(Stroke. 1996;27:695-699.)
© 1996 American Heart Association, Inc.
Articles |
Determination of Duplex Doppler Ultrasound Criteria Appropriate to the North American Symptomatic Carotid Endarterectomy Trial
From the Departments of Surgery (J.P.C., J.T.D.) and Radiology (F.J.L.), University of Pennsylvania School of Medicine, Philadelphia.
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Abstract |
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 Top Abstract IntroductionSubjects and MethodsResultsDiscussionReferences |
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Background and Purpose The North American Symptomatic Carotid Endarterectomy Trial (NASCET) demonstrated the benefit of carotid endarterectomy for symptomatic patients with
70% carotid stenosis. Screening for detection
of significant carotid occlusive disease has relied on duplex
Doppler imaging. However, traditional duplex categories (50% to
79%, 80% to 99%) are not directly applicable to NASCET. We sought to
evaluate duplex criteria for determination of 70% carotid
stenosis. Methods Duplex scans and arteriograms of 110 patients (210 carotids), performed within 1 month of each other, were reviewed by blinded readers. Arteriographic stenosis was determined by the NASCET method. Duplex measurements of peak systolic and end-diastolic velocity (PSV, EDV) were recorded, and ratios of velocities in the internal and common carotid arteries (ICA, CCA) were calculated. Receiver-operator characteristic (ROC) curves of sensitivity, specificity, positive and negative predictive values (PPV, NPV), and accuracy were determined.
Results Interobserver agreement for measurement of
arteriographic stenosis was "almost perfect" (
=0.86).
The criteria chosen for detection of 70% stenosis were
PSVICA >210 cm/s (sensitivity, 94%; specificity, 77%;
PPV, 68%; NPV, 96%; accuracy, 83%), EDVICA >70 cm/s
(sensitivity, 92%; specificity, 60%; PPV, 73%; NPV, 86%; accuracy
77%), PSVICA/PSVCCA >3.0 (sensitivity,
91%; specificity, 78%; PPV, 70%; NPV, 94%; accuracy, 83%), and
EDVICA/EDVCCA >3.3 (sensitivity, 100%;
specificity, 65%; PPV, 65%; NPV, 100%; accuracy, 79%).
Conclusions We conclude that 70% carotid
stenosis can be reliably determined by duplex
Doppler ultrasound. Individual vascular laboratories must validate
their own results.
Key Words: carotid endarterectomy • carotid stenosis • Doppler • ultrasonics
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Introduction |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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The NASCET,1 in the context of a randomized controlled clinical trial, demonstrated the benefit of carotid endarterectomy in symptomatic patients with
70% carotid diameter reduction. Currently carotid
endarterectomy is recommended for
symptomatic patients with 70% carotid
stenosis.
The diagnosis of carotid artery stenosis relies on screening by
noninvasive techniques, usually duplex Doppler ultrasound. The
determination of degree of carotid stenosis by duplex scanning
depends on measurement of Doppler-determined velocity and
spectral analysis. Most vascular laboratories use traditional
duplex criteria that characterize the carotid bifurcation as
normal, 1% to 15% stenosis, 16% to 49%
stenosis, 50% to 79% stenosis, 80% to 99%
stenosis, and complete occlusion.2 These
traditional categories were not designed for determination of 70%
carotid stenosis. Furthermore, these categories were developed
based on an arteriographic definition of stenosis different
from that used in the NASCET study. We sought to develop duplex
Doppler ultrasound criteria for determination of 70% carotid
stenosis by comparison with arteriography, using the NASCET
method of determination of carotid stenosis.
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Subjects and Methods |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Patients
Between January 1992 and January 1994, 110 patients were identified who had undergone both duplex scanning and carotid angiography at the Hospital of the University of Pennsylvania within 1 month of each other (210 carotid arteries). These patients were being evaluated for surgical treatment of carotid artery atherosclerosis and represent all patients during this interval who had complete arteriographic examinations and duplex scanning data available for review.
Arteriography
Percutaneous catheter arteriograms were obtained
in all patients with at least two-view or, in the majority of
cases, four-view biplane selective common carotid arteriograms.
Carotid arteriograms were performed with either standard cut-film
techniques or the use of high-resolution digital subtraction
imaging (1024x1024 matrix).
