Fig. 8.1
Schematic representation of the normal cardiovascular circulation (a), shunted palliation (b), and Fontan circulation (c). (a) Normal circulation: the pulmonary circulation (P) is connected in series with the systemic circulation (S). The right ventricle maintains the right atrial pressure lower than the left atrial pressure, and provides enough energy for the blood to pass through the pulmonary resistance. (b) The systemic (S) and pulmonary (P) circuits are connected in parallel, with a considerable volume overload to the single ventricle (V). There is complete admixture of systemic and pulmonary venous blood, causing arterial oxygen desaturation. (c) Fontan circuit: the systemic veins are connected to the pulmonary artery (PA), without a subpulmonary ventricle or systemic atrium: the lungs are thereby converted into a neo-portal system which limits flow to the ventricle. In the absence of a fenestration, there is no admixture of systemic and pulmonary venous blood, but the systemic venous pressures are markedly elevated. A fenestration (F) allows the systemic venous blood to bypass the Fontan portal system and limits the damming effect, thereby increasing output and decreasing congestion, but also arterial saturation. Ao aorta, CV caval veins, F fenestration, LA left atrium, LV left ventricle, PA Pulmonary artery, RV right ventricle, V single ventricle. Line thickness reflects output, color reflects oxygen saturation
Many complex cardiac malformations are characterized by the existence of only one functional ventricle (Fig. 8.2). This “single” ventricle has to maintain both the systemic and the pulmonary circulations, which during fetal life and at birth are not connected in series but remain in parallel (Fig. 8.1b). Such a circuit has two major disadvantages: diminished oxygen saturation of the systemic arterial blood and a chronic volume load of the single ventricle. The chronic ventricular volume load leads to a progressive ventricular dysfunction and remodeled pulmonary vasculature, causing a gradual attrition due to congestive heart failure and pulmonary hypertension from the third decade of life, with few survivors beyond the fourth decade.
Fig. 8.2
Different examples of functional single ventricles: (a) tricuspid valve atresia with hypoplastic right heart, ductal flow to pulmonary artery; (b) double inlet right ventricle DIRV through an unbalanced atrioventricular septal defect with common atrioventricular valve, double outlet right ventricle DORV and hypoplasia of the left ventricle; (c) double inlet left ventricle DILV with left-sided hypoplastic right ventricle, restrictive ventricular septal defect VSD acting as subaortic stenosis, transposition of the great arteries TGA, small aortic arch and coarctation of aorta; (d) hypoplastic left heart syndrome HLHS due to aortic valve atresia
In 1971 Francis Fontan [1] from Bordeaux, France, reported a new approach to the operative treatment of these malformations, separating the systemic and pulmonary circulations. In a “Fontan circulation” the systemic venous return is connected to the pulmonary arteries without the interposition of a pumping chamber (Fig. 8.1c). In this construct, residual post-capillary transit energy is used to push blood through the lungs in a new portal circulation-like system [2]. Advantages of a Fontan circuit include (near-) normalization of the arterial oxygen saturation and abolishment of the chronic volume load on the single ventricle. However, because the pulmonary impedance hinders venous return through the pulmonary vasculature, this circulation creates a state of chronic “hypertension” and congestion of the systemic veins, and results in a decreased cardiac output, both at rest and during exercise [3, 4] (Fig. 8.3). These two features of the Fontan circulation, elevated systemic venous pressure and chronically low cardiac output, are the root cause of the majority of the physiologic impairments of this circulation.
Fig. 8.3
Exercise and output: normal versus Fontan circulation: A normal subject with a biventricular circuit can increase his output by a factor of 5 (black line). In Fontan patients, output is significantly impaired both at rest and during exercise; at best (green line) the output is mildly decreased at rest, with moderate capacity to increase flow during moderate exercise. At worst (red line), the output is severely reduced at rest and barely augments during minimal exercise
Is Every Fontan Circuit the Same?
