ABSTRACT
Large studies demonstrated that moderate or severe patient-prosthesis
mismatch (PPM) occurs in 44.2% to 65% of patients undergoing aortic
valve replacement. If there is general agreement that patients with PPM
have worse outcome than patients without, it is difficult to understand
how to prevent this dangerous complication. The formula used to
calculate the effective orifice area (EOA) of an implanted aortic
prosthesis has many weak points that produce inconsistent results using
the same prosthetic valve (type and size). The observed EOA (3 to 6
months postoperatively) of a #23 biological prosthesis can range from
0.9 to 3.5 cm², making PPM prevention impossible using projected EOA,
where only the mean value is reported (1.83 cm² for the same #23
biological prosthesis). An EACTS-STS-AATS Valve Labelling Task Force has
been established to suggest the manufacturers to present essential
information on valvular prosthesis characteristics in standardized Valve
Charts. For valves used in the aortic position, Valve Charts should
include a standardized PPM chart to assess the probability of PPM after
implantation. This will not solve completely the conundrum of
prevention, but most likely it will be a step ahead.
In this issue of the Journal, Luthra et al.1, using
the effective orifice area (EOA) as provided by the manufacturers, found
that long-term survival in patients who underwent aortic valve
replacement (AVR) was influenced by the absolute EOA or indexed EOA
(iEOA). Patients with moderate or severe prosthetic stenosis (EOA ≤1.5
cm²) had globally a lower survival than patients without (when AVR was
performed by a tissue valve, but not if a mechanical prosthesis was
used). Patients with moderate or severe patient-prosthesis mismatch
(PPM) (iEOA ≤0.85 cm2/m²) had a lower survival than
patients without.
There is general agreement that a significant prosthetic stenosis can be
a disease that can affect patients’ quantity and quality of life.
However, in order to prevent PPM, when an aortic valve is replaced by a
certain type of prosthesis of a given size, we need to know in the
surgical theatre if we are imposing a disease similar to that we are
treating. For this purpose, before starting a case, we consult the
projected prosthetic iEOA charts to select the appropriate valve size in
order to predict and prevent PPM. It is common experience that those
charts are often unreliable, as in large studies moderate or severe PPM
occurs in 44.2%2, 53.7%3 and
65%4 of the patients.
The EOA of a prosthesis is calculated applying the concept of the
continuity equation that the stroke volume (SV) ejected through the left
ventricular outflow tract (LVOT) all passes through the aortic valve
area (AVA) and thus SV is equal at both sites: SV(AV) = SV(LVOT).
Because volume flow rate through any cross-sectional area (CSA) is equal
to the CSA multiplied by the flow velocity time integral (VTI) over the
ejection period, this equation can be rewritten as: AVA*VTI(AV) =
CSA(LVOT)*VTI(LVOT). Solving for AVA yields the continuity equation AVA
= CSA(LVOT)*VTI(LVOT)/VTI(AV). Calculation of continuity-equation valve
area then requires three measurements: AVA jet velocity, LVOT diameter
for calculation of a circular CSA, and LVOT jet velocity.
The greatest potential source of error in the continuity equation is the
CSA of the LVOT. It is assumed that LVOT is circular. However, it should
be remembered that LVOT becomes progressively more elliptical (rather
than circular) in many patients, which may result in underestimation of
its CSA (squared in the equation) and in subsequent underestimation of
SV and eventually AVA. The location of LVOT measurement is as well
important. LVOT diameter has to be measured as close as possible to the
aortic annulus, whereas measurements done 5 to 10 mm below yield
significant underestimation of the stroke volume and of the
AVA5.
