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(Circulation. 1996;93:594-602.)
© 1996 American Heart Association, Inc.
Articles |
Effective Regurgitant Orifice Area by the Color Doppler Flow Convergence Method for Evaluating the Severity of Chronic Aortic Regurgitation
An Animal Study
From the Oregon Health Sciences University, Portland (T.S., M.I., B.S., S.H., D.J.S.); the Laboratory of Animal Medicine and Surgery, National Institutes of Health, Bethesda, Md (M.J., I.Y.); and the School of Chemical Engineering, Georgia Institute of Technology, Atlanta (R.S.H., A.P.Y.).
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Abstract |
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 Top Abstract IntroductionMethods Results Discussion References |
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Background The aim of the present study was to evaluate dynamic changes in aortic regurgitant (AR) orifice area with the use of calibrated electromagnetic (EM) flowmeters and to validate a color Doppler flow convergence (FC) method for evaluating effective AR orifice area and regurgitant volume.
Methods and Results In 6 sheep, 8 to 20 weeks after surgically induced AR, 22 hemodynamically different states were studied. Instantaneous regurgitant flow rates were obtained by aortic and pulmonary EM flowmeters balanced against each other. Instantaneous AR orifice areas were determined by dividing these actual AR flow rates by the corresponding continuous wave velocities (over 25 to 40 points during each diastole) matched for each steady state. Echo studies were performed to obtain maximal aliasing distances of the FC in a low range (0.20 to 0.32 m/s) and a high range (0.70 to 0.89 m/s) of aliasing velocities; the corresponding maximal AR flow rates were calculated using the hemispheric flow convergence assumption for the FC isovelocity surface. AR orifice areas were derived by dividing the maximal flow rates by the maximal continuous wave Doppler velocities. AR orifice sizes obtained with the use of EM flowmeters showed little change during diastole. Maximal and time-averaged AR orifice areas during diastole obtained by EM flowmeters ranged from 0.06 to 0.44 cm2 (mean, 0.24±0.11 cm2) and from 0.05 to 0.43 cm2 (mean, 0.21±0.06 cm2), respectively. Maximal AR orifice areas by FC using low aliasing velocities overestimated reference EM orifice areas; however, at high AV, FC predicted the reference areas more reliably (0.25±0.16 cm2, r=.82, difference=0.04±0.07 cm2). The product of the maximal orifice area obtained by the FC method using high AV and the velocity time integral of the regurgitant orifice velocity showed good agreement with regurgitant volumes per beat (r=.81, difference=0.9±7.9 mL/beat).
Conclusions This study, using strictly quantified AR volume, demonstrated little change in AR orifice size during diastole. When high aliasing velocities are chosen, the FC method can be useful for determining effective AR orifice size and regurgitant volume.
Key Words: aortic regurgitation • echocardiography • blood flow • hemodynamics
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Introduction |
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TopAbstractIntroductionMethods Results Discussion References |
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Development of Doppler echocardiographic techniques has enhanced the noninvasive assessment of aortic regurgitation.1 2 3 4 5 However, most of the Doppler echocardiographic methods have been compared with cineangiographic grading of the severity of aortic regurgitation, radionuclide scintigraphy, or other Doppler flow observations.1 2 3 4 5 The severity of aortic regurgitation estimated by cineaortography depends on many factors and may differ greatly from that determined by quantitative flow measurements.6 7
Imaging of the proximal flow convergence region in the aorta for flow accelerating retrogradely across the aortic valve as imaged by magnetic resonance imaging has been reported to be useful for identifying the site of regurgitation and has been used for grading its severity.8 9 Observation of flow acceleration phenomena and use of the flow convergence method have been used experimentally and clinically for quantifying the regurgitant flow volume and flow rate mainly in mitral regurgitation using a variety of assumptions, most commonly that of a hemispheric isovelocity flow convergence surface.10 11 12 13 14 15 16 17 18 19 20 Clinical and experimental in vivo applications of this method have been suggested for evaluating the regurgitant orifice area of mitral regurgitation.21 22 23 24 However, although the size of the regurgitant orifice is a major determinant of the regurgitant severity in aortic regurgitation,5 there have been no studies exploring the measurement of regurgitant orifice area or validating the flow convergence method for estimating the regurgitant orifice area in aortic valve disease using quantified chronic aortic regurgitation. Since the flow convergence method with the hemispheric isovelocity surface assumption (or any other geometric model) may not be applicable with accuracy during the entire duration of diastole when flows are calculated with a fixed aliasing velocity, the possibility of dynamic changes in regurgitant orifice size and the applicability of using the flow convergence method for aortic regurgitation should both be investigated.
