(Circulation. 1998;98:1205-1211.)
© 1998 American Heart Association, Inc.
Clinical Investigation and Reports |
Application of Color Doppler Flow Mapping to Calculate Orifice Area of St Jude Mitral Valve
Dominic Y. Leung, MBBS, MRCP(UK);
James Wong, MD, PhD;
Leonardo Rodriguez, MD;
Min Pu, MD;
Pieter M. Vandervoort, MD;
; James D. Thomas, MD
From the Cardiovascular Imaging Center, Department of Cardiology,
Cleveland Clinic Foundation, Cleveland, Ohio. Dr Leung is now at the
Department of Cardiology, Prince Henry Hospital, Sydney, NSW, Australia. Dr
Vandervoort is now at Hartcentrum Limburg, Genk, Belgium.
Correspondence to James D. Thomas, MD, Department of Cardiology, Desk F15, Cleveland Clinic Foundation, 9500 Euclid Ave, Cleveland, OH 44195. E-mail thomasj{at}cesmtp.ccf.org
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Abstract
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Background—The effective orifice
area (EOA) of a prosthetic valve is superior to
transvalvular gradients as a measure of valve function, but
measurement of mitral prosthesis EOA has not been
reliable.
Methods and Results—In vitro flow across St Jude valves was
calculated by hemispheric proximal isovelocity surface area (PISA) and
segment-of-spheroid (SOS) methods. For steady and pulsatile conditions,
PISA and SOS flows correlated with true flow, but SOS and not PISA
underestimated flow. These principles were then used intraoperatively
to calculate cardiac output and EOA of newly implanted St Jude mitral
valves in 36 patients. Cardiac output by PISA agreed closely with
thermodilution (r=0.91, 
=-0.05±0.55 L/min), but SOS
underestimated it (r=0.82, =-1.33±0.73 L/min).
Doppler EOAs correlated with Gorlin equation estimates
(r=0.75 for PISA and r=0.68 for SOS,
P<0.001) but were smaller than corresponding in vitro
EOA estimates.
Conclusions—Proximal flow convergence methods can calculate
forward flow and estimate EOA of St Jude mitral valves, which may
improve noninvasive assessment of prosthetic mitral valve
obstruction.
Key Words: mitral valve • prosthesis • echocardiography
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Introduction
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Assessment of prosthetic heart valve function
with current techniques is imprecise. Transvalvular gradients
obtained by catheterization or Doppler
echocardiography are indicative of
prosthetic valve obstruction1 but are
highly flow-dependent. Furthermore, pressure recovery occurs in some
prosthetic designs, resulting in discrepancies between catheter
and Doppler pressure gradients.2 3 4 5 6
Analogous to native valve area, prosthetic orifice area is a
more flow-independent measure of obstruction. Unfortunately, the Gorlin
formula may be unreliable in this setting,7 and
the pressure half-time method,8 widely used in
native mitral stenosis, has not been validated for
prosthetic valves. Pulsed-wave Doppler has been used to
calculate prosthetic aortic valve area9
but is more problematic for prosthetic mitral
valves.
Analysis of the proximal flow convergence region on color flow
mapping can quantify mitral regurgitant
severity10 11 12 13 14 15 16 17 18 19 as well as low-velocity flow
across relatively large orifices such as stenosed native mitral
valves20 and atrial septal
defects.21 However, the utility of the proximal
flow convergence to measure prosthetic forward flow and
effective orifice area (EOA) is unclear.
The aim of this study was to investigate, in flow models and in the
operating room, the feasibility and accuracy of measuring flow across
St Jude valves and calculating their EOAs by analyzing the flow
convergence region proximal to the prosthesis by use of color
Doppler mapping.
