diastolic volume can be avoided.




BIDIRECTIONAL GLENN ON CARDIOPULMONARY BYPASS

In some patients, it is necessary or preferable to perform the bidirectional Glenn shunt on cardiopulmonary bypass. This is particularly true when patients require reconstruction of the pulmonary arteries, or in patients with bilateral superior venae cavae who require bilateral bidirectional Glenn shunts. In these cases, cannulation of the ascending aorta, very proximal superior vena cava, and right atrium is performed. Cardiopulmonary bypass is commenced, and previously placed systemic-pulmonary or ventricular-pulmonary shunts are closed. The previously described procedure for anastomosis of the superior vena cava to the right pulmonary artery can then be performed with the heart decompressed.


Distally Placed Shunt

If a previous systemic-pulmonary shunt has been positioned close to the takeoff of the right upper lobe branch of the pulmonary artery, the bidirectional Glenn must be carried out on cardiopulmonary bypass. The shunt is clipped proximally and divided. The pulmonary artery end of the shunt is removed, and the resultant opening in the pulmonary artery is enlarged and anastomosed to the superior vena cava. image

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Nov 14, 2018 | Posted by in CARDIAC SURGERY | Comments Off on The Fontan Principle
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