We undertook this study to (1) evaluate dynamic changes in aortic regurgitant orifice area and flow determined with the use of electromagnetic flow–based methods and (2) examine the validity of using the flow convergence method for determining the effective regurgitant orifice areas and regurgitant stroke volumes in an animal model with chronic aortic regurgitation.
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Methods |
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TopAbstractIntroductionMethods Results Discussion References |
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Experimental Preparation
Six juvenile sheep weighing 22 to 43 kg (mean, 33 kg) were studied. Eight to 20 weeks (mean, 14 weeks) before the hemodynamic and ultrasonic studies, which constitute the experimental setting for the study, the animals had undergone thoracotomy and open heart surgery with cardiopulmonary bypass. At that time, the free edge of the right coronary cusp (3 sheep) or the noncoronary cusp (3 sheep) of the aortic valve was severed with a radial incision under direct vision. Subsequent aortic dilatation and/or leaflet retraction resulted in anatomic leaflet defects and failure of coaptation. All operative and animal management procedures were approved by the Animal Care and Use Committee of the National Heart, Lung, and Blood Institute. Preoperative, intraoperative, and postoperative animal management and husbandry methods are described in detail elsewhere.25 26 After surgery, during recovery, the animals were maintained on digoxin and furosemide.
Eight to 20 weeks later, the animals were returned to the laboratory for the physiological studies, which are the subject of this study. Anesthesia was induced with intravenous sodium pentobarbital (25 mg/kg) and maintained with 1% to 2% isoflurane with oxygen; the animals were ventilated via an endotracheal tube using a volume-cycle ventilator. Repeat thoracotomy was performed.
Cardiac Catheterization and Electromagnetic
Flowmeters
A Swan-Ganz catheter was positioned in the main
pulmonary artery inserted via the femoral vein. Another
catheter was positioned in the right common femoral artery for
monitoring systemic arterial pressure and
arterial blood gases. These catheters were interfaced with
a physiological recorder (ES 2000, Gould Inc)
with fluid-filled pressure transducers (model PD23 ID, Gould
Statham). Arterial blood gases and pH were maintained
within physiological ranges. A bilateral transverse
thoracotomy was performed. After dissection, an electromagnetic flow
probe (model EP455, Carolina Medical Electronics, Inc) was placed
around the pulmonary artery just above the pulmonary
valve sinuses. Another electromagnetic flow probe (model EP455,
Carolina Medical Electronics) was placed snugly around the skeletonized
ascending aorta distal to the coronary ostia and proximal to
the brachiocephalic trunk. Both flow probes were connected to
flowmeters (model FM501, Carolina Medical Electronics), and these were
connected to the same physiological recorders
(ES 2000, Gould Inc) used for hemodynamic pressure
recordings. Aortic and left ventricular pressures
were obtained from intracavitary manometer-tipped catheters (model
SPC-350, Millar Instruments, Inc) positioned transmurally. All
hemodynamic data were recorded at paper speeds of
250 mm/s. Four consecutive cardiac cycles were analyzed for
each hemodynamic determination.
Calibration factors for the flow probes were corrected before each hemodynamic state for each animal's hematocrit according to the manufacturer's specification. The integrals of instantaneous flows over time were determined by planimetry of the flow signal recordings. The problem of the zero baseline drift was managed as follows. The pulmonary artery flow zero-level baseline was adjusted according to the contour of its electromagnetic flow probe signal; this baseline was reconfirmed by occlusive zeros. No animal had physiologically important pulmonary regurgitation. The baseline for the aortic flow recording then was adjusted until the forward minus the backward aortic flow volumes equaled the pulmonary forward flow volume. Coronary arterial blood flow during ventricular diastole was measured in 3 sheep in a preliminary study. The coronary flow rate was small (0.13 to 0.23 L/min). These values were similar to those reported by others in studies of aortic regurgitation and thus were considered to be negligible compared with the regurgitant volumes delineated in this study.5 The correlation coefficient for the regression of pulmonary forward flow versus aortic forward minus aortic regurgitant flow was .98 (SEE=0.03 L/min). Regurgitant fraction was calculated as backward aortic flow volume per minute divided by forward aortic flow volume per minute. Once the curves for pulmonary and aortic flow were properly adjusted, instantaneous regurgitant flow rates could be determined.