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Methods
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Theoretical Background
The theory of the proximal flow convergence method is well
described.10 12 13 17 18 Briefly, flow converges
on a small orifice as concentric hemispheric shells of decreasing
surface area and increasing velocity. For a specific contour of
velocity va and radius r from the orifice
(highlighted as the blue-red aliasing boundary on color Doppler),
the instantaneous flow rate Q is given by
Q=2
r2va. However,
relatively low-velocity flow across large orifices such as
prosthetic valves causes flattening of the isotachs and
underestimation of flow,22 but this can be
countered by multiplying Q by v/(v-va), where v
is the peak transorifice velocity.
 | (1) |
The isovelocity surface area can also be calculated by the
segment-of-spheroid (SOS) method23 24 using the
chord (p) from the zenith of the contour to its outer edge (Figure 1
):
 | (2) |
EOA then is given by Q/v, where v is the transorifice velocity.
Although localized high velocities and pressure recovery have been
shown in the central orifice of the St Jude valve, we recently
demonstrated that pressure recovery across the lateral orifices of
mitral prostheses is limited.25 Because most flow
passes through the side orifices, these were subsequently used in the
EOA and stroke volume calculations.
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Figure 1. Flow convergence zone proximal to St Jude mitral
valve on color Doppler (left). Right, Same frame with color
suppressed. Radius (r) is measured to level of annulus of
prosthesis, and chord (p) is a segment of a spheroid.
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In Vitro Models
Steady-Flow Model
This in vitro flow model was described in detail
previously10 : a Plexiglas model with 2 chambers
(proximal, 28x20x8.5 cm [HxWxL]; distal, 90 cm long) divided by a
septum with a mount for prosthetic heart valves. St Jude heart
valves (3 each of 23, 25, 27, 29, and 31 mm) were mounted with the
leaflets oriented vertically to eliminate gravity effect. Flow, a 1%
to 2% aqueous suspension of cornstarch, entered the proximal chamber
from an upper reservoir whose height could be adjusted to vary
transorifice pressure and flow rate and then passed through the mounted
prosthetic valve under constant hydrostatic pressure causing
steady flow. Flow rate was measured by the average of 3 to 5 timed
collections. At least 3 different flow rates were examined for each
valve size. Measurement variability was expressed as mean percent error
for all flow rates studied, given by the SD of a set of timed
collection measurements divided by their mean. Flow rates were chosen
to simulate normal mitral transprosthetic velocities.
Pulsatile-Flow Model
To examine the effects of flow pulsatility on our method, St
Jude heart valves (23, 25, 27, 29, and 31 mm) were studied in
triplicate in the mitral position of a left heart pneumatic pump model.
Cardiac output was measured by timed collections, with variability
expressed as mean percent error. Four cardiac outputs were examined for
each prosthesis, with heart rate constant at 70 bpm.
Echocardiographic Study
A Hewlett-Packard Sonos 1500 system was used with 2.5- or
3.5-MHz phased-array transducers held by an adjustable-clamp system to
yield an imaging plane perpendicular to the leaflets to show 3 distinct
orifices. Flow velocities across the center and side orifices were
interrogated separately with continuous-wave (CW) Doppler. Color
flow images of the proximal convergence zone were obtained with 3
different aliasing velocities between 17 and 41 cm/s and stored
digitally on 650-MB optical disks and recorded onto 2-in VHS
videotape.
Clinical Study
Patients
Patients in regular rhythm without significant tricuspid
regurgitation who were undergoing St Jude mitral valve
replacement were considered for the study. Cardiac output was measured
in triplicate by thermodilution. The size of the implanted
prosthetic heart valve was recorded.
Echocardiographic Examination
Intraoperative transesophageal
echocardiography was performed with HP Sonos 1500
or Acuson 128XP with 5-MHz probes when patients were stable after
weaning from cardiopulmonary bypass. Images of the
prosthesis showing 3 distinct orifices were obtained at a depth
of 6 to 10 cm. Velocities across the center and side orifices were
obtained by CW Doppler during suspended respiration, and the
pressure half-time (t1/2) of the E wave was
measured.8 Images of the proximal convergence
zone were obtained with a color aliasing velocity between 17 and 26
cm/s,11 reducing sector size to maximize frame
rate (generally 18 to 22 frames per second), and stored on optical
disks and/or VHS videotape. Thermodilution cardiac output was obtained
simultaneously.
