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Ann Thorac Surg 1998;66:1198-1203
© 1998 The Society of Thoracic Surgeons
a Division of Cardiovascular Surgery, University of Toronto, Toronto, Ontario, Canada
b Sunnybrook Health Science Centre and the Toronto Hospital, University of Toronto, Toronto, Ontario, Canada
Address reprint requests to Dr Christakis, Sunnybrook Health Science Centre, 2075 Bayview Ave, Suite H-406, Toronto, Ont, Canada M4N 3M5
Presented at the Poster Session of the Thirty-fourth Annual Meeting of The Society of Thoracic Surgeons, New Orleans, LA, Jan 26–28, 1998.
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Abstract |
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Methods. The purpose of this study was to determine the ID and external diameter of aortic valves used at our institution and compare the measurements to manufacturers’ labeled sizes. We also evaluated valve size (ID, manufacturers’ labeled size) in 527 patients undergoing isolated aortic valve replacement between 1990 and 1996.
Results. We demonstrated that no two manufacturers’ tissue or mechanical valves have the same ID or external diameter for a given labeled size. The labeled size of tissue valves was 1 to 4 mm larger than the measured ID. The labeled size of mechanical valves was 3 to 5 mm larger than the measured ID. The St. Jude HP mechanical valve has a greater ID than all other mechanical valves for each labeled size. Among 403 patients operated on for predominant aortic stenosis, those patients receiving the Toronto Stented Porcine Valve (n = 98) had a larger mean ID (22.3 ± 1.9 mm) than 204 patients receiving stented tissue valves (ID = 20.9 ± 1.9 mm) and the 101 patients receiving mechanical valves (ID = 19.3 ± 1.9 mm, p < 0.0001). However, when the manufacturers’ labeled size was used as a measure of the size, the results were greatly exaggerated in favor of the Toronto Stented Porcine Valve (ID = 26.3 ± 1.9 mm) compared with stented tissue valves (ID = 23.1 ± 2.1) or mechanical valves (ID = 23.6 ± 1.9) (p < 0.0001).
Conclusions. Manufacturers’ labeling of valves is nonuniform and may lead to erroneous comparisons and conclusions of hemodynamic differences between valves. We therefore recommend a standardized nomenclature for the size of all valves based on the ID measurement.
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Introduction |
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TopAbstractIntroductionMaterial and methodsResultsCommentAcknowledgmentsReferences |
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The internal and external diameters of the St. Jude standard mechanical aortic valve labeled as 21 mm, are 16.7 and 25.5 mm, respectively (as reported by the manufacturer). Similarly, the internal and external diameters of the Modified Orifice Hancock II porcine aortic valve, which is labeled as 21 mm, are 18.0 and 26.5 mm, respectively (as reported by the manufacturer). It is unclear what a labeled valve number refers to. Hemodynamic characterizations and comparisons, therefore, cannot be made on the basis of a number with which industry labels valves.
The aim of this study is to summarize true dimensions of commonly used aortic valves at our institution and to evaluate the size of valves that are inserted in patients undergoing aortic valve replacement. Furthermore, we propose standardization of valve sizing by industry using internal diameter to minimize confusion and to allow for scientifically legitimate assessment of hemodynamic differences between valves.
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Material and methods |
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TopAbstractIntroductionMaterial and methodsResultsCommentAcknowledgmentsReferences |
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Surgical techniques
Surgical techniques for aortic valve replacement were exceptionally uniform among the three cardiac surgeons. Ascending aortic and two-stage single venous cannulation was used to place patients on cardiopulmonary bypass. Cardiac arrest was initiated by antegrade warm-blood cardioplegia and maintained with continuous retrograde or antegrade cold-blood cardioplegia. Venting was performed in all cases with a catheter inserted into the left ventricular cavity through the right superior pulmonary vein. The technique for sizing and implanting of valves including the Toronto Stentless Porcine Valve (SPV) has been well documented previously [3].