A hydrostatic standard was used for calibration of all pressure recordings. Left ventricular and aortic pressures were recorded simultaneously. All hemodynamic recordings were performed simultaneously with the echocardiographic studies. After baseline measurements, varying degrees of severity of aortic regurgitation were produced by altering preload and/or afterload using blood transfusion and/or angiotensin infusion. The calibrations of the flow probes were readjusted before each individual hemodynamic steady state, compensating for any change in hematocrit produced by insensible fluid loss, blood loss, and/or the alteration of preload by blood transfusion. Insensible fluid loss and associated electrolyte disturbances exacerbated by the open thoracotomy were monitored by frequent (before each individual hemodynamic study) determinations of serum electrolyte and hematocrit; aberrations were avoided by continuous infusions of lactated Ringer's solution and 5% dextrose in water supplemented with potassium and calcium, as necessary. A total of 22 hemodynamic states (2 to 4 per animal) were obtained.
Color Doppler
Echocardiography
Color Doppler flow mapping was performed with a
Vingmed 775
system (Vingmed Sound) with a 5-MHz transducer placed directly near the
apex of the heart at a pulse repetition frequency of 4.0 to 6.0 kHz
(Fig 1
). Color gain was adjusted to eliminate random
color in areas without flow. The color Doppler filter was selected
with a roll-off to deemphasize velocities less than 0.08 to 0.16
m/s. Aliasing velocities of 0.44 to 0.94 m/s were selected for the
initial imaging of the flow convergence. A narrow color sector was
chosen to allow frame rates as high as 45 per second. Since the Vingmed
775 system is equipped with a digital output port allowing transfer of
color Doppler data, two-dimensional color flow images from this
system could be directly transferred into a Macintosh IIci computer
(Apple Computer, Inc) as numerical velocity assignments in their
original digital format without color assignment or conversion into
analog video format. The initial aliasing velocity could be changed by
a postprocessing software, EchoDisp (Vingmed Sound), in the
computer system after the raw digital velocity data were transferred
into the microcomputer (Fig 1).
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The aortic regurgitant
orifice was identified as the junction of the
laminar convergence flow and the regurgitant jet (vena contracta) using
two-dimensional color Doppler imaging (Fig 1
). Atypical or more
basally located positions of the transducer usually demonstrated better
flow convergence images than routine clinically used apical views did.
Guided by the two-dimensional and color Doppler imaging of the
regurgitating jet and the valve, continuous wave (CW) Doppler
recordings were obtained, recording the regurgitant
flow velocity parallel to the direction of the aortic regurgitant jet.
The CW and color Doppler data were matched for each heartbeat and
stored in the computer.
Measurements of Aortic Regurgitant Orifice Area
Electromagnetic Flow Method
Instantaneous aortic
regurgitant orifice area was derived by
dividing the instantaneous flow rate obtained by the electromagnetic
flowmeters by the corresponding CW Doppler velocity. The CW
Doppler and the flowmeter data were transferred directly into the
Macintosh computer. After the calibration of flowmeter data,
instantaneous regurgitant orifice areas with a temporal resolution of 5
to 10 ms (Fig 2) were calculated with the use of
computer software (Excel, Microsoft, Inc) with the formula
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Time-averaged (mean) regurgitant orifice areas also were calculated to compare with those obtained using the flow convergence method.
Color Doppler Flow Convergence Method
The
maximal aliasing distances from the two-dimensional
color Doppler flow convergence images during diastole
were selected and measured by frame-by-frame search of the
digital cineloops in the computer with the use of two selected ranges
of aliasing velocities of 0.20 to 0.32 m/s and 0.70 to 0.89 m/s (Fig
1). Corresponding maximal regurgitant flow rates were
calculated with
the use of the hemispheric assumption for proximal isovelocity surface
geometry.11 12 13 Maximal aortic
regurgitant orifice areas
were derived by dividing the calculated flow rates by the maximal CW
Doppler velocities obtained during the same steady states.
Regurgitant Volume and Regurgitant Fraction
Regurgitant
volume per beat also was calculated as the
product of the maximal orifice area obtained by the flow
convergence method multiplied by the velocity time integral of the CW
Doppler velocity. Aortic diameter from parasternal views and the
velocity time integral of the aortic forward flow obtained by the
pulsed Doppler mode from apical views were measured.