Data Analysis and Calculations
Steady-Flow Model
We selected 5 frames with a clear blue-red aliasing boundary to
measure r and p, assuming the valve orifice to be at the
prosthetic annular level. Prosthetic EOA was given by
Qc/v for the hemispheric proximal
isovelocity surface area (PISA) method and by
Qp/v for the SOS method.
Pulsatile-Flow Model
Stroke volume was calculated by multiplying
Qc or Qp by the
time-velocity integral normalized by the peak transorifice velocity, v.
Cardiac output was given by the stroke volumex70 bpm.
Prosthetic EOA was given by (1)
Qc/v for the PISA method, (2)
Qp/v for the SOS method, and (3) the
modified Gorlin equation:
where p is the mean gradient across the St Jude side orifice
by CW Doppler.
Clinical Study
Forward stroke volume across the prosthetic orifice was
calculated by multiplying Qp or
Qc by the time-velocity integral normalized by v.
The product of stroke volume and heart rate yielded cardiac output.
Prosthetic EOA was calculated by (1) Qc/v
(PISA), (2) Qp/v (SOS), (3)
220/t1/2 (pressure
half-time),8 and (4) the modified Gorlin
equation above. In both the clinical and in vitro studies, the
localized high velocities in the small central orifice were disregarded
and the side orifice velocities used in the
calculations.25
Statistical Analysis
In Vitro Models
Flow rates and cardiac outputs calculated by (1) PISA and (2)
SOS were each compared with timed collections by linear regression,
with the difference between calculated and measured flow (Q)
expressed as mean±SD. These 3 measurements of flow were also compared
by repeated-measures ANOVA. Calculated in vitro EOAs were reported as
mean±SD for each size, and the triplicate prostheses were compared by
ANOVA. Center and side orifice velocities were compared by paired
Student's t test. To evaluate the impact of aliasing
velocities on flow estimation in the pulsatile model, Q for 3 ranges
of va (
20, 21 to 29, and 
30 cm/s) were
compared by ANOVA, with Q for PISA and SOS in each range compared by
paired Student's t test. Q by PISA and SOS were also
correlated with va by linear regression.
Clinical Study
Cardiac outputs calculated by (1) PISA and (2) SOS were each
compared with thermodilution by linear regression, with the difference
between Doppler and thermodilution cardiac output expressed as
mean±SD. These 3 cardiac output measurements were also compared by
repeated-measures ANOVA. In vivo EOA (reported for each valve size as
mean±SD) calculated by (1) PISA, (2) SOS, and (3) pressure half-time
method were each compared with Gorlin calculations and the geometric
orifice area. Center and side orifice velocities were compared by
paired Student's t test, with the difference expressed as
mean±SD. Statistical significance was defined as a 2-tailed
P<0.05.
Interobserver and Intraobserver Variability
Ten randomly selected color Doppler images and continuous
Doppler recordings from the in vitro and clinical studies
were used to assess interobserver and intraobserver variability in
measurement of the radius r and chord p of the proximal convergence.
Variability was expressed as the ratio of the difference between the 2
measurements to their mean.26
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Results
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In Vitro Models
Steady-Flow Model
Flow rates ranged from 218 to 406 cm3/s,
with mean error in the timed collections of 1.7±1%. The peak
velocities through the center orifices (153.9±39.6 cm/s; range, 82.9
to 245 cm/s) were significantly higher than that through the side
orifices (133.1±32.4 cm/s; range, 73.5 to 190 cm/s, v=20.9±10.9
cm/s, P<0.001). The ratio of the side to center velocities
was 0.87±0.04. PISA and SOS flow rates correlated well with the timed
collections (Figures 2 and 3), but compared with PISA, SOS
underestimated true flow rate (Q=-29.8±42.3 versus 0.7±28.2
cm3/s, P<0.001), also significant by
repeated-measures ANOVA (P<0.0001). PISA and SOS EOAs
(y) correlated with but underestimated the geometric orifice
area (x) significantly (PISA:
y=0.52x+0.43, r=0.87,
P<0.001, =-1.47±0.50 cm2; SOS:
y=0.61x-0.09, r=0.87,
P<0.001, =-1.6±0.45 cm2, Table 1). There was no significant variation in
EOAs for the 3 sets of prostheses by ANOVA.