The phenomenon of pressure recovery is another problem that affects flow
velocity through the aortic prosthesis (in the continuity equation this
value is at the denominator: the higher the lower the EOA). Convergence
of flow through the stenotic aortic valve to the vena contracta converts
potential energy to kinetic energy with a resulting reduction in
pressure at the vena contracta6. As streamlines then
diverge and slow again distal to the vena contracta, there is
reconversion of some kinetic energy to potential energy with recovery of
a proportion of the pressure lost from the LVOT to the vena
contracta7. Because Doppler-based methods detect peak
flow velocity that occurs at the vena contracta, the transvalvular
pressure drop estimated by Doppler will be greater than that calculated
from simultaneous invasive pressure measurements in the LVOT and in the
aortic root distal to the vena contracta. Pressure recovery, at the same
EOA, is inversely proportional to the diameter of the ascending aorta,
as it is less pronounced when the proximal aorta is dilated because
energy is lost from nonlaminar flow and turbulence8,9.
Measurement of AVA using echocardiography does not account for aortic
size and the degree of pressure recovery and, therefore, may
overestimate the severity of prosthetic stenosis10.
Another method, intuitively more accurate and reproducible, is the
predicted iEOA, using the size and the model of the prosthesis divided
by body surface area (BSA). BSA depends on a constant (height) and a
variable (weight). It is very likely the BSA calculated with the lean
mass should be used, being the metabolic needs of the fat tissue very
small. This concept is present in the literature, as PPM in patients
with a body mass index >30 kg/m2 seems
not to be a risk factor for survival11. Recent
guidelines suggest lower threshold for PPM in obese
patients12,13.
All the problems in the correct evaluation of EOA and iEOA are evident
in several studies where patients with the same prosthesis
(Carpentier-Edwards Perimount #23, Irvine, CA, USA) showed a mean EOA
of 1.83 ± 0.413 cm², the EOA ranging from 0.9 to 3.5
cm²14. In the reference table, however, only the mean
value of 1.83 cm² is incorporated. In the PERIGON study, the EOA
measured at 3-6 months after surgery showed that there was a wide range
of EOAs for the same prosthesis (Avalus #23, Medtronic, Minneapolis,
MN, USA). The EOAs ranged from 0.92 to 2.40 cm² with a mean value of
1.51 ± 0.3 cm². Comparing the results of the measured iEOAs with the
projected iEOAs, the Authors found that the use of an iEOA chart led to
the incorrect prediction of PPM in 30% of the patients and severe PPM
in 22% of the patients15.
The fact that so many patients are misclassified as having PPM, based on
projected EOA values, is also relevant for the interpretation of studies
that examined the effect of PPM on survival. The majority of studies use
reference EOAs derived from the literature to calculate projected iEOA
values and determine the presence of PPM2-4. The
impact on long-term outcomes of PPM based on projected EOA, or the lack
of an impact, may be confounded by misclassification bias.
The solution to this conundrum is not easy. The jet velocity, measured
in the operating theatre, is not reliable, as it can be higher or lower
according to the patient’s anemic status, heart rate and ejection
fraction. All these variables will prevent any surgeon to explant a
prosthesis with high gradient trying to reimplant a larger one. The fact
that the same type and size of prosthetic valve can provide different
EOA, with/out PPM, makes PPM prevention difficult, as many components
come from the patient and are out of our control. Indexed EOA charts
provided by valve manufacturers have been severely criticized for their
inaccuracy16, being regarded by many as marketing
tools rather than useful clinical assets17. An
EACTS-STS-AATS Valve Labelling Task Force18 has been
established to suggest the manufacturers to present essential
information on valvular prosthesis characteristics in standardized Valve
Charts. For valves used in the aortic position, Valve Charts should
include a standardized PPM chart to assess the probability of PPM after
implantation. A Valve Chart like this would be surely a great
improvement, but PPM probability is still provided after dividing
patients into the classic categories. A recent study found a near-linear
relation between iEOA and mean aortic pressure gradient, a relation that
did not differ between patients with and without obesity, suggesting
that the use of lower cut-off values of iEOA to classify patients with
obesity should be reconsidered19.
The history of how to prevent PPM is not yet written and, perhaps, will
never be.