Systolic volume was calculated
echocardiographically as the product of the area of
the aorta (
xdiameter2/4) and the aortic forward
velocity time integral. Regurgitant fraction was calculated as the
ratio of the regurgitant volume to the systolic
volume.22 Three measurements of each of these
variables were performed and averaged for each
hemodynamic condition.
Observer Variability
To evaluate the effect of observer
variability on the
Doppler measurement of orifice area, 10 randomly selected
hemodynamic conditions were analyzed at
different times with the same computer by two independent observers
(T.S. and M.I.), each without knowledge of the cardiac beats selected
for analysis by the other observer or the results obtained by
the electromagnetic flow data. Beat-to-beat variability of the
flow convergence orifice area was examined with the use of 10 randomly
selected hemodynamic conditions.
Statistical Analysis
Data are presented as mean±SD.
Aortic regurgitant
orifice areas, regurgitant fractions, regurgitant volumes, and peak
regurgitant flow rates obtained by the electromagnetic flowmeters and
the flow convergence method were compared with the use of simple
regression analysis, and the difference in values obtained by
the two methods (the flow convergence value minus the electromagnetic
flow value) was calculated and expressed as mean±SD. Aortic
regurgitant orifice areas also were compared with peak aortic
regurgitant flow rates, regurgitant fractions, and regurgitant volumes
per beat with the use of simple regression analysis. In
addition, since multiple points were used from the same animal, a
multivariate regression analysis was used to
examine the relationships of data between sheep. To do this, we created
the design matrix in a spreadsheet of a statistical computer program
(StatView 1988, Abacus Concepts, Inc) by using dummy variables as
columns to encode the different sheep and used the multiple regression
function of StatView.27 28 A value of
P<.05
was considered statistically significant.
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Results |
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TopAbstractIntroductionMethods Results Discussion References |
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Aortic regurgitant volumes per beat and regurgitant fractions were within clinically relevant ranges from 7.0 mL/beat to 48 mL/beat (average, 25±12 mL/beat) and from 23% to 78% (average, 53±17%), respectively. Peak and mean regurgitant flow rates also were within clinically relevant ranges from 1.8 to 9.8 L/min (average, 5.2±2.5 L/min) and from 0.7 to 4.1 L/min (average, 2.4±1.2 L/min), respectively. Aortic root diameters were 2.1±0.4 cm.
Dynamic Change in Aortic Orifice Area
For all of the
hemodynamic conditions, aortic
regurgitant orifice sizes obtained with the use of electromagnetic
flowmeters showed rapid "opening" of the orifice in very early
diastole and then little change during the remainder of
diastole (Fig 2).
Maximal and time-averaged regurgitant
orifice areas during
diastole obtained by the electromagnetic flowmeters ranged
from 0.06 to 0.44 cm2 (mean, 0.24±0.11 cm2)
and from 0.05 to 0.43 cm2 (mean, 0.21±0.06
cm2), respectively. Simple linear regression
analysis between the regurgitant fractions, regurgitant volumes
per beat, and peak regurgitant flow rates obtained by the
electromagnetic flowmeters and the time-averaged orifice area
showed good relationships (r=.74, .87, and .92,
respectively). When a multiple regression model was used to eliminate
data tracking induced by serial measurements within the same animal, a
weaker but significant relationship was found between regurgitant
fractions, regurgitant volumes, and peak regurgitant flow rates and the
time-averaged regurgitant orifice areas (r=.66, .76, and
.89, respectively, all P<.05, Fig 3).
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Comparison of Regurgitant Orifice Areas Obtained by the
Electromagnetic Flowmeters and the Flow Convergence
Method
Aortic regurgitant orifice areas by the flow convergence method
with the use of low aliasing velocities were 0.36±0.24 cm2
and overestimated the electromagnetically obtained orifice areas
significantly, especially for larger reference areas
(difference=0.15±0.14 cm2, P<.001, Fig
4). By selecting higher aliasing velocities to use for
calculation, however, the flow convergence method results predicted the
reference mean areas more reliably (0.25±0.16 cm2,
r=.82, difference=0.04±0.07 cm2, Fig
5). There was no significant difference between the
regurgitant orifice areas using higher aliasing velocities and those
determined by the electromagnetic flowmeters. Fig 6
shows the degree of the overestimation of regurgitant orifice areas
calculated using two different ranges of aliasing velocities as a
function of peak regurgitant flow rates determined using the
electromagnetic flowmeters. As anticipated, the higher the peak flow
rate, the more severe the overestimation of the regurgitant orifice
area.
