Ann Thorac Surg 2007;84:1001-1005
© 2007 The Society of Thoracic Surgeons
New Technology
Correction of Aortic Insufficiency With an External Adjustable Prosthetic Aortic Ring
Andrew Gogbashian, MDa,
Ravi K. Ghanta, MDa,
Ramanan Umakanthan, MDa,
Aravind T. Rangaraj, MDa,
Rita G. Laurence, BSa,
John A. Fox, MDb,
Lawrence H. Cohn, MDa,
Frederick Y. Chen, MD, PhDa,*
a Division of Cardiac Surgery, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts
b Division of Cardiac Anesthesia, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts
Accepted for publication February 22, 2007.
* Address correspondence to Dr Chen, Division of Cardiac Surgery, Brigham and Women’s Hospital, 75 Francis St, Boston, MA 02115 (Email: fchen{at}partners.org).
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Abstract
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Purpose: Less invasive, valve-sparing options are needed for patients with aortic insufficiency (AI). We sought to evaluate the feasibility of reducing AI with an external adjustable aortic ring in an ovine model.
Description: To create AI, five sheep underwent patch plasty enlargement of the aortic annulus and root by placement of a 10 x 15 mm pericardial patch between the right and noncoronary cusps. An adjustable external ring composed of a nylon band was fabricated and placed around the aortic root.
Evaluation: Aortic flow, aortic pressure, and left ventricular pressures were measured with the ring loose (off) and tightened (on). Mean regurgitant orifice area decreased by 86%, from 0.07 ± 0.03 cm2 (ring loose, off) to 0.01 ± 0.00 cm2 (ring tightened, on) [p < 0.01]. The regurgitant fraction decreased from 18 ± 4% to 2 ± 1% [p < 0.01]. The ring did not significantly affect stroke volume and aortic pressure.
Conclusions: An ovine model of aortic root dilatation resulting in acute AI has been developed. In this model, application of an external, adjustable constricting aortic ring eliminated AI. An aortic ring may be a useful adjunct in reducing AI secondary to annular dilatation.
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Introduction
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Alternative surgical options to aortic valve replacement are needed for the management of aortic insufficiency (AI). Although approximately 10% of the population has AI, relatively few are referred for aortic valve replacement because of the morbidity and mortality of valve replacement [1]. Less invasive and simpler techniques may be desirable for patients with AI who have normal aortic cusps and a dilated root, but who are otherwise not candidates for aortic valve or root replacement.
Dilation of the aortic root is the most common cause of AI in North America; however, no model exists to evaluate techniques that address AI secondary to this pathology [2]. In this study we present an ovine model of AI with normal aortic cusps and aortic root dilatation. With this model we evaluated an adjustable, quantitative external aortic ring. We hypothesize that such an external constrictor device may be able to reduce aortic circumference, restore cusp coaptation, and significantly reduce AI without impairing cardiac output.
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Technology
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An external aortic ring device was designed from an adjustable, flexible, and inelastic looped nylon band (Fig 1). The band encircles the aortic root with both free ends passing through a latex buckle. The free ends are secured to a calibrated, adjustable sizer. Because the length of the nylon band is fixed, the sizer is calibrated to determine the circumference of the band around the aorta using the formula shown in Figure 1. The aortic ring circumference may be fixed by securing the nylon band to the latex buckle. This simple device was designed to evaluate the concept of reducing AI using an external aortic ring. In the future, aortic ring devices composed of more biocompatible materials and more robust sizing mechanisms can be developed for clinical implementation.
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Fig 1. Illustration of the external aortic ring experimental device. A hardened nylon band is looped around the aorta and through a latex buckle. The free ends of the band are secured to an adjustable sizer. The ring circumference can be adjusted by sliding the band through the buckle in a controlled manner. For this study, the ring was passed underneath the right and over the left coronary arteries. (LCA = left coronary artery; RCA = right coronary artery.)
