The circulatory system and the spontaneity of the contractile tissue, especially the myocytes continue to intrigue us. The evolution of the four-chambered heart and circulatory pathway had to go through many developmental steps before maturing into the present complex organ system that can regulate pressure according to need and breaks up into a capillary bed before supplying a vital organ to diffuse the perfusion pressure and create a barrier for harmful substances entering the organ.
The model is simple and unidirectional with the contractile heart acting as the pump generating the necessary drive. The complexities of capillary formation and physiologic barriers to injurious elements were a later development.
The mammalian circulation consists of arteries, arterioles, veins, venules, and interposed capillary systems. The following is a comparative picture of the evolution of circulation in different animals.
Arthropods/ Circulation in Reptilian Circulation Circulation in higher Molluscs creatures having mammals
gills
So, the lung is the organ interposed in the circulatory passage providing the necessary oxygen to the tissue. Thus, the part of circulation involved in the supply of oxygenated blood to vital organs is systemic circulation. At the same time, the deoxygenated venous blood returning to the heart is pulmonary circulation. The interposed heart has four chambers -- the thin-walled atrial chambers are for receiving blood and the thick muscular ventricles pump blood into the respective systemic and pulmonary systems consistent with the afterload of each circulatory pathway.
From the beginning, it was noticed that several patients with complex congenital conditions lived an uncomfortable and distressed post-natal life with a short lifespan. These are conditions with functionally univentricular hearts having defective inlet or outlet issues, atretic right ventricular inlet or outlet valves, and an altered pulmonary flow. An obligatory right-to-left shunt is needed for survival and the pulmonary flow is either less and the patient is cyanotic or high leading to a state of pulmonary hy[ertension and its consequences. The hypoplasia of the left heart is a situation unto itself and requires staged procedures for meaningful survival.
The normal human mammalian circulatory path is essentially in series where the systemic and pulmonary systems are completely segregated. Any abnormal communication between the chambers of the two systems leads to the formation of a parallel pathway. These parallel passages are responsible for the untoward turbulent flow offshoot adversely affecting the pulmonary flow. whether the patient is going to be cyanotic or have a high pulmonary vascular flow that depends upon --
the size of the shunt, and
whether or not the tricuspid or the pulmonary valve is atretic.
Since 1940 animal experiments have shown that a viable circulation is possible without the interposing RA and RV. In 1958, William Glenn reported in the New England Journal of Medicine (NEJM) a successful superior vena cava to right pulmonary artery anastomosis (Glenn procedure) for tricuspid atresia. The classical Glenn procedure consisted of an end-to-end or end-to-side anastomosis between the SVC and RPA with suture closure of the proximal RPA resulting in the offloading of upper-extremity (33%) blood from the heart.
The evolution of a bidirectional Glenn was quite natural as it allowed perfusion of both lungs simultaneously. The hemi-Fontan procedure refines the BDG. It is a complex surgical process that allows a better maturation of the pulmonary vasculature and unloads the supporting ventricle to a large extent. Both are stage II procedures before a Fontan should be completed. Case selection, anatomy at exposure, and the surgeons' preferences are the major factors determining the type of surgery to perform. However, there are distinct differences between the two procedures, BDG consists of the simple creation of a superior SVG-to-RPA anastomotic shunt while hemifontan is a complex process where the cavopulmonary anastomosis is done anteroinferiorly with a baffle that regulates the pulmonary flow to both lungs. Norwood and Jacobs first described the HF procedure in 1989 as stage-II palliation before the final Fontan correction for the hypoplastic left heart syndrome. Later it was seen that this procedure was adequate for other univentricular hearts where creating a long-lasting BT shunt is not necessary.
Fontan, in 1968, suggested a direct cavopulmonary connection without the intervening RA and RV chambers and the right-sided atrioventricular or ventriculoarterial valves. Initially, the operation was designed for cyanotic patients with tricuspid atresia. Later, mitral atresia patients were added. In 1971, Fontan and Baudet presented their series of 3 cases in patients affected with tricuspid atresia. With experience, the circulatory pathway described by Fontan was the best alternative for complex congenital heart defects (with univentricular physiology) where an effective two-ventricle repair was not possible.
Several factors have influenced the evolution from staged to primary repair and they may be summarised as follows:-
Advancements in Surgical Techniques:
Better Postoperative Care:
Understanding of Long-term Outcomes.
