J Thorac Cardiovasc Surg 2005;129:961-965
© 2005 The American Association for Thoracic Surgery
Prosthetic aortic valve replacement
Hans-Hinrich Sievers, MD*
Department of Cardiac Surgery, University of Schlewwig-Holstein, Campus Luebeck, Luebeck, Germany
Received for publication December 22, 2004; accepted for publication December 23, 2004.
* Address for reprints: Hans-Hinrich Sievers, MD, Department of Cardiac Surgery, University of Schlewwig-Holstein, Campus Luebeck, Ratzeburger Alle 160, Luebeck D-23538, Germany (E-mail: herzchir{at}medinf.mu-luebeck.de).
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The size-sizing problem
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Aortic valve replacement is the second most frequently and increasingly performed cardiac operation worldwide.1 Although the operative techniques and the performance of valve substitutes have remarkably improved during recent decades, the search for an ideal replacement valve still continues. Among the various characteristics used to compare the functional quality of innovative heart valve substitutes, the transvalvular pressure gradient (PG) and effective orifice area (EOA) are the most common. In this regard the publication of Eichinger and colleagues2 in this issue contributes important data for comparing novel bioprostheses in the aortic position, all the more because these data are based on a prospective randomized study at rest and exercise. However, this report also brings out some items that complicate comparison of valve substitutes with relation to size-sizing of both the patient’s aortic root and the prosthetic substitute (Figure 1), some aspects of which will be addressed in this editorial.
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Figure 1. Interrelating different items in the size and sizing problem of prosthetic aortic valve replacement. IGOA, Indexed geometric orifice area; SGOD (z), standardized geometric orifice diameter; IEOA, indexed effective orifice area; EOA, effective orifice area; GOA, geometric orifice area.
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Rationale for sizing
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Although the clinical significance of prosthesis-related postoperative obstruction remains not fully established, pathophysiologic considerations and a growing body of study data3 provide sound reasons to relieve the native valve stenosis as completely as possible, warranting optimal left ventricular mass regression,4 maximal functional improvement, and a low incidence of adverse cardiac events. Especially patients with impaired left ventricular function are reported to benefit from relief of stenosis.5 All valve substitutes, however, leave some kind of residual obstruction dependent on design, size, material, and implantation technique.
The residual transvalvular PG is the most commonly used indicator to assess the residual obstruction and thereby the functional quality of a prosthesis. However, the PG is dependent on flow (Q) and EOA as follows:
(where K is constant), indicating that the PG alone is not sufficient to allow for relevant analysis without being related to flow and EOA. The EOA is a functional characteristic of the substitute valve representing the minimal cross-sectional area effectively occupied by the transvalvular flow and is derived echocardiographically by using the continuity equation as follows:
Another nonfunctional but geometric dimension is the internal geometric orifice area (GOA). Both these dimensions have their own advantages and disadvantages,6 and both are related to the internal diameter of the prosthesis. In this regard it is noteworthy that the diameter has a remarkable influence on area, especially at low dimensions; for example, a 10% decrease in diameter from 20 mm to 18 mm roughly results in a 20% decrease in area and furthermore a considerable increase in PG because of the square relation between PG and EOA. Thus exact sizing of the patient’s annulus is indispensable to implant the largest valve possible that fits into the annulus.
EOA and GOA are indexed to the body surface area (BSA) of the patient as a rough correlate of cardiac output and flow to neutralize for flow. EOA/BSA correlates nicely to PG (r = 0.79) in an exponential fashion7 in the sense that EOA/BSA can be decreased over a wide range without significantly changing the PG until 0.85 cm2/m2 BSA, when a steep increase in PG occurs with detrimental clinical consequences.4 Related to the indexed GOA, this inflexion point is around 1.2 cm2/m2 BSA, and related to the standardized prosthesis orifice diameter (standardized geometric orifice diameter = z), it is –2.5 z.8 These values might serve as a threshold indicating patient-prosthesis mismatch (PPM), in other words, when the prosthesis is suspected of being too small for a particular patient. Because all valves have their own EOA, GOA, and geometric orifice diameter, the surgeon can easily judge from sizing the annulus whether the particular valve that he or she prefers and that will fit into the annulus is appropriate with respect to the PPM. Thus sizing also gives the surgeon an idea of the right valve and, as such, a rough estimate of the expected postoperative relief of obstruction.
Most surgeons use only some particular types of valves, getting more and more familiar with the sizing of that valve and thereby a feeling of the ease and safety of implantation.