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Figure 2. Correlation between flow rate measured by timed
collections (x axis) and flow rate calculated by
conventional hemispheric PISA method (y axis) in in
vitro steady-flow study (top). Dotted line represents line of
identity. Bottom, Differences between the 2 measurements against their
means.
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Figure 3. Correlation between flow rate measured by timed
collections (x axis) and flow rate calculated by SOS
(y axis) in in vitro study (top). Bottom, Differences
between the 2 measurements against their means.
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Pulsatile-Flow Model
Cardiac output ranged from 1.6 to 8.6 L/min, with a mean of 5.9
L/min and variation in the timed collections of 2.2±1.3%. The ratio
of side to center orifice velocity was 0.83±0.11. The cardiac output
calculated by PISA agreed closely with timed cardiac output
(y=0.997x-0.09, r=0.95,
P<0.001, Q=-0.11±0.59 L/min, P=NS), but SOS
showed poorer correlation (y=0.81x+0.52,
r=0.88, P<0.001) and significant underestimation
of flow (Q=-0.55±0.82 L/min, P<0.001), significantly
worse (P<0.0001) than the PISA calculations. PISA and SOS
EOAs correlated with but underestimated geometric orifice areas
(r=0.79 and 0.78, respectively, Table 2), without significant variation among
the 3 sets of prostheses by ANOVA.
By SOS, Q was -0.88±0.7 L/min for aliasing velocities 20 cm/s,
-0.49±0.9 L/min for 21 to 29 cm/s, and -0.28±0.8 L/min for 30
cm/s (P=0.07 by ANOVA). SOS Q was significantly worse
than PISA Q for 20 cm/s (-0.05±0.4 L/min, P<0.001)
and 21 to 29 cm/s (0.12±0.3 L/min, P=0.007) but not for
30 cm/s (-0.38±0.8 L/min, P=NS). There were opposite but
nonsignificant linear trends between aliasing velocity and Q for
PISA (r=-0.24, P=NS) and SOS (r=0.24,
P=NS).
Clinical Study
The study population comprised 36 patients (26 women, 58±11
years old). At the time of study, 24 were AV paced, with the remainder
in sinus rhythm. Heart rate was 92±10 bpm.
The peak and mean velocities through the center orifice (161±29 and
97±16 cm/s, respectively) were significantly higher than through the
side orifices (137±23 and 86±14 cm/s, respectively,
peak=23.6±11 cm/s, P<0.001 and
mean=11±6.5 cm/s, P<0.001), with
a side-to-central velocity ratio of 0.86±0.05 (Figure 4).
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Figure 4. CW Doppler examination of flow across central
and side orifices of St Jude mitral valve in a patient. Note higher
flow velocities across central compared with side orifice.
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Cardiac output by thermodilution (x) was 5.2±1.2 L/min (3.3
to 8.1 L/min), with close agreement by PISA
(y=1.018x-0.21, r=0.91,
CO=-0.05±0.55 L/min, Figure 5) but
underestimation by SOS (y=0.79x-0.21,
r=0.82, CO=-1.33±0.73 L/min, Figure 6), P<0.001 for the
significance of this difference by repeated-measures ANOVA
(P<0.001).
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Figure 5. Correlation between thermodilution cardiac output
(CO, x axis) and CO calculated by conventional PISA
method (y axis) in clinical study (top). Bottom,
Differences between the 2 measurements against their means.
