The Relevance of Prosthesis-patient Mismatch After Aortic Valve Replacement

The term valve prosthesis-patient mismatch (PPM) describes a state in which the effective orifice area (EOA) of a normally functioning heart valve prosthesis is too small in relation to the patient's body size; this situation results in high transvalvular pressure gradients.^[1] The EOA is a physiological parameter that represents the minimal cross-sectional area of the transprosthetic flow jet, and is easily measurable by Doppler echocardiography. The only parameter that has been validated to identify PPM is the indexed EOA: that is, the EOA of the prosthesis divided by the patient's body surface area.^[2] The rationale for using this parameter is that pressure gradients are essentially determined by EOA and transvalvular flow, which in turn are largely related to body size and resultant cardiac output requirements. For aortic valve prostheses, PPM is considered to be severe when the indexed EOA is less than 0.65 cm²/m² and moderate when this value is between 0.65 cm²/m² and 0.85 cm²/m². PPM is classed as not clinically significant when the indexed EOA is greater than 0.85 cm²/m². Severe PPM occurs in 2-11% of patients after aortic valve replacement (AVR), whereas the prevalence of moderate PPM among these individuals is 20%-70%.^[3]

Strong evidence shows that patients with PPM have worse functional class and exercise capacity, reduced regression of left ventricular (LV) hypertrophy, inferior recovery of coronary flow reserve, impaired blood coagulation status, and more adverse cardiac events after AVR compared with patients without PPM.^[3,4] PPM is also associated with an increased risk of both perioperative^[5,6] and late mortality.^[4,6,7,8,9] The greatest mortality risk is observed in patients with pre-existing LV systolic dysfunction.^[5,7] The interaction between LV dysfunction and PPM is consistent with the concept that the increased LV afterload caused by PPM is less-well-tolerated in a poorly functioning ventricle than in a normal ventricle. Avoidance of PPM in patients with LV systolic dysfunction is, therefore, an important priority.

The severity of PPM is an important factor in determining patient outcome. Severe PPM is generally associated with worse outcomes in all patients, whereas the effect of moderate PPM is most evident in specific subgroups of patients, including those with LV dysfunction and individuals younger than 65 years old. By contrast, moderate PPM is tolerated relatively well in sedentary patients older than 65 years with preserved LV function.^[9] These observations could be indicative of the fact that younger patients have higher cardiac output requirements, in relation to higher basal metabolism and increased physical activity, than older individuals. One study has shown that PPM reduces survival in patients with a BMI less than 30 kg/m², but not in those who are obese (BMI ≥30 kg/m²).^[9] This finding should not, however, be construed to mean that PPM is better tolerated by obese patients than by nonobese patients, but rather to demonstrate the limitations of the indexed EOA as a valid parameter to identify PPM in this context. Future studies will be necessary to elucidate this issue. A potentially interesting avenue of enquiry might be to index the EOA by fat-free mass—as measured by bioimpedance—because this index seems to best reflect cardiac output requirements independent of body weight.^[9]

Apparent discrepancies amongst studies of PPM have raised controversy about the effect of the condition on clinical outcomes.^[3] These discrepancies are often related to the choice of parameter used to define PPM. Most discordant studies have identified PPM by use of either the in vitro EOA, provided by the manufacturer of the prosthesis, or the geometric orifice area calculated from the internal diameter of the prosthesis stent. In fact, these parameters have been shown to bear little or no relation to postoperative hemodynamics or outcomes^[3,10] and, therefore, they cannot be considered appropriate to identify PPM.

We believe that every effort should be made to prevent severe PPM in all patients. Furthermore, moderate PPM should also be avoided in patients who present with preoperative LV dysfunction or severe LV hypertrophy, in physically active individuals, and in patients younger than 65 years old. Studies have demonstrated that the risk of PPM can be accurately predicted at the time of AVR from the 'projected indexed EOA'.^[3,10] This value is calculated by dividing the normal reference values of EOA for the different models and sizes of prostheses, by the patient's body surface area. Several prospective studies^[3,10,11] have demonstrated that PPM can be prevented or reduced in severity, by systematically calculating the projected indexed EOA of the prosthesis and, in the case of anticipated PPM, by considering alternative procedures. One such procedure involves insertion of a prosthesis with an improved hemodyamic performance. It is important to emphasize that, for any given aortic annulus size, hemodynamic performance can vary widely from one prosthesis model to another. Indeed, hemodynamic performance is generally superior, and thus the prevalence and severity of PPM lower, in newer versus older generations of prostheses, in mechanical versus stented bioprosthetic valves, in supra-annular versus intra-annular stented bioprostheses, and in stentless versus stented bioprosthetic valves. A second possible option involves aortic root enlargement, to allow a larger prosthesis of the same model to be accommodated. Some groups have successfully reduced the occurrence of PPM using aortic root enlargement without any increase in operative risk.^[11] Nonetheless, this procedure should be considered only where PPM cannot be avoided by use of a prosthesis with an improved hemodynamic performance, and where the risk:benefit ratio of aortic root enlargement is considered acceptable. Preoperative calculation of the projected indexed EOA can likewise be used to avoid the unwarranted use of aggressive procedures such as aortic root enlargement. The importance of these considerations becomes particularly evident in Asian patients who often have a small aortic root. This characteristic is, however, often counterbalanced by the reduced cardiac output requirement inherent to small body size.^[12] In this patient population, therefore, the implantation of a small prosthesis with a good hemodynamic performance often provides a valve EOA that is large enough to accommodate their cardiac output requirements.

We believe that strategies to avoid or reduce the severity of PPM should be individualized and take into account multiple variables, such as patient age, BMI, lifestyle, LV function, degree of LV hypertrophy, and use of concomitant procedures. For example, if moderate PPM is expected to occur in an elderly, sedentary patient with normal LV function, the benefits of doing an alternate procedure to avoid PPM might be estimated to be outweighed by the inherent risks or disadvantages of doing such a procedure. By contrast, prevention of PPM becomes an important consideration in a young, athletic individual, or if the patient has evidence of impaired LV function or severe LV hypertrophy. We should also emphasize that implantation of a small prosthesis does not necessarily result in PPM, and can be perfectly adequate in a patient with small body size.

In conclusion, PPM is a prevalent and modifiable risk factor in patients who undergo AVR, and is associated with poor hemodynamic and symptomatic status, increased mortality, and an elevated rate of cardiac events. The effect of PPM on outcomes differs markedly depending on its severity as well as on the patient's preoperative status. This variation highlights the importance of tailoring the PPM preventive strategy to the baseline characteristics of the patient.

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Nat Clin Pract Cardiovasc Med. 2008;5(12):764-765. © 2008  Nature Publishing Group

Cite this: The Relevance of Prosthesis-patient Mismatch After Aortic Valve Replacement - Medscape - Dec 01, 2008.