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Simple linear regression analysis between the regurgitant
fractions, regurgitant volumes per beat, and peak regurgitant flow
rates and the aortic regurgitant orifice areas derived from the flow
convergence method showed moderately good relationships
(r=.67, .78, and .86, respectively). When the multiple
regression model was used to account for between-sheep differences,
a weaker but still significant relationship was found between
regurgitant fractions, regurgitant volumes per beat, and peak
regurgitant flow rates and the regurgitant orifice areas derived from
the flow convergence method (r=.63, .74, and .81,
respectively, all P<.05, Fig 7).
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Other Measures of Aortic Regurgitation
Severity
Regurgitant volume, defined as the product of the orifice
area
obtained by the flow convergence method and the velocity time integral
of the regurgitant orifice velocity, and peak regurgitant flow rates
calculated with the hemispheric isovelocity surface assumption using
higher aliasing velocities showed good agreement with corresponding
data determined with the use of the electromagnetic flowmeters
(difference=0.9±7.9 mL/beat, 0.6±1.9 L/min, respectively,
Fig 8). When the multiple regression analysis was
used to account for between-sheep differences, a weaker but
significant relationship was found between them (Fig 8).
However,
regurgitant fraction calculated by the
echocardiographic method underestimated those
determined with the use of the electromagnetic flowmeters, although
these were linearly related to each other (r=.84,
difference=-0.14±0.08, Fig 8).
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Observer Variability
There was an excellent agreement between
the two observers'
measurements of aortic regurgitant orifice areas using the flow
convergence method (r=.95, P<.001,
difference=0.02±0.03 cm2). There was also an
excellent
agreement between orifice areas obtained from different cardiac cycles
under the same hemodynamic conditions
(r=.96, P<.001, difference=0.01±0.02
cm2).
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Discussion |
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TopAbstractIntroductionMethods Results Discussion References |
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In this study, the electromagnetic flowmeter method combined with CW Doppler demonstrated that the aortic regurgitant orifice, once established, changed little in area thereafter during diastole. The color Doppler flow convergence method predicted reference regurgitant orifice areas reasonably well when higher aliasing velocities were selected for computation. The product of the effective orifice area derived from the flow convergence method and the diastolic velocity time integral provided reasonable estimates of the regurgitant flow volume per beat obtained by the electromagnetic flowmeters (Fig 8
).
Dynamic Change in Aortic Regurgitant Orifice Area
Although
significant dynamic changes in mitral regurgitant orifice
areas have been reported by us and
others,23 24 29 little
information is available regarding dynamic changes in aortic
regurgitant orifice area.30 Reimold et al30
reported a significant decrease in aortic regurgitant orifice area
(exponential decay curve fit) during diastole in 17
patients by using magnetic resonance imaging and CW velocity. In our
study, in which electromagnetic flowmeters were used, however, the
aortic regurgitant orifice area changed little during
diastole.
The difference between the two studies may be explained as follows: Aortic regurgitation was created by severing the right or noncoronary cusp in our animal model. In contrast, a wide variety of aortic valvular pathology was examined in the study by Reimold et al,30 including patients with annular dilation. Aortic regurgitant orifices in patients with annular dilation and structurally normal leaflets may well decrease in area if the degree of dilation of the aortic annulus decreases with the falling pressure during diastole.
Another possible cause of the difference between the two studies is that in the study by Reimold et al, a significant decrease in regurgitant orifice area during diastole was observed in mild regurgitation (regurgitant fraction <20%, 6 of 17 patients); with moderate to severe aortic regurgitation in their study, there was much less change in regurgitant orifice size during diastole. In our animal model, none of the 22 hemodynamic states had regurgitant fractions <20%. Neglecting coronary arterial flow, which may be comparable to regurgitant flow in mild aortic regurgitation, could contribute to overestimation of aortic regurgitant flow, resulting in overestimation of regurgitant orifice area. This is particularly a problem early in diastole, since the coronary flow rate has been reported to be higher early in diastole than late in diastole in patients with aortic regurgitation.31 Thus, overestimation of the regurgitant orifice area early in diastole could result in an apparent dynamic change in orifice size. In the study by Reimold et al,30 only data from very early in diastole provided orifice areas substantially larger than later during diastole, resulting in the apparent exponential decay curve fit for the orifice area change. Determination of the orifice area early in diastole may be subject to measurement error because both regurgitant flow rate and CW velocity increase abruptly at that time. Additionally, in the previous study, CW Doppler and magnetic resonance imaging flow data were not obtained simultaneously. In our animal study, CW Doppler velocities and electromagnetic flow rates were simultaneously recorded and measured digitally during 25 to 40 time periods during each diastole in contrast to 8 to 10 points in the previously published study.
