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Peter Munk Cardiac Center, University Health Network, University of Toronto, 4N-464, Toronto General Hospital, 200 Elizabeth St, Toronto, Ontario, Canada M5G 2C4
(Email: vivek.rao{at}uhn.on.ca).
The concept of prosthesis-patient mismatch (PPM) after aortic valve replacement (AVR) remains controversial today as long-term follow-up studies with adequate power to detect survival differences continue to provide discordant results [1–5]. Our previous study in 2000 demonstrated impaired survival after 7 years in patients who had PPM defined as an indexed effective orifice area (EOA) of < 0.85 cm2/m2 [1]. Due to the fact that we used the manufacturer’s reported "in-vitro" EOA, we liberalized our definition of PPM compared with the 0.75 cm2/m2 defined by Rahimtoola’s [4] original description.
Similarly, Pibarot and colleagues [3] reported adverse hemodynamic effects of PPM in 392 patients undergoing stented AVR. Pibarot and colleagues’ [3] study revealed no survival differences, but important adverse effects of PPM on postoperative hemodynamics and symptom status. Subsequent studies by Pibarot and colleagues’ examined patients subjected to exercise echocardiography and documented severe hemodynamic compromise in patients with even moderate PPM. The study by Medalion and colleagues [5] reviewed 892 patients who received tissue and mechanical prostheses and used the calculated geometric orifice area to determine if PPM conferred a survival hazard. These authors found that overall mortality was not different at a mean of 5 years in those patients with PPM versus those who received an appropriately sized prosthesis. In this issue of The Annals of Thoracic Surgery, Nozohoor and colleagues [2] reviewed their experience with 372 patients and examined a different, surrogate endpoint, namely, diastolic function. Theoretically, any prosthesis that results in a residual obstruction to forward flow would be expected to create an outflow tract gradient and hence limit regression of left ventricular mass leading to persistent diastolic dysfunction. Indeed, the authors [2] found that PPM resulted in higher postoperative peak and mean transvalvular gradients. However, these differences did not translate into significant changes in left ventricular mass regression or echocardiographic assessments of diastolic function.
Intuitively, surgeons always attempt to insert the largest possible prosthesis in a given aortic root, so why is there a controversy? Those who believe that PPM has adverse hemodynamic effects argue for a change in operative strategy: a different prosthesis with more favorable hemodynamics (ie, stentless tissue or mechanical) or an annular enlargement procedure. In contrast, some surgeons argue that the increased operative risk attributable to a more complex operation (whether a stentless valve insertion or an annular enlargement) outweighs any potential risk of PPM. These surgeons point to the lack of convincing data implicating PPM as an independent risk factor for either short-term or long-term survival. Why are these studies negative? The most common shortcoming is inadequate power followed by an inadequate length or quality of follow-up. Indeed, in our previous study of PPM, survival curves did not diverge until 7 years of follow-up and required a sample size of 2,981 patients. Another important limitation is the choice of endpoint. The study of Medalion and colleagues [5] reported all-cause mortality in a population of patients that are at risk for noncardiac related death. Furthermore, few studies report quality-of-life data. Recalling the earlier important study by Pellikka and colleagues [6], even hemodynamically significant aortic stenosis is well tolerated for up to 5 years and only becomes dangerous when patients develop symptoms. Mortality of asymptomatic patients was only 4%. Surgeons would never make a decision regarding surgical intervention without knowledge of a patient’s symptomatic status, so why are we so willing to dismiss the concept of PPM without a consideration of a patient’s postoperative functional class?
The study by Nozohoor and colleagues [2] will no doubt add to the controversy surrounding this important subject; however, based on the previous discussion, the limitations of this study must be placed into context. Clinical follow-up is limited to an average of 1 year and echocardiographic data (determining LV mass and diastolic function), was only performed in 81% of patients at a mean postoperative follow-up of only 5 months. Importantly, echocardiograms were done in the resting state and no attempt was made to examine changes with exercise.
In the end, I firmly believe that surgeons act in their patient’s best interest, and when a more complex operation is contemplated in a high-risk, elderly patient, the decision to leave a degree of PPM may be justified. However, a similar decision-making process must be extended to the young, active, and large patient who will undoubtedly benefit from a more aggressive surgical approach to their aortic valve disease.
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