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Figure 6. Correlation between thermodilution cardiac output
(CO, x axis) and CO calculated by SOS method
(y axis) in clinical study (top). Bottom, Differences
between the 2 measurements against their means.
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Table 3 and Figure 7 summarize EOA calculated by the
different methods. EOA by the modified Gorlin equation was
significantly smaller than the geometric orifice area provided by the
manufacturer (y=0.37x+0.24, r=0.64,
P<0.001, = -2.5±0.5 cm2,
P<0.001). PISA EOA agreed closely with Gorlin valve area
(y=0.78x+0.7, r=0.75,
P<0.001, =0.3±0.28 cm2,
P<0.001) and also underestimated the geometric area
(y=0.48x+0.05, r=0.8,
P<0.001, = -2.15±0.43 cm2,
P=0.001). Similarly, SOS EOA agreed with Gorlin area
(y=0.64+0.41, r=0.68, P<0.001, =
-0.23±0.3 cm2, P<0.001) and
underestimated the geometric area (y=0.42x-0.23,
r=0.76, P<0.001, =-2.7±0.46
cm2). Orifice area calculated by pressure
half-time showed very poor (inverse) agreement with both Gorlin
(y=4.9-1.01x, r=-0.44) and
geometric valve area (y= -0.44x,
r=-0.33).
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Figure 7. EOAs of St Jude valves calculated by 4 different
methods and their corresponding geometric orifice area.
T1/2 indicates pressure half-time method; Gorlin, modified
Gorlin equation.
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Interobserver and Intraobserver Variabilities
In vitro interobserver variabilities in the measurement of
proximal convergence radius and chord were 3.3±3.8% and 3.9±2.9%,
respectively, with intraobserver variabilities of 3.5±1.4% and
2.1±1.1%, respectively. In the clinical study, interobserver
variabilities for r and p were 3.6±3.2% and 3.2±2.4%, respectively,
with intraobserver variabilities of 4.1±3.4% and 5.3±2.9%,
respectively.
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Discussion
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The proximal convergence method, a recently developed color
Doppler technique for quantification of mitral
regurgitation,10 11 12 13 14 15 16 17 18 19 has been
validated in experimental and clinical studies for calculation of
regurgitant stroke volume and effective regurgitant orifice
area.10 11 12 13 14 15 16 17 18 The same principle has been used to
calculate mitral stenotic orifice area, where there is
relatively low-velocity flow across a finite
orifice.20 Extending this concept to mitral valve
prostheses is reasonable, because the planar nature of the ring
obviates much of the need to correct for proximal flow constraint,
which is often necessary for mitral
stenosis20 and
regurgitation.27 Moreover, the
difficulties commonly encountered in locating the convergent
focus28 are eliminated, because the level of the
orifice is defined by the prosthetic annulus. However, the
finite size and relatively low transorifice velocity of a mitral
prosthesis are associated with isotach flattening near the
orifice.22 Fortunately, the resultant flow
underestimation can largely be corrected by a correction factor,
v/(v-va), where v is the transorifice velocity
and va the aliasing
velocity.22
The SOS method has been advocated as an alternative to the conventional
hemispheric modeling of the proximal convergence flow field. This
method has the advantage that only 1 measurement, p, is required,
compared with the more accurate hemielliptical
model.23 24 However, total forward flow by SOS,
although well correlated to actual flow in our models and clinical
study, showed systematic underestimation, which will merit further
study.
Effective Prosthetic Orifice Area
All prosthetic valves are inherently mildly
stenotic. Analogous to stenotic native valve area,
prosthetic orifice area, incorporating flow and pressure
gradient, gives a more flow-independent measure of prosthetic
resistance. Although the continuity equation and pressure half-time
methods8 29 are accepted Doppler techniques
for native mitral and aortic stenosis, effective
prosthetic orifice area has been more
elusive.30 Although Doppler has been used in
flow models to calculate effective prosthetic valve
areas31 and the continuity equation has been used
clinically for prosthetic aortic
valves,9 32 a reliable noninvasive method to
estimate prosthetic mitral orifice area clinically is still
lacking.
