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Ann Thorac Surg 2005;79:636-640
© 2005 The Society of Thoracic Surgeons

Original article: Cardiovascular

Does the Site of Insertion of a Systemic-Pulmonary Shunt Influence Growth of the Pulmonary Arteries?

Anjan S. Batra, MDa, Vaughn A. Starnes, MDc, Winfield J. Wells, MDb,*

a Department of Pediatric Cardiology, Indiana University School of Medicine, Indianapolis, Indiana
b Department of Cardiothoracic Surgery, Childrens Hospital Los Angeles Los Angeles, California
c Keck School of Medicine, University of Southern California, Los Angeles, California

Accepted for publication July 23, 2004.

* Address reprint requests to Dr Wells, Department of Cardiothoracic Surgery, Childrens Hospital Los Angeles, 4650 Sunset Blvd, Box 66, Los Angeles, CA 90027 (E-mail: wwells{at}chla.usc.edu).


   Abstract
Top
 Abstract
Introduction
 Material and Methods
 Results
 Comment
 References
 
BACKGROUND: The modified Blalock-Taussig shunt is a common palliative procedure for children with cyanotic congenital heart disease. The distal shunt anastomosis can be done to a branch pulmonary artery or to the main pulmonary artery. The purpose of this study was to determine if the site of shunt connection influences pulmonary artery growth.

METHODS: The records of 101 patients with a modified Blalock-Taussig shunt undergoing a subsequent cardiac catheterization between January 2000 and April 2002 were retrospectively reviewed. From the cineangiograms, the diameters of the right and left pulmonary arteries at their first branching and the diameter of the descending aorta at the diaphragm were measured.

RESULTS: If the distal shunt anastomosis was to the right pulmonary artery and there was no antegrade pulmonary flow then the left pulmonary artery was significantly smaller than if the distal connection was to the main pulmonary artery (p = 0.009). Absence of antegrade pulmonary blood flow resulted in significantly smaller right and left pulmonary artery size in general (p < 0.001). No significant differences in pulmonary artery growth were found with respect to gender, anatomic subtype, proximal shunt site, use of cardiopulmonary bypass or size of shunt. By multiple regression analysis absence of antegrade flow and the presence of right-sided shunts were statistically significant predictors of smaller left pulmonary artery and size discrepancy between right and left pulmonary artery.

CONCLUSIONS: These data suggest that in the absence of antegrade pulmonary blood flow, a modified Blalock-Taussig shunt to the main pulmonary artery may promote more uniform branch pulmonary artery growth.


   Introduction
Top
 Abstract
 Introduction
Material and Methods
 Results
 Comment
 References
 
The classic Blalock-Taussig shunt was introduced in 1945 to augment pulmonary blood flow in children with cyanotic congenital heart disease. As originally described the operation involved direct anastomosis of the subclavian artery to a branch pulmonary artery (PA) [1]. In 1962, Klinner and colleagues [2] suggested a modified Blalock-Taussig shunt (MBTS) which interposed a synthetic tube graft between the subclavian and a branch PA. Further technical modifications followed including a more central placement of the proximal anastomosis from the ascending aorta [3] or distal connection to the main pulmonary artery (MPA) rather than to a branch PA. Palliation with a MBTS provides time for pulmonary vascular resistance to drop and for PA growth before a more definitive cardiac repair. Though usually effective, there are potential complications including the risk of distortion, stenosis, or asymmetric growth of the PAs [4–9]. These risks increase over time. The purpose of this study was to assess whether the site of MBTS origin and PA connection had an effect on PA growth. The significance of concomitant antegrade flow into the main PA was also of interest.


   Material and Methods
Top
 Abstract
 Introduction
 Material and Methods
Results
 Comment
 References
 
From January 2000 through April 2002, 101 patients with a MBTS had a catheterization and angiography before a planned second surgical procedure. The defects requiring palliation included hypoplastic left heart syndrome in 38 patients, other forms of single ventricle in 28, pulmonary atresia with ventricular septal defect (VSD) in 16, pulmonary atresia with intact septum in 7, and other defects with severe pulmonary stenosis in 12 patients. Patients with pulmonary atresia and discontinuous pulmonary arteries were excluded.

Data were collected retrospectively from patient records, operative notes, cardiopulmonary bypass data sheets, and catheterization studies. We recorded age and weight at the initial BTS operation as well as the specifics of shunt placement including size of the synthetic tube graft, site of proximal and distal anastomosis, and use of cardiopulmonary bypass. Also noted was the presence or absence of antegrade pulmonary flow.

