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J Thorac Cardiovasc Surg 1997;113:1122-1123
© 1997 Mosby, Inc.
LETTERS TO THE EDITOR |
Department of Cardiology
Galician General Hospital
University of Santiago
Faculty of Medicine
Santiago de Compostela, Spain
Reply to the Editor:
In their letter, Izzat and Yim essentially voice four criticisms of our results: first, that the transvalvular pressure drops that we report for mechanical prostheses are too high; second, that we should have taken measurements during exercise or after dobutamine infusion; third, that we should have investigated the dependence of patient-prosthesis mismatch on cardiac output rather than body surface area; and fourth (and presumably as a corollary of the first and third criticisms), that our recommendation that 19 mm valves not be implanted in patients with body surface areas greater than 1.7 m2 is irrelevant.
Their first criticism is based on three points: a theoretical argument relating to the high-velocity central jet of bileaflet valves; the fact that we find no significant difference between the transvalvular pressure drops of bileaflet valves and bioprostheses (which they believe to have higher transvalvular pressure drops); and comparison with the transvalvular pressure drops reported in three papers authored by, among others, Izzat.
With regard to the first argument, we point out that, regardless of what the velocities of the lateral jets may be, that of the central jet is the result of a pressure that is a valid measure of ventricular effort and can be accurately calculated from the measured velocities (the value so calculated essentially does coincide with catheter measurements
1). With regard to the second point, we note that bioprostheses do not necessarily have similar hemodynamics 2, 3; in all the small bioprostheses in our study the flaps were mounted outside the stent, and their hemodynamic performance is better than that of tilting disc valves and at least as good as that of bileaflet valves. 1-6 With regard to the third point, we frankly do not understand the transvalvular pressure drops reported in the papers they cite 7, 8 (e.g., mean transvalvular pressure drops of 3.12 ± 3.6 mm Hg for nine 21 mm St. Jude Medical valves [St. Jude Medical, Inc., St. Paul, Minn.], 4.87 ± 3.8 mm Hg for ten 21 mm CarboMedics valves [CarboMedics, Inc., Austin, Tex.], and 8.1 ± 8.4 mm Hg for eight 19 mm CarboMedics valves), which are between one half and one sixth of the values habitually found, not only by us, but by numerous other groups. For example, De Paulis and associates 4 reported values of 20.1 ± 7.1 and 12.3 ± 3.4 mm Hg, respectively, for 19 and 21 mm CarboMedics valves; Franzen and colleagues, 5 17.0 ± 5.6 and 18.0 ± 7.7 mm Hg, respectively, for 21 mm CarboMedics and St. Jude Medical valves; and Ihlen, 1 Chambers, 6 and their coworkers, 17.1 ± 5.6 and 19.4 ± 5.6 mm Hg, respectively, for 19 mm CarboMedics valves.
In response to their second criticism, we have recently submitted for publication the exercise results obtained in the study that was mentioned in our article as still in progress. We do not believe that publication of the "disturbing" results obtained at rest with 19 mm valves should necessarily have awaited completion of the fuller study.
In response to their third criticism, transvalvular pressure drops are influenced by many interrelated factors, including valve area, systolic volume (and hence cardiac output), outflow tract anatomy, ventricular function, and body surface area. No correlation between transvalvular pressure drop and any single factor appears to be stable under variation of other factors. For example, in one of the papers by Izzat and others, 8 there was a correlation between transvalvular pressure drop and cardiac output and transvalvular pressure drop and body surface area for one prosthesis type but not for another; unpublished results of our own, for 19 mm valves but not for larger sizes, exhibit correlations of transvalvular pressure drop with body surface area both at rest and after exercise and correlation with cardiac output only after exercise. On the other hand, quite an uncontroversial correlation exists between body surface area and aortic root size, which is what determines the size of prosthesis that can be implanted without a root enlargement procedure.
In view of the foregoing, we believe it is unnecessary to reply explicitly to their fourth criticism.
We incidentally point out what must surely be an error in two of Izzat's papers, 7, 8 both of which report a cardiac output of only 2.8 L/min in their group of 10 patients with 21 mm CarboMedics valves (of the value of 4.1 L/min they report for their 19 mm group). We also take this opportunity to point out another unconvincing feature of the data reported in one of the papers by Izzat and others 7: the finding that whereas the effective area of 19 mm CarboMedics valves was 1.37 ± 0.53 cm2, that of 21 mm valves of the same type was only 1.20 ± 0.62 cm2. The problem there is not so much that the order of the mean values is the reverse of what it should logically be, but that this inversion is presumably the result of standard deviations that are much larger than those usually reported (0.1 to 0.2 cm2 for 19 mm valves 1-6). One wonders whether these large standard deviations really reflect mainly interpatient variation, especially inasmuch as no data on evaluator variability are given.
To close, we reiterate our reservations as to the routine use of 19 mm prostheses and bioprostheses. The exercise results to which Izzat and Yim look forward, and whose recent submission for publication was mentioned earlier, show that the increase in transvalvular pressure drops across small valves during effort is, to say the least, alarming.
12/8/80407
References
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