Evaluation of a Venous Unifocalization of the Bilateral Superior Vena Cava and Comparison with the Bilateral Bidirectional Glenn Procedure

Objective: A bilateral-bidirectional Glenn procedure is generally performed in patients with a functional single ventricle and a bilateral superior vena cava. In bilateral superior vena cava, unbalanced blood flow due to its unique anatomy and reduced volume due to its small aperture can cause blood stasis, unbalanced pulmonary blood flow, and thrombosis formation. Unifocalization of bilateral superior vena cava, a new surgical technique which makes pulmonary blood flow more equally distributed, was performed and evaluated. Methods: We retrospectively reviewed the clinical and surgical records of 65 patients who underwent Glenn procedure at the Juntendo University Hospital, Tokyo, from January 1997 to March 2014. Sixteen patients had bilateral superior vena cava anatomy. Unifocalization of superior vena cava was performed in 8 cases and conventional surgery in 8. Perioperative data were evaluated to compare outcomes and clinical courses between the two groups. Results: There were no significant differences between the 2 groups in age (group 1: 1.0 ± 0.5, group 2: 1.2 ± 1.1, years of age), body weight (group 1: 7.7±2.3, group 2: 6.5±4.3, kg), change in central venous pressure before and after the operation (group 1: 6.5 ± 3.1, group 2: 9.9 ± 6.2, mmHg), postoperative oxygen saturation (group 1: 82 ± 3.3, group 2: 83 ± 9.3, %), duration of surgery (group 1: 371±120, group 2: 439±168, min), or cardiopulmonary bypass time (group 1: 143±38, group 2: 131 ± 53, min). Unilateral blood flow, which is purportedly better than bilateral bidirectional Glenn procedure, was achieved without any disadvantages that are reported of the conventional procedure. Conclusion: There was no distinct advantage or disadvantage to using the new method.


Introduction
In the early 1970s, Fontan and Kreutzer independently developed strategies for palliation in patients with tricuspid atresia that involved baffling the caval veins directly to the pulmonary artery 1) 2) .Usually, patients with a functional single ventricle are treated with a series of staged surgical procedures culminating in a Fontan operation.Bidirectional Glenn procedure is used as an intermediate stage, which has been a good treatment option for patients with a single ventricle resulting in lower mortality than the single-stage Fontan procedure 3)-6) .In Glenn stage patients with a bilateral superior vena cava (SVC), the bilateral bidirectional Glenn procedure has generally been performed 7) .
When the Fontan operation is performed in patients who have previously had a bilateral bidirectional Glenn procedure, the blood flow from bilateral SVCs face the flow from the inferior vena cava.Because blood flow is divided between two SVCs, there is an imbalance in volume such that blood flow to the lungs is not equally distributed, and the risk of thrombosis formation increases 8) .
To resolve this issue, we introduced a new surgical method, superior vena cava unifocalization of bilateral superior vena cava, in a previously published report.In this method, the smaller SVC is anastomosed to the lateral side of the larger SVC, and the unifocalized SVC is anastomosed to the pulmonary artery 9) .In the current study, we hypothesized that this new surgical method,"SVC unifocalization" , would be more effective than the Glenn stage procedure in patients with bilateral SVC, because single-vessel flow from the dual SVCs would distribute to both lungs in a manner similar to that of the unilateral Glenn procedure (Figure-1).Our aims of this study were, in patients with a functional single ventricle and bilateral SVC, to compare the results of our new method with those of conventional bilateral bidirectional Glenn procedure, and to examine the efficacy of the method.

Retrospective data analysis
We retrospectively reviewed the clinical and surgical records of 65 patients who had undergone bidirectional Glenn procedure in a staged right cardiac bypass at Juntendo university hospital, Tokyo, between January 1997 and March 2014.We performed the first venous unifocalization in 2002.Since then, the novel method has been performed in patients in whom both the inferior vena cava and larger main SVC were on the same side; otherwise conventional bilateral bidirectional Glenn procedure is performed, since the outcome of the new method had yet to be established.
Sixteen of the patients had a bilateral superior vena cava.These patients were divided into 2 groups: group 1: patients who had received venous unifocalization (n = 8), and group 2: patients who had received the bilateral bidirectional Glenn procedure (n = 8).We retrospectively reviewed the clinical data including age, body weight, central venous pressure, blood oxygen saturation, time on cardiopulmonary bypass, total time of the procedure, and complications after surgery.Central venous pressure and oxygen saturation were measured at the preoperative catheter examination and postoperatively in the intensive care unit.Postoperative central venous pressure was measured at the main larger superior vena cava in all cases.-1 and 2. Patient age ranged from 4 months to 2 years in group 1 and 1 month to 3 years in group 2. Body weight ranged from 6.1 kg to 10.6 kg in group 1 and 2.78 kg to 13.5 kg in group 2. Tricuspid atresia was diagnosed in four patients in group 1; single ventricle associated with right isomerism was present in 1 patient in group 1 and 2 patients in group 2; pulmonary atresia was present in 1 patient in group 1 and 2 in group 2; double outlet right ventricle was present in 1 patient in group 1 and 2 in group 2, and transposition of the great arteries occurred in 2 patients in group 2. Shunt division, pulmonary artery plasty, common atrioventricular valve plasty, posterior descending artery ligation, and the Fontan procedure were performed subsequently in both groups.