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Technique
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Ovine Model of Aortic Insufficiency
To create AI, five ovine underwent patch plasty enlargement of the aortic annulus and root using a modified Manougian procedure in which a pericardial patch was placed between the right and noncoronary cusps [3]. To perform the patch plasty the ovine underwent median sternotomies. A section of pericardium was harvested, cured in 3% glutaraldehyde solution, and then shaped into a 10 x 15 mm elliptical patch. The aortic root was dissected free and an electromagnetic aortic flow probe (Carolina Medical Electronics, King, NC) was placed to measure aortic flow. High fidelity catheter-tip micromanometers (Millar Instruments, Houston, TX) were placed in the left ventricle through the apex and the ascending aorta through the right femoral artery. After baseline measurements ensured that each ovine had no preoperative AI, the instruments were removed. Animals were then placed on cardiopulmonary bypass and the aorta was cross-clamped, as previously described [4]. A transverse aortotomy was then made and the aortic valve was visualized. The aortic annulus was divided between the right and noncoronary cusps down to the level of the left ventricular outflow tract. The pericardial patch was sutured between the cusps, extending from the left ventricle to the sinotubular junction using a running 5-0 polypropylene suture (Fig 2). This resulted in dilatation of the aortic root and separation of valve cusps at their coaptation edges. The aortotomy was closed. Cross-clamp time averaged 20 minutes. The nylon band was positioned underneath the right coronary artery and above the left coronary artery to encircle the aortic root and the pericardial patch (Fig 1). The aortic pressure transducer was again advanced into the ascending aorta and the electromagnetic aortic flow probe was once again placed around the aortic root. Animals were weaned from cardiopulmonary bypass and hemodynamic studies were performed. All animals were cared for according to the "Guide for the Care and Use of Laboratory Animals" published by the National Institutes of Health (NIH Publication No. 86-23, revised 1996).
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Fig 2. Patch plasty enlargement of the aortic root. A pericardial patch is interposed between the right and noncoronary aortic valve leaflets, causing separation at their coaptation edges. This models a dilated aortic root as a cause of aortic insufficiency. (Arrow indicates pericardial patch; NCL = noncoronary leaflet; RCL = right coronary leaflet; RCO = right coronary ostium.)
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Experimental Protocol
For each subject, the aortic pressure, left ventricular pressure, aortic flow, and electrocardiogram signals were recorded at baseline, ring loose (off), and ring tightened (on). The on position was determined by observation of the aortic flow signal. The aorta was constricted until the regurgitant flow disappeared (Fig 3). The off position was when the ring was completely loosened. Twenty consecutive beats were recorded at each protocol position. The data was collected with the ventilator off to avoid respiratory variations. In addition, epicardial Doppler echocardiography (3.5 Mhz transducer [Siemens Acuson, Mountain View, CA]) was performed to further confirm the presence of AI with the aortic ring in the off and on positions.
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Fig 3. (a) Recording of aortic flow in one sheep with aortic regurgitation. The hatched area represents the area measured to determine regurgitant volume. (b) Aortic flow after ring device tightened.
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Data Analysis
Hemodynamic data was analyzed using the computational program Matlab (The Mathworks, Natick, MA). The severity of aortic regurgitation was ascertained by quantification of aortic regurgitant volume and estimation of regurgitant orifice area using the Gorlin equation [4, 5]. Aortic regurgitant volumes were determined from negative flow tracings of the aortic regurgitant flow signal recordings. For each variable, the mean of five consecutive cardiac cycles was calculated.
Statistics
All measurements were expressed as the mean ± standard error. The paired Student’s t test was used to compare hemodynamic variables with the device on and off. Results were considered statistically significant at a p value less than 0.05.
