A schematic of the described patient's circulation at birth is presented in . A double outlet right ventricle (DORV) is a type of ventriculoarterial connection in which both great arteries originate entirely or predominantly from the right ventricle; a large nonrestrictive VSD serves as the only left ventricular outlet [3
]. Although in the majority of cases the aorta spirals posterior and obliquely to the pulmonary artery, in 30% it is found to course parallel and anterior to the pulmonary artery, resembling transposition of the great arteries. Therefore, the RV contributed to both the pulmonary and systemic circulation (making the patient's original pulmonary and systemic circuits functionally parallel). Extensive intracardiac mixing of oxygenated and deoxygenated blood at multiple levels (ASD, VSD, and partial anomalous pulmonary venous return) was essential for the patient's survival, but contributed to significant RV volume overload as well.
A schematic diagram of the blood flow in a patient with double-orifice right ventricle and partial anomalous pulmonary venous return prior to palliation. Right ventricle drives pulmonary and systemic circulations in parallel.
Partial anomalous pulmonary venous return describes the return of some pulmonary venous blood into the systemic venous (right) atrium site rather than the pulmonary venous (left) atrium. Typically, one or both right pulmonary veins fail to incorporate into the left atrium during embryogenesis and connect instead to the venae cavae or to the right atrium. In our patient, oxygenated blood from the right pulmonary veins mixes with the systemic blood return to the right atrium, and the left pulmonary veins return oxygenated blood via the left atrium into the left ventricle.
A schematic of the described patient's circulation after the palliation is presented in . The goal of surgical palliation was to decrease the demand on the RV by separating the pulmonary and systemic circuits and placing them in series rather than in parallel series. This arrangement-systemic venous return driving pulmonary artery circulation without a ventricular interposition-is the quintessential characteristic of Fontan circulation [4
A schematic diagram of the blood flow in the presented patient after Fontan palliation. Pulmonary circulation is determined by the systemic venous return and the pulmonary vascular resistance.
The classical Fontan operation, consisting of right atriopulmonary connections, resulted in nonlaminar flow hydrodynamics (with consequential loss of the potential energy necessary to drive pulmonary artery flow), right atrial dilation, clot formation, and arrhythmias. Therefore, total cavopulmonary connections, omitting the right atrium, are preferred [6
]. The goal of cavopulmonary connections is to maintain laminar blood flow as the patient grows. Our patient had first undergone a SVC-to-pulmonary artery connection (“bidirectional Glenn”), followed later with an IVC-to-pulmonary artery connection via a “lateral tunnel” (utilizing prosthetic baffle and a portion of the right atrial lateral wall). Alternatively, an extracardiac conduit between the IVC and pulmonary artery could be used as well.
As a result, the burden of the pulmonary circulation was removed from the RV. The absence of ventricular pump results in low velocity, nonpulsatile pulmonary blood flow, driven only by venous pressures, and critically dependent on low pulmonary vascular resistance [5
]. Pulmonary artery blood flow (and, therefore, cardiac output) variation is significantly related to the respiratory cycle [8
], with marked augmentation during the inspiratory phase (in a spontaneously breathing patients) and profound decreases during the Valsalva maneuver. Hepatic blood flow augmentation appears to be the most significant contributor to increased pulmonary flow associated with spontaneous breathing [7
]. Conversely, an inverse linear correlation was found between the mean airway pressures during positive pressure ventilation and the cardiac index, underlining a delicate balance between adequate mechanical ventilator support (aimed to prevent atelectasis formation, hypercarbia, and hypoxemia, all associated with increase in pulmonary flow resistance) and the cardiac performance of a patient with Fontan physiology [5
Functionally, Fontan physiology imposes several resistors in series for blood return to the aortic circulation (). In this patient, a large and hypertrophied RV is the main contributing force to the systemic circulation, while the output of the underloaded small LV reaches the aorta via the nonrestrictive VSD. Factors able to limit the systemic cardiac output include low preload, poor diastolic relaxation, usually associated with ventricular hypertrophy, and a high afterload. Sinus rhythm and low pulmonary vascular resistance are paramount to ventricular preload. The latter represents the main resistor to the systemic venous return to the ventricle. Part of the original Fontan's “ten commandments” for patient selection, pulmonary vascular resistance remains (along with the ventricular performance) the crucial factor affecting surgical outcomes [1
]. Poor diastolic relaxation may further limit the ventricular preload and may be a predictor for short-term outcomes in Fontan patients [9
] and serves as another resistor to the systemic flow. These same factors require aggressive attempts to maintain sinus rhythm, as tachyarrhythmias are particularly poorly tolerated in patients with Fontan circulation [4
]. Pressure increases upstream of venous resistors also account for complications such as decreased lymphatic drainage, protein-losing enteropathy, “plastic bronchitis,” pulmonary congestion, and pleural effusions [4
A schematic of resistors in series (R1, R2 and R3) imposed on the blood flow in Fontan palliation. Increase in pulmonary vascular resistance and deterioration of the ventricular compliance may result in decreased pulmonary and systemic flows.
