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Ann Thorac Surg 2005;79:2103-2108
© 2005 The Society of Thoracic Surgeons

Original article: Cardiovascular

Optimal Size of a Monocusp Patch for Reconstruction of a Hypoplastic Pulmonary Root: An Experimental Study in Pigs

J.F. Matthias Bechtel, MDa, Peter E. Lange, MDb, Hans H. Sievers, MD, FETCSa,*

a Department of Cardiac Surgery, University of Luebeck, Luebeck Germany
b Abteilung fuer Angeborene Herzfehler, Deutsches Herzzentrum Berlin, Berlin, Germany

Accepted for publication November 22, 2004.

* Address reprint requests to Dr Sievers, Klinik fuer Herzchirurgie, UK SH, Campus Luebeck, Ratzeburger Allee 160, 23538 Luebeck, Germany (E-mail: sievers{at}medinf.mu-luebeck.de).


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Comment
 References
 
BACKGROUND: Transannular patching is often performed to relieve congenital pulmonary stenosis, especially in tetralogy of Fallot. Theoretically, a monocusp patch can reduce patch-related pulmonary regurgitation, but the optimal size relation between the implant and the native hypoplastic pulmonary root is not well defined.

METHODS: In 11 pigs, peak pressure gradient and regurgitation fraction across the pulmonary root were measured. During cardiopulmonary bypass, two cusps including the pulmonary artery wall were resected and the midpoint of the free margin of the remaining cusp was sutured to the sinus wall to imitate a hypoplastic pulmonary root. Transannular patching was performed using a noncoronary segment of a porcine aortic root. After discontinuation of cardiopulmonary bypass, all measurements were repeated. Thereafter, the cusp of the patch was resected, and all measurements again repeated. Anatomic dimensions were determined after the pigs had been sacrificed.

RESULTS: Regurgitation fraction increased from 0.2% ± 3.4% at baseline to 15.5% ± 6.2% after reconstruction with a monocusp patch and to 60.0 ± 18.6% after the cusp of the monocusp patch had been resected (p < 0.001). The median peak pressure gradient increased from 0 to 1 to 6 mm Hg (p = 0.013), respectively. The regurgitation fraction negatively correlated with the ratio of the length of the monocusp patch to that of the hypoplastic pulmonary root (r = -0.63, p = 0.037).

CONCLUSIONS: A monocusp patch for reconstruction of a hypoplastic pulmonary root results in significantly less regurgitation than a nonvalved patch of the same size, while the peak pressure gradient remains normal. The lowest regurgitation fraction was observed with a monocusp patch two-times the length of the circumference of the hypoplastic pulmonary root.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Comment
 References
 
Transannular patching is often necessary for reconstruction of right ventricular outflow tract (RVOT) obstruction in congenital cardiac defects, but it leads to transvalvular regurgitation. Acute pulmonary valve regurgitation of higher degrees impairs right ventricular (RV) function both clinically [1, 2] and experimentally [3, 4], and seems to be associated with increased perioperative mortality [5].

On the other hand, chronic pulmonary valve regurgitation appears to be tolerated well in most patients, but evidence is mounting that it results in RV volume overload, right-sided heart failure, reduced exercise performance, and fatal as well as nonfatal arrhythmias [6–13].

Therefore, a low degree of pulmonary regurgitation without significant accompanying stenosis is highly desirable. The use of valved patches has the capability to reduce pulmonary regurgitation as compared to nonvalved patches [14], but this is not readily achieved [15, 16]. Little is known about the geometric relationship between a monocusp patch length and the size of a hypoplastic pulmonary root that is necessary to obtain optimal results regarding hemodynamic function of the reconstructed pulmonary root. We studied the determinants of transvalvular regurgitation after transannular patching in a porcine model imitating a hypoplastic pulmonary root.


   Materials and Methods
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 Abstract
 Introduction
 Materials and Methods
Results
 Comment
 References
 
Experiments were performed on 11 pigs (weight 23.6 ± 3.5 kg) after approval of the responsible ministry. All animals received care in accordance with the Guide for the Care and Use of Labaratory Animals published by the National Institutes of Health (National Institutes of Health publication 85-23, revised 1985).