Since its original description, the Fontan circuit has undergone numerous modifications. Early on surgeons used valves (cavo-atrial, atrioventricular, or atrio-pulmonary) and created various connections between the right atrium and the pulmonary artery (anterior atrio-pulmonary connection, with or without inclusion of a small hypoplastic right ventricle, posterior atrio-pulmonary connection), with different materials (valved conduits, homografts, patches, direct anastomosis). The very high incidence of late reoperations, reaching 40 % in some series, does reflect the poor design of the first Fontan circuits and the less than ideal surgical techniques used in the early series. Most of the older circuits are no longer created and considered obsolete; however, many patients still survive with such circuits. When assessing a patient with a “Fontan circuit,” the clinician needs to know exactly which connection has been made and what material has been used.
During the last decade, the total cavo–pulmonary connection (TCPC) has emerged as being superior [5]. The caval veins are connected to the pulmonary artery, bypassing not only the right ventricle but also the right atrium (Fig. 8.4a–c). The superior caval vein is connected to the pulmonary artery (bidirectional Glenn shunt or partial cavo-pulmonary connection [PCPC]). There are two variants to connect the inferior caval vein: the lateral tunnel and the extra cardiac conduit. Introduced in the mid-1980s, the lateral tunnel provides a tubular path between the inferior caval vein and the pulmonary artery, consisting of a prosthetic baffle and a portion of the lateral atrial wall. This circuit has growth potential and can therefore be created in children as young as 1 year; it leaves a minimal amount of atrial tissue exposed to high pressure, which over time may cause atrial arrhythmias. The extra cardiac conduit was introduced in 1990, and consists of a tube graft between the transected inferior caval vein and the pulmonary artery. This circuit leaves the entire atrium at a low pressure, has no or minimal atrial suture lines, and can be performed without aortic cross clamping or even cardiopulmonary bypass; however, this conduit has no growth potential and therefore will be offered to patients large enough to accept a graft adequate for an adult’s inferior caval vein flow.
Fig. 8.4
Schematic representation of treatment strategy from birth to Fontan circulation. (a) Patient of with tricuspid valve atresia and hypoplastic right heart (cf. Fig. 8.2a): first palliation consists of stenting the arterial duct, followed by partial cavo-pulmonary connection (PCPC); at the age of about 3 years the Fontan circulation or total cavo-pulmonary connection (TCPC) is completed by connecting the inferior caval vein through an extracardiac conduit to the pulmonary artery; a small fenestration is created between the conduit and the right atrium. (b) Patient of Fig. 8.2b (DIRV DORV AVSD): initial palliation consists of banding of the pulmonary artery at the age of 4–6 weeks; Fig. 8.4 (continued) PCPC at 6 months; TCPC at 3 years. (c) Patient of Fig. 8.2c (DILV TGA coarctation): initial palliation consists of neonatal banding of the pulmonary artery with arch reconstruction; PCPC with Damus-Kaye-Stansel operation at 6 months; TCPC at 3 years. (d) Patient of Fig. 8.2d (HLHS): initial palliation consists of either Norwood operation (complex arch reconstruction, atrial septectomy, Sano shunt from RV to pulmonary artery) or hybrid procedure (stent in duct, bilateral banding branch pulmonary arteries); PCPC ± Norwood arch repair at 6 months; TCPC at 3 years
How to Build a Fontan Circuit?
At birth, it is impossible to create a Fontan circulation. The pulmonary vascular resistance (PVR) is still elevated for several weeks, and the vessels—caval veins and pulmonary arteries—are usually too small, making any cavo-pulmonary shunt impossible during that period. Even when resistance has fallen, a staged approach is preferred connecting the superior and inferior caval veins at separate occasions. Such a staged approach allows the body to adapt progressively to the different hemodynamic conditions, and reduces the overall operative morbidity and mortality. A staged approach also allows a better patient selection and intermediate preparatory interventions.
Initially in the neonatal period, management must focus on—if not provided to some degree by nature-unrestricted flow from the heart to the aorta (if required: coarctectomy, Damus-Kaye-Stansel, Norwood repair), a well-balanced limited flow to the lungs (if required: pulmonary artery banding, shunt (modified Blalock-Taussig, central), stent in duct), and unrestricted return of blood to the ventricle (if required: Rashkind balloon septostomy, atrial septectomy) (Fig. 8.4a–c). The infant is then allowed to grow for several months. During this time, the heart is submitted to a chronic volume overload which is beneficial for development of the pulmonary vasculature, but if excessive detrimental for ventricular function (see below). The infant will have mild oxygen desaturation which is inversely related to mild cardiac failure (Fig. 8.1b).
