The Problem of Severe Valve Prosthesis-Patient Mismatch in Aortic Bioprostheses: Near Extinction?

Valve prosthesis-patient mismatch (PPM) was first described in 1978 by Rahimtoola as follows: “Mismatch can be considered to be present when the effective prosthetic valve area, after insertion into the patient, is less than that of a normal human valve.” Hence, the main consequence of PPM was observed to be higher than expected gradients through normally functioning prosthetic valves. However, it also became evident that the degree of obstruction and resulting gradients due to PPM for a given prosthesis could vary widely from one patient to another and was essentially related to the varying relation between the effective orifice area (EOA) of the prosthesis and the patient’s cardiac output requirements. Hence, the parameter proposed to characterize PPM severity has been the indexed EOA (EOAi; i.e., the EOA of the prosthesis divided by the patient’s body surface area). To this day, it is the only parameter that has been consistently related with postoperative gradients as well as clinical outcomes. On this basis, an EOAi ≤ 0.85 cm 2 /m 2 is now widely accepted as the threshold for PPM in the aortic position, with values between 0.65 and 0.85 cm 2 /m 2 classified as moderate PPM and those <0.65 cm 2 /m 2 as severe PPM. Depending on studies, the reported prevalence of moderate PPM varies between 20% and 70%, whereas that of severe PPM is between 2% and 15%. It should nonetheless be noted that the prevalence of severe PPM has had a tendency to decrease substantially over the past decade because of (1) increased recognition and awareness that, notwithstanding associated conditions, severe PPM is invariably associated with adverse outcomes and that it should thus be avoided as much as possible; (2) more widespread implementation of preventive strategies designed to avoid PPM; and (3) improved design and hemodynamic performance of newer generation prostheses.

Indeed, there is now a strong body of evidence showing that aortic PPM is an important risk factor with regard to various clinical outcomes, including improvement in functional class, regression of left ventricular (LV) hypertrophy, and both early and late survival. However, as opposed to most other risk factors associated with adverse clinical outcomes after aortic valve replacement, PPM is modifiable and can be largely avoided by calculating, at the time of operation, the minimum EOA that the prosthesis to be implanted should have to avoid PPM. And in case that requirement cannot be fulfilled by the type of prosthesis first considered to be implanted, one or more of the following strategies can be considered: (1) implantation of a prosthesis providing a larger EOA relative to the patient’s aortic annular size, (2) use of a stentless prosthesis, (3) aortic root enlargement allowing the implantation of a larger prosthesis, (4) use of transcatheter aortic valve implantation, which for a given annular size usually provides a larger EOA than that provided by surgically implanted prosthesis.

Despite considerable evidence that PPM significantly influences prognosis and may be successfully prevented, this issue nonetheless remains controversial. Indeed, some authors still challenge the appropriateness of using preventive strategies to avoid mismatch. The arguments used to defend this position are as follows: (1) EOA is only one of the many factors that must be taken into account when selecting a valve replacement device for an individual patient, (2) The projected EOAi of the prosthesis to be implanted has a low accuracy in predicting PPM, (3) The surgical techniques available to prevent PPM are generally more invasive, and a projected EOAi that falsely predicts the presence of PPM may cause the patient to undergo unnecessary risks; this argument is based on the premise that the main option available to avoid PPM is not the choice of a better performing prosthesis but rather the performance of more complicated procedures such as aortic root enlargement or the use of a stentless prosthesis.

Appearing in this issue of JASE are two reports pertaining to the performance of a new low-profile bioprosthesis (Trifecta; St. Jude Medical, St. Paul, MN). The study of Yadlapati et al . presents a comparison between the Trifecta prosthesis (n = 75 patients) and an earlier generation stented bioprosthesis (Epic; St. Jude Medical) (n = 49 patients) with regard to hemodynamic performance at rest, prevalence of PPM, and the occurrence of adverse clinical events. Their hypothesis was that because of its improved design, the Trifecta would be superior to the Epic prosthesis, and indeed, their results show that the Trifecta prosthesis exhibits larger EOAs, lower gradients, a lesser prevalence of PPM, and better clinical outcomes. The other study, by Levy et al ., pertains to resting and exercise hemodynamics in 85 patients having received Trifecta prostheses. The results show very satisfactory hemodynamics on both counts and a low prevalence of PPM. We believe both reports provide relevant information with regard to the controversy surrounding PPM.