Patient characteristics are shown in Tables
There were no significant differences in age (group 1: 1.0 ± 0.5, group 2: 1.2 ± 1.1, years of age), body weight (group 1: 7.7 ± 2.3, group 2: 6.5 ± 4.3, kg), change in central venous pressure before and after operation (group 1: 6.5 ± 3.1, group 2: 9.9 ± 6.2, mmHg), postoperative oxygen saturation (group 1: 82 ± 3.3, group 2: 83 ± 9.3, %), duration of surgery (group 1: 371 ± 120, group 2: 439 ± 168, min), or cardiopulmonary bypass time (group 1: 143 ± 38, group 2: 131 ± 53, min).Preoperative pulmonary artery index was shorter in group 1 (group 1: 269 ± 120, group 2: 365 ± 124, p = 0.03) (Table-3).There were no significant differences in any other indices.Although the differences were not significant, there were 4 mortalities (1 early in group 1, and 1 early and 2 late in group 2), and 6 morbidities (2 in group 1, 4 in group 2).In group 1, 7 patients received the Fontan procedure.In the patient that died early, the larger SVC was located on the left side, and the inferior vena cava was located on the right side.On the patientʼs right side, the left larger SVC was anastomosed to the right smaller SVC.Post-operatively, central venous pressure was elevated, and severe edema of the face was observed.Two patients had phrenic nerve palsy in group 1.In group 2, 4 patients received the Fontan procedure, and 1 is awaiting the Fontan procedure.The patient with early death died due to low output syndrome, and the 2 late deaths were due to respiratory infections. 2 cases of mediastinitis and 2 cases of long-standing pleural effusion were seen in Group 2 (Figure -2).
None of the cases presented pressure gradients by echocardiography on postoperative day 3.

Discussion
The bidirectional Glenn (BDG) procedure is an important intermediate stage preceding the Fontan operation.Generally, the bilateral BDG is performed in patients who have bilateral superior vena cava.In the bilateral SVC anatomy, the imbalance of flow through the pulmonary artery can cause stasis of blood and thrombosis formation, because the small apertures of the bilateral SVC reduces blood flow volume 8) .The clinical impact of thrombosis in the cavopulmonary circulation is a higher risk of mortality after BDG and/or increased hospitalization time 10) 11) .In addition, there is a concern that blood flow from the inferior vena cava might not distribute equally after the Fontan procedure, because there is a dimensional mismatch between the baffle and the connecting vessels.It was not until 1994, when Srivastava and colleagues 12) conclusively demonstrated that pulmonary arterialvenous fistulae occurred due to the absence of "hepatic factor"or"mesenteric factor" , and that an unidentified factor in the hepatic venous drainage inhibited the recruitment and dilation of preexisting pulmonary arteriovenous connections 12)-14) .Unequally distributed inferior vena cava blood flow to the lungs can lead to pulmonary arteriovenous malformations.Currently, with the widespread use of the bidirectional Glenn procedure as an intermediate stage in the palliation of children with single-ventricle physiology, pulmonary arteriovenous malformations continue to be a cause of considerable morbidity.Although the bidirectional Glenn procedure provides excellent hemodynamic palliation for these children, its durability is often limited by the development of progressive cyanosis due to pulmonary arteriovenous malformations 15) 16) .These facts suggest that undisturbed pulmonary blood flow from SVCs and the distribution of blood flow from the inferior vena cava are important factors for the prognosis of patients with a single ventricle.Iyer GK, et al has reported there was a lower operative mortality and a higher conversion rate to the Fontan circulation in children undergoing a unilateral bidirectional Glenn than in children undergoing bilateral bidirectional Glenn 17) .
In patients with bilateral SVCs, our new method would generate undisturbed blood flow similar to that of unilateral BDG but without anatomical or surgical limitations.We believe our new method would resolve some of theissues that occur in bilateral-SVCs patients.
Although we speculated that blood flow from the inferior vena cava, which includes the hepatic factor, might be distributed better than with the unilateral Glenn procedure, proof that the new method creates more equally distributed blood flows could not be demonstrated.In a future study, we will measure the equality of blood flow distribution in the two methods by using hydrodynamic simulation.Although evaluation of a new method requires patient follow-up early and long-term, we predict a positive outcome based on literature 18) .

Study limitations
One limitation of this study was the lack of some useful information, which includes quantitative data such as echocardiography or cardiac angiography, as well as data on flow volume ratio between the SVCs and the IVC.Another major limitation of this study was the small sample size, which was not sufficient to establish the validity of this surgical method.However, evaluation of a novel method on a rare disease such as bilateral superior vena cava is valuable in itself.Further study with a larger sample size, long-term follow-up, and additional data including flow volume ratio and quantitative analysis is essential to show the applicability of this method in various anatomical subsets.

Conclusion
Although we had hypothesized that the new surgical method,"SVC unifocalization" , would be more effective than the Glenn stage procedure in patients with bilateral SVC, the results of this study demonstrated that there was no distinct advantage or disadvantage to using our new method.Therefore, further clinical study was warranted.

Figure- 1
Figure-1 Unifocalization of bilateral superior vena cavaSmaller SVC was anastomosed to the lateral side of larger SVC, then the unifocalized SVC was anastomosed to the pulmonary artery.Unilateral Glenn-like flow was obtained.

Figure- 2
Figure-2 Clinical course of two groups

Table - 3
Comparison of peri-operative variables in group 1 (unifocalization of the superior vena cava procedure) and group 2 (bilateral bidirectional Glenn procedure)