After anesthesia was induced with 500 mg of ketamine hydochloride intramusculary, the pigs were intubated and mechanically ventilated with 21% oxygen (Assistor, Draeger, Luebeck, Germany). The arterial partial pressures of oxygen and CO2 and the pH were checked and kept within normal values. Anesthesia was maintained with {alpha}-glucochloralose (0.5 mg/kg body weight per hour). For measurement of aortic pressure, a 7F catheter was placed into the ascending aorta. For injection of contrast material, a 7F catheter was placed into the right ventricle. For measurement of right ventricular pressure (RVP) and pulmonary artery pressure (PAP), 5F microtransducers (Millar Instruments, Houston, TX) were used. All catheters were advanced into their designated position from the femoral vessels.

The pigs were placed in supine position, turned 45 degrees to the right and 30 degrees head-low, and rotated 10 degrees to the left. This position allows optimal radiologic assessment of the right ventricle in pigs. Baseline measurements of RVP and PAP and of the transvalvular regurgitation were performed. Regurgitation was assessed using videodensitometry by R-wave triggered injection of 4 mL of contrast material (76% Urografin; Schering, Berlin, Germany) into the right ventricle during 1 second. Videodensitometry is a contrast-material-dilution technique. The ratio of changes in amplitudes of the densitogram of an electronic window covering the silhouette of the right ventricle represents the systolic forward flow and the reflux of contrast material and allows calculation of the regurgitation fraction. The regurgitation fraction was determined from 6 consecutive beats before and after application of the contrast material. Our group has previously found that the videodensitometric assessment of the regurgitation fraction is ±5% precise [17, 18]. Images (50 per second) were recorded from a lateral view.

Median sternotomy was then performed. Following systemic anticoagulation with heparin, the ascending aorta and the superior and inferior vena cava were then cannulated, and hypothermic (29°C) total cardiopulmonary bypass (CPB) was established using a bubble oxygenator (BOS-5; Bentley Labaratories, Irvine, CA). The left vena azygos (which drains into the coronary sinus in pigs) was ligated. Cardiac arrest was induced and maintained by cold (4°C) crystalloid cardioplegia (St. Thomas Hospital solution, 20 mL/kg body weight), which was repeated in 20-minute intervals.

The left and anterior cusps of the pulmonary valve including the corresponding arterial wall were resected, and the middle of the free margin of the remaining right cusp was attached to the sinus wall using a 6–0 poly-propylene suture (Fig 1). This way, pathoanatomic conditions characteristic for the hypoplastic pulmonary root in Fallot’s tetralogy with two rudimentary cusps were created, and we further refer to it as hypoplastic pulmonary root.



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  		Fig 1. First, a hypoplastic pulmonary root was surgically created (right side of the figure). The native pulmonary root was incised by dividing the anterior sinus of the pulmonary valve. Then the anterior and the left cusps of the pulmonary valve, with their corresponding sinus wall, were resected; the residual cusp was divided into two hypoplastic cusps by suturing the middle of the free margin of the cusp to the sinus wall. The dotted line is referred to as "length of the hypoplastic pulmonary root." Second, the hypoplastic pulmonary root was reconstructed using a monocusp patch (left side of the figure). The monocusp patch was excised from a stentless porcine aortic valve prosthesis. Therefore, the anatomy of the cusp, the commissures, and the sinus was preserved. The interrupted line is referred to as "length of the monocusp patch." The numbers indicate the sequence of the first stitches; this way it was assured that the cusps of the hypoplastic pulmonary root and of the monocusp patch were on the same height and coapted as much as possible after the reconstruction had been accomplished.

 
The hypoplastic pulmonary root was then reconstructed using a monocusp patch made from the noncoronary segment of a porcine glutaraldehyde-fixed aortic root (Fig 1). The length of the circumference of the monocusp patch varied randomly according to the heart valve donor. Special emphasis was spent on implanting the monocusp patch in such a way that the cusps of the hypoplastic pulmonary root and of the monocusp patch were on the same height and coapted as much as possible. Continuous 5–0 poly-propylene sutures were used for fixation of the patch.

After rewarming, CPB was discontinued. After stable hemodynamics (aortic pressure within the baseline range for 15 minutes) had been achieved, the sternum was closed, and all measurements were repeated.

Thereafter, the sternum was reopened, and CPB was again established in 7 pigs. The pulmonary artery was reopened and the cusp of the monocusp patch was excised. The pulmonary artery was then closed again using 5–0 poly-proplene sutures. After CPB had been discontinued and stable hemodynamics again had been achieved, the sternum was closed, and all measurements were again repeated.

Then, the pigs were sacrified. The reconstructed pulmonary root was excised, and the circumference of the pulmonary artery was measured at the level of the sinotubular junction. Then, one of the anastomotic sides was opened and the reconstructed pulmonary root/artery was unrolled into a single plane. No tension was applied in doing so. The length of the circumference of the hypoplastic pulmonary root and of the monocusp patch was then measured using a calibrated ruler.