Essentially the Fontan correction is the final stage of a series of operations done to give the patient a lifestyle that compares to more than 80% of the normal for that age group. The overall results in a congenital care unit are --
Freedom from arrhythmia: 85.1% of patients were free from arrhythmia 10 years after the procedure.
Freedom from thromboembolism: 92.9% of patients were free from thromboembolism 10 years after the procedure.
New York Heart Association (NYHA) class: 53% NYHA class I at recent follow-up.
events: 41% of patients were free of serious events at the age of 40 years.
The modern trend favours staging the Fontan procedure for optimum results as the pulmonary circuit slowly matures and develops in an environment of high venous pressure before the final stage of operation. It must be mentioned that the BDG predates a Fontan cavopulmonary connection by several years and was the sole method for unloading the ventricle and augmenting the pulmonary circulation at the same time. The hemi-Fontan procedure was formally introduced in 1994 when better maturation of the pulmonary tree was observed. The hemi-Fontan procedure is also favoured when the pulmonary pressure is high and there is some degree of ventricular hypertrophy.
HEMI-FONTAN
The hemi-Fontan surgery is complex, but it effectively avoids the direct onslaught of systemic venous pressure on the immature pulmonary arteries, as a baffle excludes the intervening right and right ventricular chambers. This procedure is more acceptable in the pediatric units of the advanced centres, and the final total cavopulmonary connection (as suggested by Fontan) is easier and requires fewer extraneous resources.
A BDG, however, is technically easy as a primary preparatory procedure, but the only final option remains an extra-cardiac inferior vena cava (IVC) to pulmonary artery connection. The establishment of BDG as a pre-Fontan preparatory surgical stage was not easy. The procedure unloaded the functioning ventricle and made the pulmonary vascular tree tolerable to a higher pressure. As mentioned before, the BDG was discovered quite some time before a total cavopulmonary connection, now better known as the Fontan, was initially used to treat cyanotic children with tricuspid atresia. Rodbard and Wagner first described right ventricular bypass in 1949. The remarkable pioneering contribution of many other surgeons, namely, Carlo Carlon, Francis Robicsek, Nikolai Galankin, Tigran Darbinian, Harris Shumacker, and Evgeny Meshalkin should be remembered. Most were dog experiments, but it must be noted that Calron first propounded the idea of cavopulmonary anastomosis in 1950 and Shumaker was the first to attempt this in 1954 clinically. Ultimately, the Yale team [ed by William Glenn in 1958 tasted success with an anastomosis of the superior termination of SVC and distal right pulmonary artery with closure of the proximal part. There was a gradual shift in attitude and a bi-directional SVC to the pulmonary artery is becoming a common choice as pulmonary vascular preparation before the final stage in Fontan correction for univentricular hearts. Later, it was also found that although pulmonary arterio-venous malformations are higher in a BDG patient, particularly after the Kawashima procedure, such malformations usually occur in patients with alternate pulmonary vascular supply from the liver. A debate is about the choice of the preliminary staging operation, a simpler BDG, or the surgically complex hemi-Fontan is on. Selection depends upon many factors for acceptable results that may be summarised as follows:--
Case selection,
Prior training and the thought process of the surgeon,
Ease and intra-atrial conversion from hemi-Fontan,
Extra-cardiac lateral tunnel cavopulmonary connection with a conduit only option left for the completion stage in BDG.
Less extraneous material is required for a final stage cavopulmonary connection if hemi-Fontan was done as the first stage. On the other hand, a total cavopulmonary conversion of a BDG requires more materials for the creation final stage.
Good bidirectional blood flow distribution in BDG, while a lateral extracardiac tunnel with a synthetic tube of appropriate length and diameter. This makes this procedure require more resources during the pulmonary vascular development and maturation is better with hemi-Fontan.
Choussat's "Ten Commandments" are criteria for selecting patients for the Fontan procedure. These guidelines were first published in 1977 by Choussat et al. They have been modified over time by various centres. These are:--
The patient is at least 4 years old,
A normal heart rhythm,
A normal right atrial volume,
Ventricular function adequate
Normal systemic venous return,
No atrioventricular valve leak,
Adequate pulmonary artery size,
No distortion of pulmonary arteries from prior surgery,
Pulmonary artery pressure (PAP) less than 15 mm Hg,
Pulmonary vascular resistance index (PVRI) less than 4 Wood units/m.