Together, sizing holds an eminent place in the interrelation between the choice of an appropriate large prosthesis and the ease and safety of implantation.
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Surgical anatomy of the aortic root
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Discussing sizing, sizers, prostheses, and implantation techniques makes consideration of the anatomy of the aortic root as the counterpart for sizers and prostheses indispensable. The aortic root is a complex 4-dimensional structure,9 following a highly sophisticated and still not completely understood function.10,11 Through 3 segments of an ellipse, the cusps are attached to the wall of the aorta supported by thickened, dense fibrous tissue. These fibrous thickenings form an annulus that has a crown-shaped configuration from the lateral aspect and a ring or annulus-like appearance when viewed from the top. These 3 attachments confine the sinuses on one side as the most proximal part of the aorta and the intervalvular trigones on the other side, which at least hemodynamically belong to the left ventricle. Therefore the aortic root incorporates elements of the aorta and the left ventricle, outlining its bridging function. There is, however, no solid continuous anatomic circular annulus or ring in which to place the anchoring sutures as a geometric exact counterpart for the perfect sewing rings of the prostheses. Nevertheless, for practical reasons, the term aortic annulus exists in the surgical field and will continue to do so. It consists of 2 different but somewhat overlapping components (Figure 2).12 First is the surgical annulus, which includes the proximal parts of the elliptical fibrous attachments of the cusps. This anatomic substrate is the strongest to warrant safe anchoring of the sutures. Second is the basic annulus, consisting of the nadirs of the elliptical attachment of the cusps, the septal muscle, the ventricular membranous septum, and the distal end of the aortomitral curtain, together termed sometimes the ventriculoarterial junction9 and defining the smallest cross-sectional area between the left ventricle and the aorta. As such, this basic annulus defines the width of the root as measured from the sizers and also the seating of the circular prostheses because they are fixed with sutures through the nadirs of the annulus. These considerations provide some insight into the complexity and discrepancy between the size and configuration of the aortic annulus and the sewing ring of the prostheses. Furthermore, it becomes clearer now that the definition of the position of the sewing ring in relation to the aortic annulus of the patient is more complex, as commonly used and as shown in Figure 3. For example, a so-called supra-annular prosthesis is only supra-annular at the nadirs of the suture line, but its sewing ring has to cross the surgical annulus in the area of the commissures, where it takes a subannular position.
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Figure 2. Opened-out view of the aortic root after excision of the cusps modified according to the method of Harlan and coworkers.18 The aortic annulus from a surgical point of view consists of 2 components, the surgical annulus (red interrupted lines), including the proximal part of the fibrous attachment of the cusps for anchoring the sutures, and the basic annulus (blue interrupted lines), indicating the smallest cross-sectional area of the aortic root and also the position of the circular prosthesis.
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Figure 3. Designation of dimensions of heart valve substitute sewing ring configurations according to International Organization for Standardization 5840. Additional schematic drawing of a longitudinal section of the aortic root through the nadir of the annulus shows the complexity of this issue; for example, on the right column tissue annulus diameter and internal orifice area are equal, suggesting that the tissue annulus extends to this level. So why is the lip in the sewing ring necessary? IOA, Internal orifice area; TAD, tissue annulus diameter; ESRD, external sewing ring diameter.
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Suture techniques
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Not only are the dimensions of the annulus of the patient important for determining the size of the prosthesis, but the suture technique is also important. Ventriculoarterial mattress sutures with Teflon pledgets (Figure 2) decrease the annulus diameter by at least 1 mm; this is less if simple standard sutures or continuous sutures are used. Sometimes, if a large enough valve cannot be implanted in the conventional manner, mattress sutures through the aortic wall above the annulus in the area of the noncoronary sinus can be placed, implanting the prosthesis in a true supra-annular position in this area. This might serve to safely anchor the prosthesis but also increases the risk of getting some undesired tissue beneath the prosthesis, potentially disturbing the flow through the valve. Also, the sequence of tying the sutures might influence the proper seating of a prosthesis preferably performed in a face-to-face mode. Careful decalcification is important to fully use the available width of the annulus. This increases the diameter of the annulus up to 2 mm, allowing a larger prosthesis to be implanted and also minimizing the risk of paravalvular leaks.