An important consequence of chronically elevated central venous pressures, especially in the IVC basin, is the development of gradual hepatic congestion with attendant dysfunction and coagulopathy. Both pro and anticoagulant arms may be affected; reduced production of proteins C, S, and antithrombin III may predispose the sluggish venopulmonary blood flow to thrombus formation [4
]. Dehydration and infection may further increase the risk of a fatal pulmonary thromboembolism [4
]. Covert chronic pulmonary microembolism (in up to 18% of the patients) may lead to pulmonary vascular occlusive disease and may require chronic anticoagulation [4
]. Spontaneous contrast formation is readily diagnosed by TEE and may be indicative of an increased risk for thromboembolism [8
Therefore, the challenge of the intraoperative management of a patient with Fontan physiology is to maintain adequate perfusion pressure and cardiac output with minimal alterations in the pulmonary vascular resistance, cardiac rate and rhythm, and systemic venous blood return. Team approach and a thorough preoperative discussion with cardiologist and surgeon are paramount. Coagulopathy, iatrogenic, spontaneous, or mixed, frequently is a confounding factor. Risks and benefits of each anesthetic modality should be carefully weighed for each patient. Preservation of spontaneous ventilation and auxiliary effects of work of breathing on pulmonary blood flow, achieved with neuraxial, regional or local anesthetic, should be carefully balanced against the risks of coagulopathy and sudden changes in afterload and venous return. If general anesthesia is contemplated, untoward effects of positive pressure ventilation on systemic venous return and pulmonary hemodynamics should be carefully considered and minimized [12
TEE provides an invaluable perioperative diagnostic and monitoring guidance, far superior to 2D transthoracic examination [8
]. During the intraoperative echocardiographic examination, the midesophageal four-chamber view allows for the evaluation of the venopulmonary connections, the (bi) ventricular geometry, and performance. Low-velocity, laminar flow from the IVC towards the pulmonary artery, is examined for evidence of thrombi or obstruction. Because the pulmonary circulation is in series with the systemic circulation, such events would compromise the pulmonary circulation and may lead to decreases in ventricular preload, cardiac failure, and hypotension. The atria are examined for thrombus formation and function: sinus rhythm is especially important for the chronically underloaded ventricles.
Forward rotation of the transducer visualizes the ventricular outflow tracts and allows for further evaluation of the geometry and function of both ventricles. Ventricular hypertrophy and function, as well as the size and location of the VSD can all be visualized in this view. The absence of the fibrous continuity between the semilunar and atrioventricular valves (aortic and tricuspid in our patient) is characteristic of the RV origin of the aorta. Examination of the ventricles' geometry, performance, and outflow is facilitated by the deep transgastric long-axis view.
In conclusion, a perioperative TEE examination of an adult patient with Fontan physiology should provoke a “train of thought”, linking the images to the perioperative physiologic considerations. A thorough familiarity with the preceding corrective and palliative surgeries for the primary pathology is crucial for the correct echocardiographic interpretation.