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(Circulation. 2005;111:3186-3187.)
© 2005 American Heart Association, Inc.

Editorial

Is Prosthesis–Patient Mismatch a Clinically Relevant Entity?

Tirone E. David, MD

From the Division of Cardiovascular Surgery, Toronto General Hospital, and the University of Toronto, Toronto, Ontario, Canada.

Correspondence to T.E. David, MD, Toronto General Hospital, 200 Elizabeth St–4N457, Toronto, Ontario M5G 2C4, Canada. E-mail tirone.david{at}uhn.on.ca

Key Words: Editorials • aortic valve • prosthesis • stenosis

In this issue of Circulation, Koch and associates¹ from the Cleveland Clinic published a study on the relationship between prosthetic valve size and the Duke Activity Status Index (DASI) after aortic valve replacement (AVR). The study was conducted with 1014 patients operated on from 1995 through 1998, completed 1 year later, and published now. DASI scores increased from a mean of 29 preoperatively to 46 postoperatively after a mean follow-up of 8.3 months. The investigators could find no obvious relationship between prosthetic valve size and postoperative DASI score. One of the limitations of the study was that prosthetic valve gradients and effective orifice areas were not measured by echocardiography to quantify prosthesis–patient mismatch. It is also interesting to note that more than two thirds of the patients were in New York Heart Association functional classes I and II preoperatively, and yet the DASI mean score before surgery was only 29 out of a maximum of 58.2. For this reason alone, it must be assumed that factors other than the aortic valve disease played a role in the DASI score. Nevertheless, this is not the first study to suggest that valve size plays no role in the clinical outcomes of AVR.

See p 3221

Valve prosthesis–patient mismatch (PPM) is a term introduced by Rahimtoola in 1978 to describe a condition in which the in vivo prosthetic valve effective orifice area is smaller than that of the native valve.² According to this broad definition, every patient with a prosthetic heart valve has PPM because the leaflets of both mechanical and bioprosthetic valves are mounted into frames that occupy space in the periphery of the valve where the loss of effective orifice is greater than in its central portion. This loss of effective orifice area may or may not be clinically significant depending on the size and type of prosthetic valve implanted; however, as with native heart valve disease, the thresholds between normal valve orifice and pathophysiologically important stenosis are quite broad and the clinical and hemodynamic consequences are variable.

Native aortic stenosis is considered severe when the peak systolic gradient exceeds 50 mm Hg in the presence of normal cardiac output or the effective orifice area is <0.5 cm²/m². Most patients with this degree of aortic stenosis remain symptom-free for many years and their likelihood of survival is good as long as they remain asymptomatic.^3,4 Once symptoms develop, the prognosis changes dramatically, with a 2-year survival rate <50%.^5,6 Thus, in adults with native aortic stenosis, in contrast to those with aortic regurgitation, clinical outcome is most closely related to the presence or absence of symptoms.

Prosthetic aortic valve stenosis should not behave differently if similar degrees of left ventricular obstruction are present. Because all prosthetic heart valves are intrinsically stenotic, there must be a degree of obstruction that is harmful to the patient. Therefore, do obstructive prosthetic valves increase operative mortality, impair functional recovery, and affect survival after AVR?