Dynamic Change in Aortic Versus Mitral Regurgitant Orifice
Areas
In contrast to the orifice area in aortic
regurgitation, the regurgitant orifice area in mitral
regurgitation appears to change significantly during
systole.23 24 The difference in dynamic changes of
orifice
areas for aortic versus mitral regurgitation may be
related to the fact that aortic regurgitation occurs
during left ventricular relaxation, which is a more static
phenomenon during most of diastole, compared with mitral
regurgitation, which occurs during dynamic left
ventricular contraction. The dynamics of mitral
subvalvular support and the dynamic changes in the mitral
annulus size compared with the more static structural support of the
aortic root and leaflets also may be contributing factors to the
difference between how the two types of valves behave when
regurgitation is present. Therefore, the
instantaneous aortic regurgitant orifice area during almost any part of
the diastolic period appears to be well related to the
severity of the aortic regurgitation, while any single
instantaneous mitral regurgitant orifice area during systole may not be
representative of the severity of the
regurgitation.24
The aliasing velocity that appears most accurately applied in these methods varies with CW transorifice velocities and regurgitant flow rates.18 24 Therefore, at any fixed aliasing velocity, the flow convergence method used with the hemispheric isovelocity surface assumption (or any other static geometric assumption) can be expected to determine most accurately a regurgitant orifice area only at the point in time when the transorifice velocity and aliasing velocity are matched and a hemispheric assumption is valid.18 24
Applicability of the Flow Convergence Method for Evaluating the
Effective Aortic Regurgitant Orifice Area
Several noninvasive methods
for evaluating the severity of aortic
regurgitation, such as CW deceleration slope, pressure
half-time, color Doppler jet area, and proximal size of the
jet, have been compared with angiographic grading. However,
angiographic grading in itself may be at variance with actual
regurgitant volumes.1 2 6 7
In contrast, our study
compared the flow convergence data with electromagnetic
flowmeter–measured regurgitant flow rates and volumes.
The clinical use of magnetic resonance imaging techniques to image flow convergence for grading the severity of aortic regurgitation has been reported recently.8 9 30 Although these techniques have no interrogation angle problems, their limited acquisition rates may be associated with underestimation of the maximal size of the flow convergence. In addition, all the other reports that used magnetic resonance imaging techniques (except the most recent report by Reimold et al30 ) were not truly quantitative studies but used semiquantitative estimates of regurgitant severity.
For mitral regurgitation, the quantitative application of the color Doppler flow convergence method with the simple hemispheric isovelocity surface assumption has been used to attempt to provide clinical information regarding dynamic change in effective regurgitant orifice size as well as mean mitral regurgitant orifice area.21 22 23 Vandervoort et al21 reported a good correlation between effective regurgitant orifice areas calculated by the flow convergence method and the pulsed Doppler velocity time integral method in patients with mitral regurgitation. However, for aortic regurgitation, before our present study there have been no strictly quantitative studies for validating the flow convergence method.
In the present study, in which
higher aliasing velocities (0.70 to
0.89 m/s) were used, the flow convergence method using maximized
two-dimensional flow convergence imaging and maximal CW velocity
provided reasonably accurate estimates of regurgitant orifice area
compared with those derived from the electromagnetic flowmeters.