Data are presented as mean ± standard deviation, except when otherwise noted. Comparisons for paired measurements were made using the Friedman and Wilcoxon tests. Determinants of transvalvular regurgitation after trans-annular patching were determined using Pearson’s correlation and linear regression analysis. A p value less than or equal to 0.05 was considered significant. All analyses were performed using SPSS for windows, release 9.0 (SPSS, Chicago, IL).


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Comment
 References
 
Creation and Reconstruction of a Hypoplastic Pulmonary Root
After resection of the left and anterior cusps, the circumference of the hypoplastic pulmonary root was 16 ± 3 mm corresponding to a diameter of a hypoplastic pulmonary artery of ~ 5 mm, typical for tetralogy of Fallot in infancy.

The monocusp patch had a length of 21 ± 3 mm (p = 0.008 as compared to the circumference of the hypoplastic pulmonary root). The mean ratio of monocusp patch length to that of the hypoplastic pulmonary root was 1.4 ± 0.4. After reconstruction, the circumference of the pulmonary artery did not differ significantly from the circumference before the surgical creation of a hypoplastic pulmonary root (ratio of preoperative to postoperative circumference = 0.94 ± 0.12; p = 0.11 for comparison of the preoperative and postoperative circumferences).

Peak Pressure Gradient
The peak pressure gradient across the pulmonary root at baseline, after reconstruction with a monocusp patch and after resection of the cusp of the monocusp patch is illustrated in Figure 2. There was a hemodynamically unimportant, but statistically significant increase from baseline to the reconstructed state with the intact monocusp patch (median 0–1.4 mm Hg, p = 0.012), but no further statistically significant change after resection of the cusp of the monocusp patch (median 6.1 mm Hg, p = 0.237).



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  		Fig 2. Peak pressure gradient across the pulmonary root at baseline, after reconstruction with a monocusp patch, and after the cusp of the patch had been resected. Depicted are the individual values at each measurement. The lines between the open circles and closed circles to the right indicate the corresponding peak pressure gradients in individual pigs observed with the monocusp patch and after the cusp had been resected. In most animals, there was only a very modest change of the peak pressure gradient.

 
Regurgitation Fraction
The median regurgitation fraction at baseline was 0.6% (minimum-maximum: -3.1% to 8.0%). After implantation of the monocusp patch, the regurgitation fraction increased significantly and further increased signficantly after the cusp of the monocusp patch had been resected (Fig 3).



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  		Fig 3. Regurgitation fraction across the pulmonary root at baseline, after reconstruction with a monocusp patch, and after the cusp had been excised from the patch. Values are mean ± standard deviation.

 
Determinants of Hemodynamics After Implantation of the Monocusp Patch
The regurgitation fraction revealed a significant negative correlation with the ratio of the length of the monocusp patch to the length of the hypoplastic pulmonary root (r = –0.63, p = 0.037; Fig 4). There was no significant evidence that the peak pressure gradient also was dependent on this ratio. No further anatomic variable showed a significant correlation with the regurgitation fraction across the hypoplastic pulmonary root after reconstruction with a monocusp patch.



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  		Fig 4. Scatterplot showing the relation between the ratio of the length of the MCP to the length of the HPR and the regurgitation fraction. A significant negative correlation was observed. Depicted are the individual measurements (closed circles), and the regression line is also shown. (MCP = monocusp patch; HPR = hypoplastic pulmonary root.)

 

   Comment
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Comment
References
 
Our data obtained in a porcine model of a hypoplastic pulmonary root show that the regurgitation fraction after reconstruction is significantly smaller if a monocusp patch is used as compared to a nonvalved patch of the same size. Our data further demonstrate that the regurgitation fraction after reconstruction is dependent on the length of the monocusp patch relative to the length of the hypoplastic pulmonary root. The best result was obtained with a monocusp patch roughly two times as wide as the hypoplastic pulmonary root.

The impact of pulmonary regurgitation after reconstruction of the RVOT has been controversely discussed for decades. Experimentally, acute pulmonary valve regurgitation is associated with depressed RV function and may contribute to postoperative complications [1–5]. It therefore seems advisable that acute pulmonary regurgitation has to be avoided in order to obtain optimal perioperative results.