Valves measured
The following aortic valves were used for implantation:
Mechanical: Medtronic Hall, Carbomedics Standard, Carbomedics Supra-annular TopHat, St. Jude Standard, St. Jude HP.
Tissue valves: Toronto SPV, Carpentier Edwards Pericardial (Model 2700), Hancock II Porcine, Hancock II Modified Orifice.
Internal and external valve dimensions
To assess true valve dimensions, we measured the internal and external diameters of all valves. The internal diameter of an aortic valve refers to the largest diameter or potential space at the base of each valve, measured from inner mechanical or tissue surface to inner mechanical or tissue surface. The external diameter of an aortic valve refers to the largest external diameter of each valve as measured from the outer undisturbed cuff surface to outer cuff surface. Diameters were measured using a digital caliper micrometer (Mitutoyo, Plastic Vernier Caliber, model CD-6-CS; MSC Manufacturing, New York, NY). The resolution of these calipers was ±0.01 mm with an accuracy of ±0.04 mm. Measurements were performed in the operating room or laboratory, subject to valve availability. A minimum of five different valves were used for all valves labeled as greater than 19 mm. Only three 19-mm labeled valves were available for measurement.
Measurements of the internal diameter of mechanical valves were performed by placing the caliper hooks just touching the inner carbon surface of each valve without scratching or compressing the surface. An average of five measurements performed by a single investigator was used for each valve at five equidistant locations around the ring. The largest internal diameter was measured for valves lacking perfectly circular outflow (Carbomedics Standard and Supra-annular valves).
Measurement of the internal diameter of tissue valves was performed by placing the caliper hooks at the base of each valve just touching the area where the valve tissue was sutured to the stent or cuff. With stentless valves, this measurement was performed at the base of each valve at the interface of aortic wall and valve tissue. Measurements were performed without any compression or distortion of tissue, allowing caliper hooks to just touch the measured surface.
External diameters of all valves were measured with the caliper hooks just touching the outer surface of the cuff of each valve. The largest portion of each cuff was selected as the site of measurement. No distortion or compression of the cuff was allowed. When the cuffs were prefolded by the manufacturer (Carpentier Edwards Pericardial), no manipulation or unfolding of the valves was attempted.
In addition to measurements performed by the investigators, internal and external valve dimensions were solicited from all manufacturers when this information was available. The measurements in this study represent measurements performed by the investigators, as there was minimal deviation from manufacturer supplied information. These measurements were used in assessing the 527 patients undergoing aortic valve replacement. Measurements were rounded off to the nearest 10th of a millimeter, as accuracy measurements were within ±0.04 mm.
Clinical assessment of true valve sizes
During the study period (1990 to 1996) 527 isolated aortic valve replacements were performed at our institution. The manufacturers’ labeled valve sizes were recorded and collected prospectively for all cases. Patients were divided into two groups based on the preoperative aortic valve lesion (aortic stenosis or mixed disease, and aortic insufficiency). For each valve type implanted, the average labeled valve size and the average measured internal diameter were evaluated. Comparisons were also performed assessing differences in the average labeled and measured internal diameter size for the stented valves, stentless valves, and mechanical valves.
Statistical analysis
The average differences of diameters (internal and external), as performed with calipers by investigators, and measurements provided by manufacturer were calculated for each industry-labeled valve size. Differences in the average labeled size and measured internal diameter for each valve type were assessed by repeated measures analysis of variance.
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Results |
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Figure 2 compares the external diameter of aortic valves to the manufacturers’ labeled size for each valve.
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Assessment of clinical valve sizing
Among the 527 patients undergoing isolated aortic valve replacement during the study period, 403 patients had aortic stenosis or mixed valve disease as their major preoperative lesion and 124 patients had predominantly aortic insufficiency.