At the age of several months (4–12 months old), most centers will introduce a partial cavopulmonary connection (PCPC) or bidirectional Glenn shunt: the superior caval vein is connected to the pulmonary artery (bilateral if present). If no other blood flow is directed to the lungs, the volume load for the heart is significantly decreased to half or even less than normal for the body surface area (BSA). The patient at this stage will remain slightly cyanotic, as the desaturated blood from the inferior caval vein is still allowed to flow to the aorta.
At the age of several years (1–5 years, depending on the preference of the center, growth of vascular structures, and cyanosis at rest and during exercise), the Fontan circuit is completed by connecting the inferior caval vein to the pulmonary artery. As mentioned above, two techniques are currently used: the lateral tunnel and the extra cardiac conduit. Frequently a small fenestration is created between the tunnel-conduit and the pulmonary atrium, either routinely or only in “high risk” patients [6]. Such fenestration will allow a residual right-to-left shunt, thereby limiting caval pressure and congestion, and increasing the preload of the systemic ventricle and cardiac output, at the expense of cyanosis. Such fenestration has been shown to reduce the operative mortality and morbidity associated with pleural drainage; the fenestration can later be closed percutaneously (weeks-months) after adaptation of the body to the new hemodynamic condition.
Cardiac Output in the Setting of a Fontan Circulation
By creating a TCPC, a new portal system is made. A portal system occurs when one capillary bed pools blood into another capillary bed through veins without passing through the heart, as for example in the hepatic portal system and the pituitary portal system. The Fontan neo-portal system dams off and pools the systemic venous blood. As a result, transit of blood through this neo-portal system is dependent on the pressure gradient from the systemic post-capillary vessels to the pulmonary post-capillary vessels (Fig. 8.1c). Since there is no pump to transmit energy to the system, small changes in the static resistances and dynamic impedances of the structures within this portal system have a profound impact on the blood flow.
Although the heart itself may function well, the inherent limitations of the Fontan neo-portal system determine the degree of circulatory compromise. It is this neo-portal system that is the major limiting factor of flow and the underlying cause of venous congestion and diminished cardiac output. The heart, while still the engine of the circuit, cannot compensate for this major flow restriction: the suction required to compensate for the damming effect of the Fontan portal system cannot be generated [7]. The heart therefore no longer controls cardiac output nor can it alter the degree of systemic venous congestion. However, in cases in which the systemic ventricle functions poorly, the heart can make an already compromised circulation worse. Figure 8.5a, b illustrates the relationship between output, ventricular contractility, and PVR in a normal and a Fontan circulation. In a normal subject, output at rest is minimally influenced by ventricular function, except when severely depressed; mild changes of the PVR will not influence the output as these changes are neutralized by the right ventricle. In Fontan patients, the PVR is the primary modulator of cardiac output: small changes have a profound impact; systolic performance will only impact output at rest when cardiac function is severely depressed. If the ventricular function is not severely depressed, squeezing harder will not result in more output.
Fig. 8.5
Relationship of output at rest, ventricular function and PVR. (a) Modulation by PVR: in a normal subject (black line), output at rest is minimally influenced by ventricular function, except when severely depressed. In Fontan patients (colored lines), PVR is the primary modulator of cardiac output. As in a two-ventricle system, systolic performance will only impact output at rest when cardiac function is severely depressed. If ventricular function is not severely depressed, squeezing harder will not result in more output. (b) Modulation by ventricular function. In a normal subject (black line), cardiac output is not influenced by a mild increase of PVR up to 5 Woods Units. In all Fontan patients (colored lines), an increase in PVR is invariably associated with a decrease in cardiac output. If PVR is low, a reasonable output is achieved in patients with normal or moderately depressed ventricular function (green and yellow lines). However, severely depressed ventricular function invariably results in low output (red). EF ejection fraction, F Fontan, LV normal left ventricle, PVR pulmonary vascular resistance, UVH univentricular heart
The components that make up the Fontan neo-portal system are thus critically important in the overall function of the Fontan circuit. These include the veno-arterial Fontan connection itself (atrio-pulmonary in older patients), pulmonary arteries, pulmonary capillary network (including precapillary sphincters), pulmonary veins, and the veno-atrial connection. Impairment at any level of this portal system will have profound consequences on the output of the Fontan circuit, much more than a comparable dysfunction in a two-ventricle circulation. These impairments include, but are not limited to: stenosis, hypoplasia, distortion, vasoconstriction, pulmonary vascular disease, loss or exclusion of large vessels or microvessels, turbulence and flow collision, flow mismatch and obstruction by external compression.