Blais and colleagues⁷ defined PPM as "severe" when the prosthetic aortic valve effective orifice area was <0.65 cm²/m², as "moderate" when the area ranged from 0.65 to 0.85 cm²/m², and "normal" when it was >0.85 cm²/m². These levels of PPM had a profound effect on operative mortality rate in a consecutive group of 1266 patients who had AVR and had undergone echocardiographic assessment of prosthetic valve function.⁷ Ejection fraction <40%, infective endocarditis, urgent/salvage operation, cardiopulmonary bypass >120 minutes, and PPM were independent predictors of operative mortality.⁷ The overall operative mortality rate was 4.6% in that series, and moderate PPM had a risk ratio of 2.0, whereas severe PPM had a risk ratio of 12.6. PPM was particularly harmful in patients with impaired left ventricular ejection fraction. A recent review of the Society of Thoracic Surgery National Cardiac Database disclosed a cohort of 33 611 patients who had isolated AVR between 2000 and 2003, and the prosthetic valve effective orifice area was an independent predictor of operative mortality.⁸ Most studies that addressed this issue showed that operative mortality for AVR is increased when a small prosthetic heart valve is implanted.^9–17 Although the majority of these studies used multivariate analysis to determine the incremental risk of small prosthetic valves on operative mortality, the reality is that patients with small aortic annulus are usually older, have more severe left ventricular hypertrophy with impaired diastolic function, and frequently have coronary artery disease. In addition, AVR in a patient with a small aortic root is technically more demanding than is AVR in a larger root, and the surgeon’s experience plays a crucial role in the outcome. Therefore, because multiple measurable and nonmeasurable factors affect the operative mortality of AVR, it is difficult to quantify the role of small prosthetic heart valves on operative outcomes. Patients with anatomically small aortic annulus (transverse diameter <18 mm by echocardiography) or narrow left ventricular outflow tract (<15 mm) are rare but present a formidable challenge to the surgeon because no commercially available prosthetic heart valve fits in an aortic annulus that small; even if a stentless biological valve is implanted, the residual gradient across the outflow tract is so high that a poor outcome is inevitable. Luckily, these patients are not common, and the treatment for them is extensive septal myectomy and reconstruction of the aortic root to accommodate a hemodynamically adequate prosthetic valve.

The effects of PPM on functional improvement after AVR are even more difficult to evaluate because concurrent cardiovascular and noncardiovascular diseases affect clinical outcomes. Pibarot and colleagues were the first investigators to point out the deleterious effect of PPM of bioprosthetic valves on functional status after AVR.^18,19 More recently, Ruel and associates analyzed the longitudinal outcomes of AVR in 1563 patients and found that PPM (effective orifice area <0.80 cm²/m²) was an independent predictor of postoperative congestive heart failure, and larger prosthetic valves had a beneficial effect on symptoms.²⁰ In that study, PPM had no effect on overall survival. In their article in the present issue of Circulation, Koch et al¹ find no correlation between prosthetic aortic valve size and postoperative DASI after AVR.

Finally, does PPM affect survival? Blackstone and associates⁹ examined this issue in a cohort of 13 258 patients who had AVR with various types of heart valves. After adjusting for other risk factors, they found that valve size had no effect on survival up to 15 years. In a study by Hanayama and colleagues¹⁰ on 692 patients who had AVR and postoperative echocardiographic assessment of the prosthetic valve, severe mismatch (effective orifice area <0.60 cm²/m²) had no effect on survival up to 7 years after surgery. Even more intriguing in that study is the fact that left ventricular mass index had no effect on survival. In that series, however, the estimated 7-year survival rate of patients with severe mismatch was 95%; they had a mean age of 62 years at the time of surgery, 38% had coronary artery disease, and two thirds were in New York Heart Association functional classes III and IV. These are variables that predict poor survival after AVR; however, most published studies show that PPM has no effect on survival after AVR.^11,21,22

In conclusion, obstructive aortic prosthetic valves (effective orifice area <0.80 cm²/m²) may increase operative mortality and impair functional recovery after AVR, but because there are so many variables that interfere with postoperative clinical outcomes, it is difficult to establish the importance of the hemodynamic performance of the valve. I believe that small prosthetic aortic valves should be avoided in large and physically active patients as well as in those with impaired left ventricular function to reduce the operative risk and optimize functional recovery. Moreover, PPM should be considered as a mechanism to explain the lack of functional improvement after AVR.

	Footnotes

 
The opinions expressed in this article are not necessarily those of the editors or of the American Heart Association.

	References

Top References

 

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