The long-term effects of pulmonary regurgitation are another concern. Our group has earlier reported that pulmonary valve regurgitation impairs RV function [19], but the degree of chronic pulmonary regurgitation has been difficult to determine for a long time. More recent studies, using echocardiography and magnetic resonance imaging, allow a more precise measurement of the degree of chronic pulmonary regurgitation [20] and indicate that it may indeed have a detrimentral effect on RV function and survival [6–13]. Videodensitometry, as in our study, allows a very precise assessment of the regurgitation fraction [17, 18].

Surgery for the correction of congenital defects of the RVOT must therefore aim at a optimal hemodynamic performance early and late after reconstruction. No currently available surgical strategy reliably achieves this goal; the long-term results are sometimes complicated by the need for reoperations or the development of heart failure [21–23]. There is some hope that tissue engineering will allow the implantation of viable, growing and durable materials.

The use of a transannular patch for reconstruction of the RVOT is somewhat feared because it was reported that the perioperative mortality and the reoperation rates are increased [24]. However, if total correction of tetralogy of Fallot is performed, transannular patching often cannot be avoided [23]. Our data indicate that the use of a monocusp patch is hemodynamically superior to a nonvalved patch and that the geometric relation between the monocusp patch and the hypoplastic pulmonary root is the major determinant of the degree of pulmonary regurgitation early after reconstruction. In our experiments, the highest degrees of regurgitation were obtained with the smallest (relative to the hypoplastic root) monocusp patches used, most likely because the free margin of the monocusp was not long enough to coapt completely with the hypoplastic pulmonary valve cusps.

The lowest degrees of pulmonary regurgitation were observed with the largest monocusp patches. In this study, the largest monocusp patch was 2.1 times as wide as the hypoplastic pulmonary root, and the resulting peak pressure gradient was <10 mm Hg. Contemporary synthetic or xenogeneic patch materials and valve substitutes are stiffer than native cusps; this may not only have a negative influence on the pressure gradient but also on valve opening and closing characteristics [25]. Therefore, the hemodynamic characteristics of monocusp patches that are even larger than the ones used in the present study cannot be predicted from our data.

It is important to note that the variation of the circumference of the surgically created hypoplastic pulmonary root as well as the length of the monocusp patch was relatively narrow (as shown by the small range of values) but nevertheless had a large impact on the regurgitation fraction; therefore, exact size determination and suturing are required in any attempt to replicate the data or transfer our results to humans. The measurements in this study were made post mortem after opening one suture side. This method will inevitable differ from all invasive or noninvasive measurements obtained during life because of the loss of tension. However, our main finding relates to the ratio of the size of the monocusp patch to that of the hypoplastic pulmonary root and will therefore be less affected by the method of measurement.

Limitations of the Study
We have only studied the immediate effects of the geometry on the function of the reconstructed pulmonary valve. The long-term function of a monocusp patch as large as in our study has not been studied, and it is not known whether, when and how it will fail. However, we believe that it will be very important in many cases to avoid acute postoperative pulmonary regurgitation after transannular patching, and our study provides a simple rule for achieving this goal.

Another limitation of our study, however, is that it is unknown whether our results obtained in a porcine model can be simply carried forward to humans, especially as in those patients who need surgery because of pulmonary stenosis, marked right ventricular hypertrophy is found. This is not present in our animal model, but we believe that acute valvular regurgitation is not significantly affected by the degree of right ventricular hypertrophy. The porcine semilunar valves are very similar to their man counterparts, and the experimental model used was designed to mimic a hypoplastic pulmonary root as it is observed in patients with congenital defects of the RVOT as closely as possible [26]. This special characteristic of our model was achieved by suturing the middle of the free margin of the residual native cusp to the wall of the pulmonary artery thereby creating two rudimentary cusps that were severely limited in their mobility. In addition, the geometric rule found is very simple which – in our opinion – raises the possibility that a similar dependency may exist in humans.

In our study, a glutaraldehyde-fixed porcine monocusp patch was used which indicated preserved anatomy of the leaflet, the commissures and the sinus. There is some evidence that the use of xenogeneic monocusp patches have a limited long-term function when used for RVOT reconstruction in infants and children [27, 28]. Therefore, we would recommend the use of allogenic material in humans.

Today, valved-patches used for RVOT reconstruction are often constructed from pericardium or synthetic material [29, 30]. Although it is unknown whether our results can be transferred to such monocusp patches, we strongly believe that a similar, simple geometric rule will exist even with materials that have different physical properties.

In conclusion, our data suggest that by meticolous adherence to basic geometric relations the degree of pulmonary regurgitation early after reconstruction of a hypoplastic pulmonary root with a monocusp patch can be reduced to a minimum.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Comment
 References
 

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