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Clinical Experience
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Hemodynamic variables for all animals are shown in Table 1. No significant AI was present in any of the ovine at baseline. All five ovine tolerated patch plasty of the aortic root and were successfully weaned off cardiopulmonary bypass. Aortic insufficiency developed in all five ovine with a mean hemodynamic regurgitant orifice area of 0.07 ± 0.03 cm2 and a mean regurgitant volume of 3.2 ± 1.9 mL/beat, with a corresponding regurgitant fraction of 18 ± 4%. Application of the ring decreased the hemodynamic regurgitant orifice to 0.01 ± 0.00 cm2 (p < 0.01; Fig 4, left panel). This represented an 88% decrease in hemodynamic regurgitant orifice area. Similarly the regurgitant volume decreased to 0.4 ± 0.2 mL/beat (p < 0.05). This corresponded to a reduction of regurgitant fraction to 2 ± 1% (p < 0.05) as a result of the external ring (Fig 4, right panel). There was no statistically significant change in aortic pressure with the ring tightened versus loosened (p = 0.24).
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Fig 4. Acute effects of the external aortic ring device in sheep. Depicted are the individual values for each animal: (left) regurgitant fraction (%) decreased with application of the aortic ring (p < 0.01), and (right) the hemodynamic regurgitant orifice area (cm2) decreased with application of the aortic ring (p < 0.01).
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Two-dimensional color flow Doppler echocardiograms were obtained with the ring off and on. Echocardiographic images corroborated the findings obtained by the hemodynamic recordings (Fig 5). Significant AI was present with the ring off. Aortic insufficiency occurred centrally and between the right and noncoronary cusps where the patch was placed (Fig 5b). This AI disappeared with the ring on.
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Fig 5. Two-dimensional epicardial echocardiograms: the acute effect of the aortic ring device on aortic regurgitation. (a) Ring off: diastolic regurgitant color flow Doppler signal extending into left ventricle. (b) Ring off: short-axis view demonstrating central and eccentric flow between the right and noncoronary cusps. (c) Ring on: trivial regurgitant flow during diastole with almost complete elimination of aortic insufficiency. (d) Ring off: reproducible, regurgitant flow in diastole after loosening of device. (Arrow indicates regurgitant Doppler jet; LA = left atrium; LV = left ventricle; RV= right ventricle.)
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Comment
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In this study our purpose was to present a new model of AI and evaluate a simple, less invasive valve-sparing technique to treat AI secondary to aortic root dilatation. Previous animal models of AI involved removal of normal aortic valve tissue and are not representative of the clinical pathology of interest here [6]. Our model creates acute AI due to aortic root dilation and failed cusp coaptation. The pericardial patch increases annular circumference by 20% and leads to dilation up to the sinotubular junction. The aortic cusps themselves are normal but do not coapt. We believe that no such animal model has been previously described to more closely resemble the human pathology.
We demonstrated that reducing the aortic annular circumference with an external constricting device corrected the regurgitant volume and orifice area to near normal values. Our adjustable nylon band allowed for gradual and appropriate tightening guided by real-time monitoring of systemic hemodynamics. We were able to restore normal valve cusp coaptation without detrimentally affecting the mean aortic pressure or cardiac output.
The concept of adjusting aortic root circumference to eliminate AI is not new. In 1958, Taylor and colleagues [7] published their technique of "circumclusion," in which a silk suture was placed and tightened around the base of the aorta in 11 patients with AI. However, at that time no objective techniques (ie, intraoperative echocardiography or quantitative real-time measures of aortic flow) existed to guide the degree of aortic constriction. Their results were inconsistent due to lack of such precise techniques to assess their therapy. In 1984, Frater [8] demonstrated that adjustment of the intercommissural distance at the sinus rim level may eliminate AI. Presently, repair methods such as commissural and subcommissural annuloplasty alter the aortic root circumference by plication of the aortic wall with pledgeted "U" stitches [9].