The restriction of the cardiac output imposed by the neo-portal system can be partially reversed by bypassing the pulmonary vasculature. A Fontan fenestration allows flow to bypass the Fontan neo-portal system and results in an increase in cardiac output and a decrease in venous congestion. However, while a fenestration can increase the overall output, it does so at the expense of diminished arterial oxygen saturation. Nevertheless, in the setting of a fenestration, the increase in cardiac output can result in an increase in peripheral oxygen delivery even if the saturation is mildly diminished. Figure 8.6 shows the relationship between output, congestion, and arterial saturation in a successful and a failing Fontan circuit, and the effect of partial improvement by a fenestration. In a successful Fontan circuit, the low resistance portal system will cause a mild decrease of output with a modest increase in the systemic venous pressures, making a fenestration unnecessary. In a failing Fontan circuit, inclusion of a high vascular resistance portal system will decrease the cardiac output and create venous congestion of an unacceptable degree; a fenestration will attenuate these changes, but in patients with an increased PVR an acceptable compromise may not be possible.
Fig. 8.6
Effect of various degrees of pulmonary bypassing in a Fontan circuit on systemic output, saturation, and systemic venous congestion. A “good Fontan” with low neo-portal resistance (green lines) has an output (thick solid line) of about 80 % of normal for BSA, with a high saturation (dotted line) and a mildly elevated CVP (thin line). The “bad Fontan” with a high portal resistance has an output with a similar saturation but with a very low to unacceptable output despite a high CVP. Partial bypassing of the Fontan portal system by a fenestration invariably increases systemic output and decreases systemic congestion, but in the “bad Fontan,” this occurs at an unacceptable degree of cyanosis. CVP central venous pressure
Functional Impairment After the Fontan Operation
The restriction of the cardiac output and the inability to power the blood through the pulmonary vasculature results in a circulation in which the ability to perform exercise is reduced. Under resting conditions, the cardiac output in a patient with a Fontan circulation is approximately 70–80 % of normal. During exercise, the limitations of the Fontan circuit are substantially magnified such that the small difference in cardiac output at rest becomes a large difference during activity (Fig. 8.2). At peak exercise, a well-trained athlete with a normal heart can increase blood flow through the lungs by up to fivefold (see also Chap. 15). This is accomplished through a substantial increase in the right ventricular systolic pressure (up to 70 mmHg! [5]) as well as with pulmonary blood flow acceleration coupled with a decrease in the PVR. In a patient with Fontan physiology, there is no physiologic mechanism to allow for a similar increase in cardiac output. The maximal mean venous pressure rarely reaches 30 mmHg; there is no blood acceleration and the pulmonary vascular reactivity and the ability to recruit reserve vessels is attenuated or absent [8]. Together, these limitations result in a diminished ability to augment cardiac output in response to increased metabolic demand, and therefore limit the ability of a patient with a Fontan circulation to perform exercise.
Through childhood and until puberty, the mean maximal exercise capacity for patients with a Fontan circulation is in the range of 65 % predicted for gender and age [9]. While successful Fontan patients may remain stable for many years, poor Fontan patients suffer an accelerated increase of PVR, and increasing filling pressures of the ventricle as a result of chronic preload deprivation and disuse dysfunction. Longitudinal studies of late adolescents and young adults demonstrate this point well; as patients progress to late adolescence and early adulthood, exercise capacity tends to continue to decline by about 2.6 % per year [10–12].