Operative Technique
The median age and weight at MBTS was 6 days (1 day to 75 months old) and 3.3 kg (2 to 20 kg), respectively. Most patients (81%) were operated in the first month of life. All shunts were performed through a median sternotomy and a partial or complete thymectomy was carried out. The size of the shunt and sites for proximal and distal connection were selected based on individual patient anatomy and the preference of the operating surgeon. The proximal anastomosis came from the innominate in 86 and from the ascending aorta in the remaining 15. When the shunt arose from the aorta the anastomosis was performed in a side-to-side fashion with the end of the graft oversewn. A 3.5 mm shunt was used in 46 patients, a 4.0 mm in 48, and a 5.0 mm in 7 patients. Anastomosis was completed with a running 8.0 polypropylene technique. The site of distal anastomosis was to the right branch PA in 81, and the MPA in the remaining 20. Those patients having a shunt to the MPA without cardiopulmonary bypass had a partial occlusion clamp placed proximally on the MPA such that there was still adequate ductal flow to the PA branches during the anastomosis.

The MBTS was performed on cardiopulmonary bypass in 68 patients including 56 of 79 (82%) with shunts to the RPA and 12 of 20 (60%) with the distal connection to the MPA. For patients done without cardiopulmonary bypass, heparin (150 U/Kg) was given before the application of vascular clamps. Patients were started on Aspirin in the immediate postoperative period.

Morphometric Analysis from Angiography
All patients had cardiac catheterization and pulmonary angiography before their second surgical procedure. The digital images were studied to record PA size, and any distortion or stenosis related to the shunt. The diameter of the RPA and LPA were measured proximally (at the confluence) and at the hilum. The diameter of the DA was measured at the diaphragm. Measurements were made with Quantitative Coronary Analysis Software (Quantcor, version 4.0; Siemens Medical Inc., Malvern, PA) connected directly to a Simmons Acom. PC version 3.1 browser. All values were obtained during ventricular systole and when possible, from the same angiographic frame. Angiographic catheter sizes were used to calculate a magnification coefficient to adjust for measured versus actual size.

Pulmonary artery distortion was defined as tenting or kinking by the shunt. PA stenosis was measured as the percentage of narrowing of the luminal diameter, and a constriction greater than 50% was considered to be major.

Statistical Analysis
We compared outcome variables: RPA/DA, LPA/DA, difference (RPA – LPA)/DA, and sum (RPA + LPA)/DA. We analyzed the differences of these outcome variables between subgroups defined by size, insertion and origin of shunt, diagnosis, gender, and use of cardiopulmonary bypass using the multivariate analysis of variance (MANOVA). If MANOVA was significant at {alpha} = 0.05, each dependent variable was tested with the univariate t-test or ANOVA, adjusted for multiplicity by stepdown bootstrap permutation method. The same method of adjustment was applied for comparing more than two subgroups. For continuous independent variables we studied both the Pearson's and Spearman's correlation coefficients between them and the dependent variables, and tested whether they are significantly different from zero. Finally, we conducted the stepwise multiple regression analysis for each outcome variable to determine the risk factors associated with it.


   Results
Top
 Abstract
 Introduction
 Material and Methods
 Results
Comment
 References
 
Pulmonary Artery Size
When comparing the site of distal MBTS anastomosis, a shunt taken to the RPA resulted in a significantly smaller LPA/DA index versus a shunt to the MPA (p = 0.02). There was also a larger difference between RPA/DA and LPA/DA when the shunt went to the RPA (p = 0.003) as opposed to more uniform growth of both branch PAs in the presence of antegrade flow through the MPA. Patients with pulmonary atresia (no antegrade pulmonary flow) had smaller branch PAs on both the right and left side versus those with antegrade flow (p = 0.001). Patients with hypoplastic left heart syndrome undergoing the Norwood procedure had smaller LPA/DA index when compared to the RPA/DA index. These differences are detailed in Table 1.


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  		Table 1. Subgroup Comparisons
 
No significant difference in branch PA size was found with respect to age or weight at the MBTS, gender, anatomic subtype, size of the shunt, site of proximal anastomosis, use of cardiopulmonary bypass, or interval between the shunt and presecond stage catheterization (Tables 1 and 2).


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  		Table 2. Correlation Coefficients Between Outcome Variables and Continuous Independent Factors
 
Table 3 presents the findings of a stepwise multiple regression analysis of factors potentially influencing the size of the branch PAs after MBTS. RPA growth was positively influenced by the presence of concomitant antegrade pulmonary flow. Better growth of the LPA correlated with both antegrade flow and distal connection of the shunt to the MPA. Likewise the difference between growth of the RPA and LPA was more significant if the site of distal shunt anastomosis was to the RPA, and sum of RPA and LPA was greater with antegrade flow and location of the shunt connection on the MPA.