Of the 403 patients with preoperative aortic stenosis or mixed disease, 302 received tissue valves and 101 received mechanical valves. Of the 124 patients with predominant aortic insufficiency, 68 received tissue valves and 56 received mechanical valves. Figure 3 demonstrates the mean manufacturers’ labeled size implanted in the 403 patients with aortic stenosis or mixed disease. The mean SPV size implanted is more than 3.5 mm larger than the mean size of implanted stented tissue valves. The mean labeled valve size for patients who received mechanical valves appears to be similar except for patients receiving Björk-Shiley or Medtronic Hall valves.
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Comment |
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TopAbstractIntroductionMaterial and methodsResultsCommentAcknowledgmentsReferences |
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The real criterion for whether a valve is adequate or not depends on the surgeon’s forecast for the expected postoperative transvalvular gradient. The number with which a valve is labeled should therefore be directly correlated to expected postoperative gradients. Furthermore, to allow for adequate comparison of hemodynamics between valves, all valves should ideally follow the same numbering system.
The internal diameter of the valve or the potential free space for blood flow is a very important determinant of postoperative gradients. Additional variables that influence postoperative gradients include "leaflet material" (tissue versus mechanical), size, shape, leaflet type (eg, single, disc, bileaflet), the opening and closing angles of leaflets, the obstructive material in the outflow tract (muscle bar, cuff, ring, subvalvular or supravalvular stenoses), size and flexibility of stents in tissue valves, compliance characteristics of sinuses and aortic root and ascending aorta, stress absorption by sinuses, and patient-related factors such as body size and cardiac output. Of all the variables influencing postoperative gradients, the only factor that is common to all valves and that can easily have a number assigned to it is the internal diameter of the valve.
It can also be argued that the external diameter of a valve is an important factor influencing the size of the valve implanted. For a given annulus or root, the external diameter will determine how big a valve can be inserted. However, the external diameter of a valve reflects sizing issues. Sizing is a mechanical event comprised of hand–eye coordination, a sizing instrument, experience with sizing, the surgeon’s philosophy or bias, and the extent of annular decalcification, among other issues. These issues vary from surgeon to surgeon, and from prosthesis to prosthesis, and cannot be easily controlled. In addition, the external diameter of a valve may not influence the size of the valve inserted as much as the position in which the valve is inserted in the outflow tract. The Toronto SPV has been designed to be inserted in an intraannular position. The Carpentier Edwards pericardial and other stented tissue valves can be inserted in a supraannular position, thus allowing the aortic sinuses to take up the bulk of cuff tissue. Potentially, one could insert an SPV and Carpentier Edwards with dissimilar external dimensions, but similar internal diameters into the same annulus. The external diameter of the valves in this situation would not reflect the true size of the valve that could be implanted.
Results of this study
In this study we have determined that the number with which manufacturers label valves does not correspond to the internal dimension of the valves. There is no uniform nomenclature for valve size labeling. In a few situations, the number with which the manufacturer labels the valve corresponds to the external diameter. In the majority of cases, the labeled size of valve is unrelated to any hemodynamically significant dimension. A nonuniform nomenclature of valve sizes can result in erroneous hemodynamic comparisons and conclusions when comparing different manufacturers’ valves [4–8]. Our institution has previously reported the larger sizes used when implanting the Toronto SPV [3, 9]. In the present study, we demonstrated that the mean internal diameter of SPVs was larger than that of stented tissue valves implanted in patients proceeding to aortic valve replacement. However, when the manufacturers’ labeled size was used to assess the mean size of valve implanted, the differences were erroneously magnified. We have demonstrated in this study that a 25-mm SPV has a smaller internal diameter than a 23-mm Carpentier Edwards pericardial valve. Patients, however, were not randomized, and the aortic annulus size influences the type of valve implanted. Therefore, no conclusions or inferences can be made from these data.