There is no doubt that if all the commandments could be followed the outcome would have been better, but in reality, the clinician has to proceed without many of these. Fontan and Baudet published a series of 3 cases in Thorax in 1971. Guillermo Kreutzer from Buenos Aires, Argentina any knowledge of Fontan's experience, performed a similar procedure in July 1971 without placing a valve in the Inferior Vena Cava inlet and introduced the concept of "fenestration" – leaving a small atrial septal defect to serve as a pop-off valve for the circulation. Fontan's publication appeared in January while Kreutrer presented his work on a 3-year-old dying cyanotic child before the AATS in July -- practically they were contemporaries independently working, on different continents, on the same problem -- hence the surgery is also known as the Fontan-Kreutzer procedure. The idea to stage the procedure was a later proposition based on finding better results if sufficient time is given for the maturation and development of the pulmonary vasculature. The first Fontan procedure was performed in 1968 and consisted of creating anastomoses between the superior vena cava and the right pulmonary artery and between the inferior vena cava and the left pulmonary artery employing valvular homografts. Subsequent modifications included:--
Operating at a lower age (at present, the Glenn/hemiFontan anastomosis is performed at the age of approximately 4–6 months, and the Fontan procedure is performed when the patient is approximately 2–3 years old),
Direct atrio-pulmonary and cavo-pulmonary connections,
Including other complex congenital heart conditions where a biventricular repair cannot be done,
Multi-staging,
Popularisation of the extra-cardiac lateral tunnel -- Bjork modification -- total cavopulmonary anastomoses.
Performance of hemi-Fontan and subsequent intra-atrial cavo-pulmonary connections by trained surgeons and at centres experienced in the peri-operative care of such patients.
The Fontan circulatory path is unnatural, and the direct transmission of the high systemic venous pressure has certain specific complications that require early handling by an expert caregiver. The constant high venous pressure in the pulmonary vessels and sluggish blood flow make the patient more amenable to clot formation and obstruction. Gewling has hypothesized the creation of a neo-portal system that conspicuously lacks the rhythmic pump drive of a muscular ventricle. The postcapillary energy of systemic circulation drives the flow through pulmonary capillaries resulting in a pulmonary vascular pressure always higher than the normal wedge pressure. Thus exercise-dependent variation in cardiac output is problematic and may herald early systolic/diastolic dysfunction and heart failure. The oxygen saturation hovers around the '90s and is suitable for leading a sedentary life, i.e, 90% of normal. The "failing Fontan" patient may have intractable:--
systolic and/or diastolic dysfunction of the single ventricle,
cardiac dysrhythmias,
conduction disturbances,
thromboembolic complications,
edemas and effusions into body cavities,
chylothorax,
liver fibrosis and cirrhosis,
arteriovenous malformations in the lung.
kidney failure,
protein-losing enteropathy,
plastic bronchitis.
Over the years, observation of post-TCPC patients with an alternative circulatory pathway reveals some interesting facts that require being told to the patient and party. Also, these are interesting facts for psychiatric and genetic counselling if such a patient becomes pregnant. Summarised these traits are:--
Exercise tolerance is limited,
Low NYHA functional class,
IQ is slightly less compared to the average population,
Higher incidence of mental disorders like anxiety, depression, and ADHD,
Though hematocrit is always high, chances of malnutrition, a lower than normal muscle mass, and vitamin-D deficiency are a stark possibility requiring iron, vitamins and other supplements.
The risk of giving birth to a child with a serious heart defect is approx. 6:1000 -- I. abnormal karyotype ii. microdeletion in chromosomes, iii. environmental factors are the implied factors.
The Fontan-Kreutzer procedure is a pioneering and unique surgical correction that creates a somewhat alternative circulation for complex congenital cardiac conditions that failed to thrive. What started as a remedy for tricuspid atresia could now be effectively applied to managing scores of children born with univentricular physiology. This contribution was immeasurable and congenital heart defects were armed with a new armamentarium of surgical correction that allowed new near-normal lives for millions. Admitted that there were some restrictions and complications specific to this type of circulation can be a botheration, and the need for future counselling becomes necessary, the Fontan pathway is to be lauded.
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