ASBTRACT
Much has changed since the introduction of surgical valve repair in the
1950s, from the introduction bioprosthetic valves to percutaneous
approaches to valve repair. Yet, despite substantial advancements in
bioprosthetic valve technology, there has been a lack of direct,
independent comparison between bioprosthetic mitral valve devices,
accompanied by a marked heterogeneity in approaches to the sizing and
selection thereof. Wang et al. have hence endeavoured to evaluate,
head-to-head, the technical successes and biomechanical outcomes
associated with three different bioprosthetic mitral valves (Epic,
Abbott, IL; Mosaic, Medtronic, MN; Mitris Resilia, Edwards Lifesciences,
CA) in a porcine model, under standardised haemodynamic and anatomical
conditions. With a robust experimental technique, they have made clear
the heterogeneity in both sizing and biomechanical properties between
bioprosthetic mitral valves, and have further emphasised the need for a
uniform approach to the manufacturing and sizing of bioprosthetic
valves.
COMMENTARY
We read with great interest the manuscript by Wang, Caranasos, and
O’Neill et al. titled ‘Comparison of a new bioprosthetic mitral
valve to other commercially available devices under controlled
conditions in a porcine model’ .1 The authors astutely
identified a gap in existing research concerned with long-term outcomes
associated with bioprosthetic mitral valves (particularly, in terms of
echocardiographic gradients across the prosthesis, true-annular opening,
and propensity for left ventricular outflow tract [LVOT]
obstruction).1 They also highlighted issues with
mitral bioprosthetic valve sizing that have been identified by the Valve
Labelling Task Force as requiring regulatory evaluation.
Manufacturer-labelled dimensions for mitral bioprostheses appear to lack
consistency and are not evidence based; further, factors such as device
size selection, implantation technique, and instructions for use vary
greatly between manufacturers.1,2 In view of this,
Wang et al study sought to address this gap in research by comparing
outcomes associated with three different mitral valve bioprostheses.
When anatomically feasible, management of mitral valve pathologies
should be geared to securing optimal long-term outcomes. It follows
therefore that a robust analysis advocating the use of bioprosthetic
mitral valves should be centred on a high-risk population profile (e.g.
groups with blunted life expectancy, or incapacity for anticoagulation).
Henceforth, an early feasibility experimental study, featuring
controlled haemodynamic and anatomical variables, would need to be
conducted to allow a head-to-head comparison between commercially
available prostheses. To this effect, an evaluation of bioprosthesis
efficacy against these portrayed endpoints would facilitate
comprehensive appraisal of devices in question.
Wang and colleague’s study is, arguably, affected by several
limitations. Firstly, the study was carried out using a smaller number
of animals, restricting the number of bioprosthesis types and sizes
tested, and precluding the statistical analysis of reported outcomes.
This low sampling bias permitted a limited scope around the longevity of
valve in the face of structural deterioration which could necessitate
reintervention. Furthermore, the acuity of the study prevented the
long-term outcomes of each bioprosthesis from being evaluated, for
example in terms of paravalvular leak, ventricular remodelling, or
ejection fraction.4 These limitations are highlighted
and accounted for the in their study.1
Undoubtedly, there exists great heterogeneity in bioprosthetic mitral
valve sizing and application between manufacturers. This would come as
no surprise to the experienced cardiac surgeon or interventionalist – a
similar issue has been reported in aortic valve
prostheses.5 Interestingly, Wang et al. found that
different bioprostheses of the same size (namely, 27 mm Epic and Mosaic)
were associated with drastically different valve gradients, peak
velocities, LVOT, and SOA.1 The extent to which LVOT
may be attributed to differences in prosthesis strut protrusion into the
LV, or from abnormal subvalvular positioning of the prosthesis, leading
to outlflow tract obstruction, is unclear.
Bothe et al. highlight that the bioprosthesis-specific sizers provided
by the manufacturer vary greatly, and indeed, the numbers used to label
prosthesis-specific sizers are often arbitrary numbers, rather than
metrics – making any effort towards standardisation on this front more
complicated yet. For example, some manufacturers base sizer metrics on
the external stent diameter, without accounting for the sewing ring,
while others do.
A key message highlighted by Wang et al. is that considering the
significant heterogeneity between individual patient anatomical and
biomechanical characteristics, and between mitral valve bioprosthesis
sizing practices, a standardised, evidence-based strategy is required.
Yet, the potential for untoward colliding bias between exposure and
outcomes in the attribution and confirmation of thereof should be kept
in view. Perhaps a concept derived through practiced assessment of
annuloplasty rings, as highlighted by Ender et al., would also be a
viable option. They suggested an approach using superimposed
computer-generated models of annuloplasty rings onto live, 3D
echocardiographic images, and thus far have reported good results in
terms of correlation with ring size determined
intraoperatively.6 Yet, while these and other observed
approaches are novel and require further investigation, they help pave
the way for a more evidence-based mitral valve bioprosthesis sizing and
selection protocol. This would mandate further efforts to avail against
patient-prosthesis mismatch which is detrimental for patient quality of
life.
In conclusion, the manuscript by Wang et al. succinctly highlights an
area in dire need of further investigation and standardisation. Their
clinical feasibility experiment using a porcine model has produced
valuable findings towards the development of a standardised approach to
mitral valve bioprosthesis sizing and selection.