The present study demonstrated that the pressure half-time method
has little value in measuring effective prosthetic orifice
areas, actually showing an inverse relation to valve size. We have
previously shown that the pressure half-time method is unreliable
immediately after balloon valvuloplasty33 because
of abrupt changes in transvalvular gradient and chamber
compliance. In the present study, the pressure half-time was
measured immediately after mitral valve replacement, when sudden
changes in net chamber compliance and transvalvular gradient
might have contributed to the inaccuracy of the pressure half-time
method for effective prosthetic valve area.
The effective prosthetic valve areas in our clinical study are
consistent with some published studies31
but smaller than others.34 35 36 Baumgartner et
al31 calculated EOA in a pulsatile-flow model
with the Gorlin formula and Doppler gradients and obtained values
similar to our in vivo EOA. However, in the same study, EOAs were
significantly larger when catheter gradients were used with the Gorlin
formula. Similarly, Yoganathan and his group obtained significantly
larger EOAs with the catheter-based Gorlin formula both in
vitro35 and in vivo,36
related to the overestimation of catheter gradients by
Doppler34 due to the phenomenon of pressure
recovery.25 Interestingly, Baumgartner et al
observed larger EOAs in their steady in vitro model than in the
pulsatile one. This is supported by a preliminary study by Trujillo et
al,37 who showed that for rigid circular
orifices, Doppler continuity EOAs during steady flow are
consistently larger than in pulsatile-flow conditions. Further
preliminary data show flow dependency of EOA in pulsatile
conditions.37 38 In our clinical study, the
relatively fast heart rate and therefore lower stroke volume observed
just after cardiopulmonary bypass may contribute to smaller
calculated EOAs. In addition, the geometry surrounding the
prosthesis in vivo and the presence or absence of the mitral
subvalvular apparatus may also have important
effects on the calculated EOA.
Limitations
The major limitation in all studies examining effective
prosthetic orifice area is the lack of a proper, universally
accepted "gold standard." Various references have been used,
including geometric, Gorlin, continuity, and half-time EOAs. However,
true in vivo EOA may vary for a given prosthesis depending on
physiological conditions, shown by the range in
reported EOA in our and prior studies. In vitro results suggest that
flow pulsatility affects EOA. Our data showed consistent
results across 3 valves of each size, suggesting relatively little
actual interprosthesis variance.
Clinical Application
Although we would not recommend this technique for routine
assessment of mitral prostheses (because of the need for
transesophageal echocardiography),
we have found it quite useful in relatively subtle or questionable
situations of prosthetic obstruction. When a leaflet is
completely stuck by thrombus or pannus, this is usually evident by
direct 2-dimensional imaging and a high transprosthetic
gradient in the presence of a low-output state. In contrast, the PISA
EOA is helpful for assessing nonobvious obstruction or, conversely,
ruling it out when a high gradient occurs in a high-output state.
Conclusions
This is the first study to demonstrate that mitral
prosthetic EOA can be measured noninvasively by proximal flow
convergence methods. We examined the St Jude prosthesis because
it is the mostly commonly implanted prosthetic valve; because
its hydrodynamic profile is among the most complex, this technique is
likely to be applicable to other types of prostheses. This may improve
the management of patients with mitral prostheses by allowing accurate
detection and assessment of prosthetic valve obstruction
independent of transvalvular flow.
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Acknowledgments
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This study was supported in part by grant NCC9-60 from the
National Aeronautics and Space Administration, Houston, Tex, with valve
prostheses provided by St Jude Medical, Inc, Minneapolis,
Minn.
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Footnotes
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Presented in part at the 44th Annual Scientific Session of the American College of Cardiology, March 19–22, 1995, New Orleans, La, and at the 6th Scientific Sessions of the American Society of Echocardiography, June 14–16, 1995, Toronto, Canada.
Received December 19, 1997;
revision received May 14, 1998;
accepted May 20, 1998.
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Abstract
Introduction