In a recent study [10] we compared our experience with SPVs and stented tissue valves and demonstrated that the mean manufacturers’ labeled valve size for SPVs was 26.6 ± 2.1 mm, compared with the mean manufacturers’ labeled valve size for stented tissue valves of 24.0 ± 2.9, (p < 0.01). We could not, however, demonstrate any differences in peak or mean gradients, nor in left ventricular mass regression between stented and stentless valves 3 to 6 months postoperatively. In retrospect, if we had chosen internal diameter as a measure of valve size for the stented and stentless group, we would not have demonstrated a difference in the mean size of valve implanted. This would make our conclusion, that there was no difference in gradients and left ventricular mass regression between the two groups, more meaningful and logical.
David and associates [11] in a case-controlled and risk-matched study comparing Hancock II porcine tissue valves to the Toronto SPV demonstrated better hemodynamics in patients receiving SPVs for each manufacturers’ valve size subgroup. As the current study illustrates, a 25-mm Hancock II porcine valve should not be compared with a 25-mm SPV because the stentless valve has a smaller internal diameter. The hemodynamic results would probably have been even more favorable for the SPV had the investigators compared valves according to internal diameter. Future assessment of hemodynamic differences between valves may also be confounded and inappropriate if the current nomenclature for valve sizing is not altered. Investigators in the future may wish to compare the postoperative transvalvular gradient differences between the St. Jude 21-mm HP and the Carbomedics 21-mm supra-annular TopHat mechanical valves. Such a comparison would be inappropriate because the internal diameter of a 21-mm St. Jude HP is significantly larger than a 21-mm Carbomedics TopHat, unless the investigators could demonstrate that in all cases both valves would be the largest that could be inserted for each patient’s aortic root.
Proposed nomenclature for aortic valve sizes and protocol for assessment of hemodynamic performance differences between valves
The internal diameter of a valve is a major determinant of gradient, correlates with a surgeon’s idea of valve size, and can be practically assigned a number with which hemodynamic differences can be assessed. It would be difficult or impossible to determine the reason for hemodynamic differences between two valves if both valves did not have the same internal diameter. The manufacturers’ current system of sizing valves is not uniform and has resulted in inappropriate hemodynamic comparisons between valves. We propose that the valve industry change their labeling of aortic valves to the internal diameter of each valve. Furthermore, valve sizing instruments are a visual and sensory aid that assist in determining which valve to implant in a given aortic root. The actual size of the valve sizing instrument is not very meaningful, whereas the internal diameter of the valve that the sizer corresponds to may provide the surgeon with more important information. We therefore propose that valve sizers be labeled by the internal diameter of the valve they correspond to. To allow surgeons to compare the external diameter of each valve, one could alternatively also label sizers with their true external diameter.
To conclude that one manufacturer’s valve is hemodynamically superior to another manufacturer’s valve, investigators must compare valves according to the size of valve that can actually be fitted and implanted in a given patient’s aortic root. To scientifically prove that one manufacturer’s valve is hemodynamically superior to another’s, we propose all future hemodynamic assessments be performed in a prospective, randomized manner. We propose that investigators size the aortic root of a prospective study patient with sizers from both manufacturers’ valves, irrespective of which valve is eventually implanted. The investigators would therefore be able to compare the mean internal diameter of the valve that was inserted with the mean internal diameter of the valve type that could potentially have been inserted in the valve population.
Conclusion
Manufacturers’ labeling of valves is nonuniform and at times haphazard. This has led to erroneous comparisons and conclusions of hemodynamic differences between valves. We therefore recommend a standardized nomenclature for the size of all valves based on the internal diameter.
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Acknowledgments |
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TopAbstractIntroductionMaterial and methodsResultsCommentAcknowledgmentsReferences |
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References |
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TopAbstractIntroductionMaterial and methodsResultsCommentAcknowledgmentsReferences |
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p = 0.0001 STD versus MCH, for named valve size *p = 0.0001, SPV versus STD and SPV versus MCH.