There are several reasons why adults with a Fontan circuit in the current era do not represent the current cohort of patients. Many of the original candidates for a Fontan operation, the now adult cohort, were suboptimal for this type of surgery from a hemodynamic standpoint, with many significant residual lesions and sequelae related to the original cardiac malformation and palliative procedures. A shunt procedure performed during the period from the 1960s to the 1980s was evaluated based on the goal of the long-term relief of cyanosis: “the pinker the better.” Often a second aortapulmonary shunt was created to augment pulmonary blood flow after the first shunt was deemed inadequate. The potential that these shunts could induce pulmonary vascular disease, ventricular hypertrophy and dysfunction, or pulmonary artery distortion was not—as it is now—a principal concern of the surgeon. Currently, the success of a shunt is evaluated by obtaining acceptable relief of cyanosis without significant volume overload of the ventricle and by the induction of adequate pulmonary growth without causing changes of the PVR. In addition, in the modern era of palliation, the systemic to pulmonary shunt is designed to last 4–6 months, enough time for the PVR to drop such that a PCPC can safely be created. Figure 8.7a–c illustrates the different loading conditions of the single ventricle at the various stages of palliation, highlighting the differences in management before the 1990s (typically two aortapulmonary shunts prior to the full Fontan circulation) and after (typically one shunt, then partial and later complete Fontan circulation). However, with the current staged strategy for functionally single ventricle, only a limited period of controlled pulmonary “overflow” is allowed to stimulate pulmonary arterial growth. The only time of significant pulmonary overflow is immediately after birth when a shunt or band is placed. In many situations the systemic to pulmonary shunt is made as small as possible to avoid volume overload of the ventricle and potential cardiac damage. If flow to the lungs is too low for too short a period, it may lead to inadequate development of the pulmonary vascular bed, high PVR and, eventually a poorly functioning Fontan circulation (Fig. 8.8). Similarly, hybrid procedures involving neonatal banding of branch pulmonary arteries may cause distortion and inadequate growth or even loss of the distal pulmonary arteries.
Fig. 8.7
Cardiac output versus time in the normal left ventricle and the univentricular heart (UVH) managed before and after the 1990s: the same story but expressed with different reference frame: in absolute value (a), related to BSA (b), and to ventricular size (c). (a) Cardiac output expressed in absolute value: The black line shows output of a normal ventricle which increases proportional to growth. At birth the volume load to the UVH is about 250–300 % of that of a normal left ventricle. Prior to the 1990s, a neonatal and infant large shunt was created with significant increase in output (red line); the shunts were abolished at the time of the Fontan operation, and the Fontan portal dam reduced even further preload. After the 1990s, a small neonatal shunt is created for a short time, and the ventricle is progressively unloaded both at the Glenn and the Fontan operation (green line). (b) Cardiac output related to BSA. Black line: output of normal remains at 100 % for BSA. This representation assumes only dilation and stretch without any overgrowth of the ventricle. Fig. 8.7 (continued) The patient with a UVH is born with a large ventricle (volume load of 250 % of normal for BSA). Prior to the 1990s (red line), the preload to the ventricle is augmented shortly after birth by a shunt procedure to ±350 % of normal for BSA. The patient slowly outgrows his shunt, thereby gradually reducing the volume overload. A second shunt was created, augmenting the volume overload again. As this patient again outgrows his shunt, a Fontan circuit is made, reducing the volume load to <80 %. After the 1990s (green line), a small neonatal shunt was created for a short time; the patient slowly outgrows his shunt; the ventricle is progressively unloaded both at the Glenn and Fontan operation (green line). (c) Cardiac output related to ventricular size in univentricular heart (UVH) managed before and after the 1990s. This representation assumes adapted overgrowth of the ventricle in every stage in function of chronic preload, A: output of normal remains at 100 % for ventricular size. The patient with a UVH is born with an appropriate ventricle for volume load (100 % of normal for ventricular size). Prior to the 1990s (red line), the preload to the ventricle was augmented shortly after birth by a shunt procedure to ±150 %. The patient slowly outgrows his shunt, and adapts his ventricle, thereby gradually reducing the volume overload to ±100 % for its size. A second shunt was created, augmenting the volume overload again to 150 %. As this patient again outgrows his shunt, a Fontan circuit is made, reducing the volume load to 25 % of its “due” preload. After the 1990s (green line), a small neonatal shunt was created for a short time; the patient slowly outgrows his shunt; the ventricle is progressively unloaded both at the Glenn and Fontan operation in much milder steps avoiding acute unloading and severe deprivation (green line)
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