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  		Table 3. Stepwise Multiple Regression Analysis
 
Pulmonary Artery Stenosis and Distortion
Of 101 shunts studied, 77 exhibited no important distortion or stenosis of the pulmonary arteries. Significant stenosis (>50% lumen narrowing) was present in 20 patients. Patients with right-sided shunts had the highest incidence of stenosis (n = 19) including 14 proximal LPA stenosis and 5 RPA stenosis. Among the 20 patients with a shunt placed to the MPA only one had stenosis of the LPA (5%). Distortion of the pulmonary artery at the site of the shunt, without other pulmonary artery stenosis was noted in 3 patients with a right-sided shunt and 1 with a centrally inserting shunt. The incidence of PA distortion was low for the patient population as a whole and did not correlate with the insertion of the shunt. The proximal LPA in the region of the ductus was the most frequent site of narrowing. Ductal tissue contraction may have played a major role in this problem.


   Comment
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
References
 
Many criteria have been used to assess the adequacy of pulmonary artery growth after palliation with a systemic to pulmonary shunt. These include simple measurement of PA size [10], PA size indexed to body surface area [11, 12], and the ratio of the pulmonary artery to the diameter of the ascending [13] or more commonly the descending aorta [4, 5, 8, 11, 14]. We selected the ratio of the diameter of the pulmonary artery at the first lobar branching to the diameter of the descending aorta at the level of the diaphragm. Previous reports have shown that with growth, the ratio of the pulmonary artery to the descending aorta and the ratio of the sum of the pulmonary arteries to the diameter of the descending aorta remains constant with the latter index approximating one.

When the site of distal MBTS anastomosis is to one of the branch PAs, there may not be optimal growth of the contralateral pulmonary vessel. Published series report conflicting results with regard to both growth and distortion of the PAs after MBTS [7, 8, 11] Even the earliest reports suggested that a more centrally placed shunt may produce better distribution of blood flow and thus enhance PA growth [15]. Our study supports this hypothesis but additionally shows the importance of antegrade pulmonary flow. More recently, the Sano modification utilizing a right ventricle to pulmonary artery conduit is gaining favor for repair of hypoplastic left heart syndrome. Preliminary studies show that pulsatile conduit flow provides a stable systemic circulation as well as adequate pulmonary blood flow [16]. How pulmonary artery growth with the Sano modification compares with a centrally inserted shunt needs further investigation.

Though previous studies have suggested no difference between right and left PA/DA index after MBTS, the majority of the patients in these series have had concomitant antegrade pulmonary blood flow [9, 17]. Our study found that a discrepancy between the size of the PAs is present only in patients without antegrade flow and a shunt to the RPA. In patients with a centrally inserting shunt or the presence of antegrade flow the PA/DA ratio was consistent with previously reported normal values [8, 12].

Ishikawa and coworkers [18] described a differential in PA growth following MBTS with distal connection to a branch PA. This was a small series of patients operated when they were over 1 year of age. The smaller contralateral pulmonary artery was felt to be due to decreased flow though Waldman and associates [19] have shown that children with congenital pulmonary atresia may experience spontaneous narrowing of the proximal PA at the site of ductal entry with subsequent hypoplasia of the distal pulmonary vessel. They postulated that ductal tissue within the PA wall maybe responsible for this constriction, which may progress to discontinuity. Our study raises the question of whether antegrade flow has the potential to decrease the incidence of this problem. However it is also the case that the PDA was surgically ligated at the time of MBTS in all of our patients. This may not have been the situation in Waldman's series.

This report includes a large number of patients with hypoplastic left heart syndrome (HLHS), all of whom had a right-sided shunt and the absence of antegrade pulmonary blood flow. Because of this there was concern that the discrepancy in the size of the PAs, which we observed, could have been related to the HLHS defect itself. However multivariant analysis found that the location of the shunt was a much stronger predictor of PA size as compared to anatomic subtype.

It is recognized that MBTS performed in small children may lead to PA distortion with a wide variation in frequency (0% to 36%) [20–25]. We observed a low incidence of pulmonary artery distortion with shunts going to the RPA and MPA (4% vs 6% respectively). This could be related to the fact that we used relatively small shunts (3.5 or 4 mm) that might reduce the chances of kinking or flattening of the perianastomotic PA. It could also be a function of the short time that most of the shunts were in place.

We recognize limitations in the design of this study. This was a retrospective study. Data on the initial size of the PA was not available because cineangiograms are not routinely done before a shunt placement. Measurement of the size of MBTS was limited to its diameter and not its length. However, the length of the MBTS is known to be a much less powerful determinant of shunt flow than shunt diameter.

In conclusion, our findings suggest that in patients where a MBTS is the only source of pulmonary blood flow, a shunt inserting on the MPA may promote more uniform branch pulmonary artery growth. When antegrade flow is present, the shunt placement on a branch PA or MPA should give equally good results.


   References
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 References
 

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