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Ann Thorac Surg 1997;63:933-934
© 1997 The Society of Thoracic Surgeons
Sunnybrook Health Science Centre, University of Toronto, Toronto, Ontario, Canada
The study by Sawant and colleagues in this issue of The Annals [1] addresses the influence of small valves on long-term survival. Sawant and colleagues followed up 104 patients who received 19-mm St. Jude mechanical aortic valve replacements (mean of 5.48 ± 3.73 years), and demonstrated 5- and 10-year survival rates of 80.6% ± 8.3% and 61.6% ± 15.0%, respectively, similar to other reports of larger mechanical aortic valve prostheses. They also demonstrated that a body surface area of 1.7 m2 or greater was not a determinant of long-term mortality, suggesting that patient-prosthesis mismatch does not influence long-term outcome. Sawant and colleagues concluded that implantation of a 19-mm St. Jude mechanical prosthesis in small aortic roots is a reasonable alternative to other more complex procedures for inserting larger valves. Initially, their results and conclusions appear counterintuitive. However, the paucity of studies examining these issues requires that this report be more closely scrutinized.
As with most studies, review of the data by different investigators may lead to different conclusions. The 104 patients with 19-mm valves were selected from a population of 593 patients undergoing aortic valve replacement, a "small valve" prevalence of 18%. This extremely high rate is inconsistent with other previously reported series, suggesting a selection bias or an unusual patient population, and that the conclusions are not generalizable.
On close examination of the data, the reader will in fact discover significant selection bias and an inhomogeneous study group: the patients in this study are elderly (42% 
70 years old), female (89%), and with a small body surface area (1.61 ± 0.16 m2), which may preclude patient-prosthesis mismatch. The most striking evidence of inappropriate bias is that one quarter (24%) of the patients underwent both mitral and aortic valve replacement. This is a different patient population than straightforward aortic valve replacement for aortic stenosis, which may have influenced Sawant and colleagues' results and conclusions. Patients may have undergone operation for reasons related to mitral valve pathology and not necessarily severe aortic stenosis. Aortic valve replacement may have been coincidental or a secondary indication as revealed by the absence of data on gradients in this subpopulation, and the absence of the primary indications for operation. Furthermore, at least 9 patients underwent aortic valve replacement for aortic insufficiency. This would result in implantation of a stenotic 19-mm aortic valve in patients with a volume-overloaded ventricle, being converted to pressure overload hypertrophy. Little is known about the long-term effects of this subentity after aortic valve replacement. For the results of a study to be widely applicable, the patient population must be homogenous, which is clearly a major deficiency of this article.
In addition, the follow-up period may have been too short to make valid conclusions. Because only 15 patients were at risk for death at 10 years, the study may be underpowered. Although Sawant and colleagues conclude that body surface area of 1.7 m2 or greater was not a determinant of long-term survival, only 16 patients were at risk at 5 years of follow-up, and none were available for follow-up at 10 years. Therefore, the ability to answer whether patient-prosthesis mismatch influences long-term survival is also underpowered. Freedom from cardiac death was reported as 94.0% ± 5.2% at 5 years and 78.8% ± 14.7% at 10 years. However, 13 of the 18 late deaths (72%) were cardiac deaths.
To date, there are no controlled or randomized data in the surgical literature to suggest that implantation of a small aortic valve or patient-prosthesis mismatch reduces long-term survival. What then is the basis for our intuitive beliefs that smaller valves are detrimental to patients? After aortic valve replacement for aortic stenosis, the beneficial correlation of clinical improvement and significant reduction of transvalvular gradients is widely accepted. Complete abolition of gradients is not possible because of the obligatory sewing ring and design of valves. However, it has also been assumed that prosthesis size must be matched to patient size. Small valves may not provide clinical or hemodynamic benefit to a large patient in whom a high cardiac output across a small effective orifice area produces high transvalvular gradients. Mechanical aortic prostheses have consistently reported lower resting gradients after aortic valve replacement than tissue valves, probably due to complete opening of mechanical disks at all levels of cardiac output and to a more efficient ratio of sewing ring to effective outflow dimension. Nicoloff and associates [2] studied 22 patients with aortic stenosis by catheterization after aortic valve replacement with St. Jude mechanical prostheses. They demonstrated mean gradients of 6 ± 6 mm Hg for 21-mm valves and 3.5 ± 3.5 mm Hg for 29-mm valves. Resting gradients, however, may not be representative of the patient's status during exercise. Exercise capacity may be limited due to high exercise-induced gradients, especially in patients with small aortic prostheses. Wortham and colleagues [3] studied 17 patients with 19- and 21-mm St. Jude mechanical valves and demonstrated a mean resting gradient of 20 ± 3 mm Hg, which increased to 38 ± 4 mm Hg during exercise. It is unclear from the study by Sawant and colleagues whether the patients' exercise capacity was diminished, which could imply elevated transvalvular gradients.
It has been suggested that aortic valve replacement results in substitution of one disease for another with persistent, but not progressive aortic stenosis. Left ventricular hypertrophy may not resolve completely after valve replacement, especially if small valves are implanted. The long-term effects of residual left ventricular hypertrophy after aortic valve replacement are not well established. Concerns have been raised by various investigators, especially for patients with small aortic roots. He and associates [4] reported 30-year follow-up of 404 patients undergoing aortic valve replacement with small aortic roots (valve size 
21 mm). They demonstrated with a multivariable analysis that mismatch between body size and prosthesis is a negative determinant of long-term survival (body surface area <1.7, 50%; body surface area 1.7, 7%; p < 0.01) in patients with coronary artery disease. Lund and associates [5] provided additional indirect evidence that long-term survival after aortic valve replacement may be limited by residual transvalvular gradients. They demonstrated that of 176 late deaths, 23.9% were due to sudden cardiac-related events, and 30.1% were due to congestive heart failure. They concluded that suboptimal hemodynamic function of small valves and narrow aortic roots may have contributed to excess long-term mortality. In studies that have measured left ventricular mass index between 5 and 10 years after aortic valve replacement, compelling evidence exists that left ventricular hypertrophy persists due to the obstructive nature of aortic valves [6].
The lack of direct evidence that small aortic valves diminish long-term survival leads to a relevant and still unanswered question: can we improve patient survival by simply palliating from severe aortic stenosis to moderate fixed aortic stenosis? And is this survival comparable with that of patients receiving larger valves? The poor natural history of untreated severe aortic stenosis is only relevant for patients with symptoms of congestive heart failure, syncope, or angina. By relieving these symptoms with aortic valve replacement (even with small valves), is it possible that long-term survival is improved? Perhaps by decreasing gradients from 90 to 150 mm Hg to 30 to 50 mm Hg with 19-mm St. Jude valves, there is adequate reduction of left ventricular wall stress and hypertrophy, which may improve intermediate survival in elderly, small, inactive patients. As long as patients remain asymptomatic, perhaps even the long-term results may be positively influenced. This study, unfortunately, does not address these hypotheses due to the inhomogeneous make-up of the patient population and the relatively short follow-up period.
Footnotes
Address reprint requests to Dr Christakis, Division of Cardiovascular Surgery, Sunnybrook Health Science Centre, 2075 Bayview Ave, Suite H-406, Toronto, ON M4N 3M5, Canada.
References
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