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J Thorac Cardiovasc Surg 1997;113:1121-1122
© 1997 Mosby, Inc.

LETTERS TO THE EDITOR

Assessment of the hemodynamic performance of small-size aortic valve prostheses

Department of Surgery
Division of Cardiothoracic Surgery
The Chinese University of Hong Kong
Hong Kong

To the Editor:

We read with interest the recent paper by Gonzàlez-Juanatey and associates ¹ investigating the influence of the size of aortic valve prostheses on hemodynamics and change in left ventricular mass. They concluded that 19 mm aortic prostheses continue to create significant obstruction of the left ventricular outflow tract and fail to bring about significant reduction in left ventricular hypertrophy.

One major point of criticism to this study is that each valve size group incorporated three very small subsets of patients receiving three different valve prostheses: two pericardial bioprostheses and one mechanical bileaflet prosthesis, yet the three subsets were grouped together when the hemodynamic variables were compared.

Doppler assessment of valve prostheses is complicated by an important phenomenon that greatly influences gradient and orifice area results—the nonuniform local velocities resulting from the unique geometry of each valve. ² A good example is bileaflet prostheses, in which the two leaflets partition the orifice of the valve into three unequal smaller orifices with a nonuniform velocity distribution and a higher velocity jet between the two leaflets. This jet is detected by the Doppler beam, which records higher transvalvular gradients than those measured by a catheter. This, however, is not the case in bioprostheses, in which Doppler ultrasonography accurately measures transvalvular pressure drop. ² It should be borne in mind, therefore, that comparing studies of valves of different designs and flow patterns may not always be valid.

Gonzàlez-Juanatey and associates did report that there were no significant differences in the gradients between patients with mechanical prostheses and patients with bioprostheses. Nevertheless, the lack of a difference is likely due to higher gradients in the bioprosthesis group, bringing a balance to the Doppler-exaggerated gradients of mechanical valves. This theory is conceivable because bioprostheses are known to generate higher gradients than mechanical prostheses of equal size. ³ It may also explain the significantly higher gradients reported by the authors for each size group compared with similar groups in previous studies. ^4-6

Another issue of concern is the simplistic use of gradients measured at rest in the assessment of prosthetic valve function. Such measurements may be misleading because, in addition to valve type and size, gradients are dependent on flow; hence small prostheses are known to produce high gradients at high cardiac outputs that are not seen at rest. ⁷ Performance of a valve prosthesis should, therefore, be evaluated under a range of flow conditions, and hemodynamic alterations induced by exercise or dobutamine stress have been proposed as satisfactory approaches for assessing valvular function. Indeed, we have used the latter method to evaluate and compare the performance of small mechanical and bioprosthetic aortic valves. ^4-6 Not only was the performance of small prostheses found to be satisfactory, but we also demonstrated repeatedly that patient-prosthesis mismatch is not related to body surface area but primarily to cardiac output.

Although we entirely agree with the authors that a size 19 mm prosthesis should probably not be implanted in a physically active patient, our data disagree with their recommendation that it should be used only in patients with a body surface area less than 1.7 m². We look forward to the results of the exercise hemodynamics that Gonzàlez-Juanatey and associates are currently performing and hope that larger groups of patients will be studied and analyzed separately to make the data more informative.

12/8/80409

References

1. Gonzàlez-Juanatey JR, Garcia-Acuña JM, Fernandez MV, Cedón AA, Fuentes VC, García-Bengoechea JB, et al. Influence of the size of aortic valve prostheses on hemodynamics and change in left ventricular mass: implications for the surgical management of aortic stenosis. J Thorac Cardiovasc Surg 1996;112:273-80.[Abstract/Free Full Text]
   
2. Chambers J, Fraser A, Lawford P, Nihoyannopolous P, Simpson I. Echocardiographic assessment of artificial heart valves: British Society of Echocardiography position paper. Br Heart J 1994;71(suppl):6-14.[Free Full Text]
   
3. Dumesnil JG, Yoganathan AP. Valve prosthesis hemodynamics and the problem of high transprosthetic pressure gradients. Eur J Cardiothorac Surg 1992;6(suppl 1):534-8.
   
4. Izzat MB, Birdi I, Wilde P, Bryan AJ, Angelini GD. Evaluation of the hemodynamic performance of small CarboMedics aortic prostheses using dobutamine-stress Doppler echocardiography. Ann Thorac Surg 1995;60:1048-52.[Abstract/Free Full Text]
   
5. Izzat MB, Birdi I, Wilde P, Bryan AJ, Angelini GD. Comparison of the hemodynamic performance of St. Jude Medical and CarboMedics 21 mm prostheses using dobutamine stress echocardiography. J Thorac Cardiovasc Surg 1996;111:408-15.[Abstract/Free Full Text]
   
6. Kadir I, Izzat MB, Wilde P, Reeves B, Bryan AJ, Angelini GD. Dynamic evaluation of the 21 mm Medtronic Intact aortic bioprosthesis by dobutamine echocardiography. Ann Thorac Surg. In press.
   
7. Teoh KH, Fulop JC, Weisel RD, et al. Aortic valve replacement with a small prosthesis. Circulation 1987;76(Suppl):III123-31.
   

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