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Labeled and actual size of sizers and prostheses
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There is now growing evidence that in the majority of cases the actual sizer and valve dimensions vary considerably from the labeled diameters.13–16 Furthermore, the labeled size is unrelated to any hemodynamically significant dimension.17 The manifold possible interrelations (Figure 1) between the different labeled and actual dimensions, the configuration of the sizers and the aortic annulus, and the lack of a consistent pattern of these dimensional discrepancies seem to render the sizing confusion unsolvable. For example, the sizers for the 23-mm Perimount and 23-mm Mosaic bioprosthesis have actual diameters of 22.3 mm and 20.6 mm, respectively, and the diameter for the 25-mm Mosaic is 22.6 mm. Therefore from the sizer point of view, a 23-mm Perimount and a 25-mm Mosaic could have been used in most patients with a 23-mm aortic annulus. Taking into account these sizing problems could have had also influenced the results in the article by Eichinger and colleagues,2 and indeed, Seitelberger and associates18 did not find any difference in hemodynamic performance between the Perimount and Mosaic valves if the patient’s annulus, as measured with a metric sizer, was chosen as the reference dimension. But why do manufacturers label sizers up to 2 mm larger than the actual tissue annulus diameter of the prosthesis? Probably they wish to avoid the surgeon choosing too big a prosthesis that will not fit into the annulus and as such might cause a difficult situation requiring removal of the valve and reimplantation with a smaller one or placement of the valve in a tilted position, leading to impaired hemodynamics. Whatever the reason, these remarkable discrepancies in sizing do not allow comparison of different valves only by the labeled size of valves.
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Summary
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For surgeons, it might be advisable to know the functional and geometric characteristics, especially the GOA and the EOA, of the prostheses they use and to get an impression of the completeness of relief of obstruction by calculating the indexed EOA and indexed GOA for that particular patient in relation to the threshold value of 0.85 cm2/m2 and 1.2 cm2/m2, respectively. Lower values might increase the risk of adverse cardiac events, especially in younger patients who want to live an active lifestyle after valve replacement.
After complete decalcification of the annulus, measurement of the actual size of the annulus with a metric sizer for the records is indicated. However, this size should be ignored when asking for valve sizers of the particular valve because they do not have the same dimensions. The largest possible valve is identified, and the indexed EOA/GOA is calculated. Especially in small annuli, appropriate suture and tying techniques should be applied, and in younger and active patients, a PPM (indexed EOA <0.85 cm2/m2 or indexed GOA <1.2 cm2/m2) is favorably avoided, when necessary, by performing an annuloplasty. It is most important to find a passable compromise between the desire to implant a large valve and the ease and risk of proper implantation. Not only the dimensions but also the patient’s age, condition, comorbidities, expected lifetime, lifestyle, and so on are additional essential determinants for decision making to adapt the right valve for a particular patient.
Manufacturers should provide surgeons with the dimensions of the valve according to International Organization for Standardization standards and the GOA and EOA of their valve, preferably derived from in vivo data. The labeled size of the valve should reflect the tissue annulus diameter because this gives the surgeon a quick idea about the appropriate valve. These data must be clearly outlined on the valve container. To be more versatile, smaller valves should be closer graduated in sizes like 20, 21, 22, and 23 mm because these are the most critical diameters. Sizers themselves should imitate the configuration of the sewing ring of the corresponding valve and take into account the space needed for a particular suture technique. Sizers should be marked in addition to the labeled size of that valve with its EOA and GOA. This allows for quickly calculating the indexed EOA and GOA by dividing by BSA and thus is very useful to roughly assess the efficiency of the prosthesis in relation to PPM and the operative technique for the particular patient.
For postoperative evaluation, different prostheses should be compared by relating the patient’s actual annulus diameter to the PGs at rest and exercise in relation to flow, as well as the EOA and GOA.
Sizing confusion is likely to persist if we as physicians and especially our working groups do not define appropriate standards for valve and sizer dimensions to be reported by the manufactures and postoperative investigators. These standards should also consider characteristics of the aortic annulus, its relation to the sewing and mounting ring of the prosthesis, and implantation techniques. This might aid in providing more reliable parameters for comparing different prostheses and ultimately help in choosing the right valve for the varying needs of our patients.
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References
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- Eichinger WB, Botzenhardt F, Keithahn A, et al. Exercise hemodynamics of bovine versus porcine bioprostheses. a prospective randomized comparison of the Mosaic and Perimount aortic valves. J Thorac Cardiovasc Surg 2005;129:1056-1063.[Abstract/Free Full Text]
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Top
The size-sizing problem
Rationale for sizing