Because regurgitant orifice areas were derived from regurgitant flow
rates calculated using the hemispheric assumption, our study
fundamentally evaluated the predictive applicability of the flow
convergence methods for estimating regurgitant flow rates and by
extension regurgitant orifice areas (Fig 8). More importantly,
the
estimated effective regurgitant area by the flow convergence method
correlated well with regurgitant flow rates even when the multiple
regression model was used to eliminate associations based on several
data points from one animal. The aortic regurgitant orifice area can be
measured directly with the use of two-dimensional echo images when
the orifice geometry is simple and planar, especially when multiplane
transesophageal echocardiography is
used. However, clinically encountered regurgitant orifices are usually
geometrically complicated. Thus, the concept of using effective
regurgitant orifice areas as determined by the flow convergence method
in the present study should be useful for evaluating patients with
aortic regurgitation.
Study Limitations
We used epicardial echocardiography to
select
the best position of the echo transducer in this animal study to obtain
good alignment for Doppler imaging of the aortic flow convergence
and interrogation of the CW signals of regurgitation
(Fig 1). Under clinical conditions, however, such alignment may
not be
possible for some patients with aortic regurgitation.
Perhaps as a result of this imaging problem, Yamachika et
al32 described less clinical success with obtaining the
flow convergence in patients with aortic regurgitation
as compared with mitral regurgitation. Calcification of
the aorta and/or aortic valve may hinder clinical flow convergence
imaging; this may be a particular problem in elderly patients with
aortic regurgitation. In our present animal study,
routine clinically used apical views rarely provided good flow
convergence images. However, atypical or more basally located positions
of the transducer usually did demonstrate good flow convergence images.
It should be emphasized that good alignment is essential in order to
obtain optimal images of the aortic regurgitant flow convergence. Thus,
care must be taken to obtain the best position and the direction of the
transducer to maximize the flow convergence image using atypical apical
views, high right parasternal views, and/or omniplane
transesophageal echocardiography
with or without transgastric views when necessary. Otherwise,
quantitative assessment may not be sufficiently accurate to estimate
regurgitant orifice area.
When we selected lower aliasing velocities, the flow convergence images may have been more affected by the color Doppler filters than the images obtained with the use of higher aliasing velocities. This may be one of the reasons that the flow convergence method significantly overestimated the reference orifice areas with lower aliasing velocities, since elimination of lower velocity clutter signals would bring the mean pixel velocities up proportionally. As in mitral regurgitation, selection of appropriate aliasing velocities was critical for estimating actual regurgitant flow rates and regurgitant orifice sizes. In our study, higher aliasing velocities provided excellent estimates of orifice areas and regurgitant volumes per beat. However, for milder degrees of aortic regurgitation, the flow convergence method underestimated the severity. Thus, conclusions drawn from our animal model study, which had no data from steady states with a regurgitant fraction <20%, may not be applicable for clinically mild aortic regurgitation. In clinical situations with irregularly shaped orifices and differing aortic wall boundaries causing potential flow constraint,33 optimal aliasing velocities may be different from the range we found most useful in our study. Thus, the results from our animal model of aortic regurgitation in which regurgitant orifices were surgically created may not be directly applicable for the entire spectrum of aortic and aortic valvular pathology, which may lead to regurgitation.
For a slitlike orifice similar to that of a regurgitant bicuspid valve, we recently observed an elongated flow convergence isovelocity surface using three-dimensional reconstruction methods for color flow data.34 Even in view of the angle dependency problems of color Doppler flow mapping, three-dimensional observations should provide more accurate estimates of the isovelocity surface geometries and thus aid in defining orifice areas and regurgitant volumes for more geometrically complicated aortic regurgitant orifices.
Conclusions
In our study, the electromagnetic flowmeter
method demonstrated
little change in regurgitant orifice area during diastole,
although the constancy of regurgitant orifice size may not always be
the case for the various causes of aortic regurgitation
encountered clinically, especially those cases with mild
regurgitation. Also in this animal study, the flow
convergence method using higher aliasing velocities provided good
estimates of regurgitant orifice areas with regurgitant fractions
>20%. In addition, the product of the color Doppler
regurgitant orifice area and the CW Doppler velocity time integral
of the regurgitant orifice flow predicted regurgitant flow
volumes.
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Acknowledgments |
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This study was supported in part by a grant from the National Heart, Lung, and Blood Institute (HL-43287), National Institutes of Health, Bethesda, Md.
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Footnotes |
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Reprint requests to Michael Jones, MD, National Heart, Lung, and Blood Institute, Bldg 14E, Room 1074A, Bethesda, MD 20892.
Received May 24, 1995; revision received August 7, 1995; accepted September 14, 1995.
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References |
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TopAbstractIntroductionMethods Results Discussion References |
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