Similar-Size Bioprostheses Are Not Created Equal

Earlier studies of prosthesis hemodynamics reported that, overall, stented bioprostheses exhibited a higher prevalence of PPM and thus higher postoperative gradients than similar-size mechanical prostheses. From these observations came the paradigm that more complicated procedures, such as aortic root enlargement or the implantation of a stentless prosthesis, would often be required to avoid PPM in patients not deemed to be suitable candidates for mechanical prostheses. The present studies, however, provide further confirmation that changes in the design of prosthesis may contribute to significantly improve its intrinsic hemodynamic performance. The study of Yadlapati et al . was a single-center study comparing two bioprostheses made by the same manufacturer but with completely different designs. Both were implanted in a supra-annular position, and the same sizer was used in both cases. The superior hemodynamic performance of the Trifecta prosthesis therefore cannot be attributed to differences in surgical technique or labeling of sizers but is rather likely due to an improvement in the intrinsic characteristics of the prosthesis. This observation is further supported by the fact that, size for size, the LV outflow tract diameter measurements before operation did not differ significantly between patients receiving either type of prosthesis. Indeed, the leaflets of the Trifecta prosthesis are mounted around the stents of the prosthesis, and as a result, the area available for blood flow through the valve is larger than when the leaflets are mounted inside the stents, as is the case in most other bioprostheses. The results of Levy et al . as well as of other recent series are consistent with these observations. By analogy, the results of Yadlapati et al . can also be compared with the results of Botzenhardt et al ., who studied two differently designed bioprostheses from the same manufacturer and similarly found that the prosthesis with the improved design was definitely better in terms of both hemodynamic performance and prevalence of PPM. Hence, size for size, the intrinsic design of a bioprosthesis can definitely make a difference in terms of hemodynamics, and from this standpoint, all bioprostheses are thus not created equal.

This point is further illustrated by Table 1 , which lists the average EOA values for different types of bioprostheses. For instance, it can be seen that the observed EOA values for size 21 and size 23 bioprostheses, which were the most frequently implanted sizes in both Yadlapati et al .’s and Levy et al .’s studies, vary from 1.2 to 1.8 cm- for size 21 and from 1.4 to 2.1 cm 2 for size 23, depending on the type of prosthesis (variations in size-labeling criteria among different manufacturers are not taken into account). Such differences are all the more important because the relation between EOAi and gradients is curvilinear and because gradients tend to increase exponentially when EOAi is <0.85 cm 2 /m 2 . This phenomenon is further illustrated by Figure 6 in Yadlapati et al .’s report when the results of the two groups are viewed globally. Finally, it should be emphasized that the improved hemodynamics provided by the Trifecta prosthesis do not appear to have been obtained at the expense of increased risk or a more complicated surgical procedure.

Table 1

Illustrative mean values for effective orifice areas in different types of aortic bioprostheses

Prosthetic Valve size (mm) Reference
19 21 23 25 27 29
Aortic stented bioprostheses
Mosaic 1.1 1.2 1.4 1.7 1.8 2.0
Hancock II 1.2 1.3 1.5 1.6 1.6
Carpentier-Edwards Perimount 1.1 1.3 1.5 1.8 2.1 2.2
Carpentier-Edwards Magna 1.3 1.7 2.1 2.3
Biocor (Epic) 1.3 1.6 1.8
Mitroflow 1.1 1.3 1.5 1.8
Trifecta (Yadlapati et al .) 1.1 1.7 1.9 2.7 2.9 2.4
Trifecta (Levy et al .) 1.8 2.0 2.2
Aortic stentless bioprostheses
Medtronic Freestyle 1.2 1.4 1.5 2.0 2.3
St. Jude Medical Toronto SPV 1.3 1.5 1.7 2.1 2.7

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May 31, 2018 | Posted by in CARDIOLOGY | Comments Off on The Problem of Severe Valve Prosthesis-Patient Mismatch in Aortic Bioprostheses: Near Extinction?

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