Although this study represents a preliminary step in an ovine model and further technique and technology development is required, a number of potential clinical applications can be envisioned. We believe this technique might be applicable in patients with AI secondary to root dilatation who are not candidates for traditional aortic valve or root surgery due to prohibitive comorbidities. Residual or recurrent AI after aortic remodeling surgery, the Ross procedure, or valve replacement using stentless aortic valves is typically secondary to root or aortic dilatation. An aortic ring, either placed prophylactically at the original operation, or as an intervention to correct such a development, may be considered. Another potential application might exist for patients who present for cardiac surgery (ie, coronary bypass or mitral valve surgery) with moderate AI. At the present time, aortic valve replacement is not indicated here because the morbidity and mortality of current techniques is too high to justify such an operation [1]. However, aortic insufficiency is a progressive disease, and early intervention might be desirable. A less invasive technique may be an attractive management option for these patients (even at a lower threshold of AI) compared with traditional indications for intervention.
This investigation represents a feasibility study and demonstrates the concept of using an external aortic ring to reduce AI secondary to a dilated aortic root in an ovine model. However, the model has limitations with important differences to human pathology. The pericardial patch primarily affects coaptation of the cusps adjacent to the patch. In clinical practice, dilatation is typically more uniform and affects all three cusps. However, because the band exerts radial force uniformly to the aortic tissue, this limitation should be inconsequential to our data interpretation or study goals. In practice, aortic tissue will also be more calcified and less compliant than ovine tissue in this model. In patients with heavily calcified aortic roots, this technique may not be applicable. This study only evaluates the acute effects of this device. The device used in this study was devised to test our hypothesis and is not the most appropriate for long-term implantation. The long-term durability and possible erosion of an external aortic ring into the aorta or adjacent structures must be evaluated through chronic studies. These studies should use a more robust ring composed of biocompatible materials. In addition, we chose to position the ring below the right coronary artery and above the left because dissection under the left coronary artery increased animal mortality in our experience. Any actual clinical practice of an aortic ring would likely involve placing the ring above both coronaries or below both coronaries.
In summary, we have developed a new in vivo ovine model of aortic root dilatation that results in acute AI. Our study demonstrates that reduction of aortic root circumference by an external aortic ring reduces AI in this model. In the future, an external adjustable aortic ring may prove useful for managing AI secondary to dilatation of the aortic root when valve or root replacement is not possible.
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Disclosures and Freedom of Investigation
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The technology described in this study was conceived, developed, and constructed by the authors independent of any external organizations. All authors had full control of the design of the study, methods used, outcome measurements, analysis of data, and production of the written report.
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Disclaimer
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The Society of Thoracic Surgeons, the Southern Thoracic Surgical Association, and The Annals of Thoracic Surgery neither endorse nor discourage use of the new technology described in this article.
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Acknowledgments
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This work was supported in part by the Brigham and Women’s Hospital Cardiac Surgery Research Fund and the Brigham and Women’s Hospital Department of Surgery.
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References
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- Manouguian S, Abu-Aishah N, Neitzel J. Patch enlargement of the aortic and mitral valve rings with aortic and mitral double valve replacement: experimental study J Thorac Cardiovasc Surg 1979;78:394-401.[Abstract]
- Reimold SC, Byrne JG, Caguioa ES, et al. Load dependence of the effective regurgitant orifice area in a sheep model of aortic regurgitation J Am Coll Cardiol 1991;18:1085-1090.[Abstract]
- Gorlin R, Gorlin SG. Hydraulic formula for calculation of the area of the stenotic mitral valve, other cardiac valves, and central circulatory shunts. I Am Heart J 1951;41:1-29.[Medline]
- Reimold SC, Aranki SF, Caguioa ES, et al. An external aortic root device for decreasing aortic regurgitation: in vitro and in vivo animal studies J Card Surg 1994;9:304-313.[Medline]
- Taylor WJ, Thrower WB, Black H, Harken DE. The surgical correction of aortic insufficiency by circumclusion J Thorac Surg 1958;35:192-205.[Medline]
- Frater RW. Aortic valve insufficiency due to aortic dilatation: correction by sinus rim adjustment Circulation 1986;74:I136-I142.[Medline]
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Abstract
Introduction