The percent stenosis, determined by the arteriogram, was calculated from direct measurements of the maximum stenosis (minimal residual lumen diameter [MRL]) in the carotid bifurcation region (distal CCA and proximal ICA) made with the use of a hand-held magnifier marked in 1-mm increments. This was compared with the diameter of the normal-appearing ICA distal to the bifurcation (DL) with the technique described for the NASCET study. We calculated diameter stenosis using the MRL and DL in the equation [1-(MRL/DL)]x100.
Observers were blinded to both the results of the duplex study and the other observers' readings. The first 70 vessels were evaluated by three blinded readers, and an interim calculation of interobserver agreement was made. As a result of the "near perfect" agreement of the three observers (see "Results"), a single observer completed the remaining 140 carotid arteries, providing a total of 210 carotid arteries for evaluation with complete duplex and arteriographic data.
Duplex Doppler Ultrasound
Duplex Doppler ultrasound studies were performed on a
Hewlett-Packard Sonos 1000 Color Duplex System with the use of a
7.5-MHz linear array transducer with 5.6-MHz Doppler frequency. The
cervical ICA, CCA, and external carotid arteries were examined.
Velocity waveforms were obtained routinely from the CCA at the base of
the neck, just proximal to the carotid bifurcation; the proximal, mid,
and distal ICA; and the external carotid artery at a 60° incident
angle. In addition, velocity waveforms were obtained from any location
at which stenosis was suspected by either B-mode appearance or
color flow mapping. The highest PSV and EDV were recorded from each
location.
Analysis
Maximal PSV and EDV in the carotid bifurcation region (distal
CCA or ICA [PSVICA, EDVICA]) were used
for comparison with maximal angiographic stenosis. The maximal
carotid bifurcation PSVICA and EDVICA were
compared with the maximal PSV or EDV in the proximal CCA low in the
neck (PSVCCA, EDVCCA) and their ratios
(PSVICA/PSVCCA,
EDVICA/EDVCCA) calculated.
Sensitivity was defined as the number of true-positive studies divided by the sum of true-positive and false-negative studies; specificity was defined as the number of true-negative studies divided by the sum of false-positive and true-negative studies; PPV was defined as the number of true-positive studies divided by the sum of true-positive and false-positive studies; NPV was defined as the number of true-negative studies divided by the sum of true-negative and false-negative studies; and accuracy was defined as the sum of true-positive and true-negative studies defined by the total number of studies performed.
ROC curves were generated to predict a 70% angiographic
stenosis. These curves describe sensitivity, specificity, PPV,
NPV, and accuracy of each criterion (PSVICA,
EDVICA,
PSVICA/PSVCCA,
EDVICA/EDVCCA).
Interobserver variability for interpretation of arteriographic
stenosis was assessed with the statistic, in which the
degree of agreement between different readers was defined by the scale
of Landis and Koch3 : <0.00, poor; 0.00 to 0.20, slight;
0.21 to 0.40, fair; 0.41 to 0.60, moderate; 0.61 to 0.80, substantial;
and 0.81 to 1.0, almost perfect.
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Results |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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A total of 210 carotid arteries were evaluable. A
70%
arteriographic stenosis was present in 69 cases (33%), not
including 17 occluded ICAs (8%). Interobserver variability for the
first 70 carotid arteries evaluated by three observers was almost
perfect (=0.86).
Plots of degree of arteriographic stenosis versus the cardinal
duplex parameters (PSV, EDV,
PSVICA/ PSVCCA,
EDVICA/EDVCCA) are shown in Figs 1 through 4
.
From these scatterplots the sensitivity, specificity, PPV, NPV, and
accuracy for various values and combinations of the
PSVICA, EDVICA,
PSVICA/PSVCCA, and
EDVICA/EDVCCA were determined and are
shown in Figs 5 through 8 and Table 1. Suggested criteria for determination of 70%
carotid stenosis are shown in Table 2.
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Combined Criteria (Table 1
)
When all four criteria for 70% stenosis are met
(PSVICA >210 cm/s, EDVICA >70 cm/s,
PSVICA/PSVCCA >3.0,
EDVICA/EDVCCA >3.3), a sensitivity and
NPV of 100% are achieved. The specificity (75%) and PPV (72%) are
somewhat lower, with an overall accuracy of 85%. Of 69 arteries with
70% stenosis, only 13 (19%) met all four criteria. The five
false-positive cases identified (there were no false-negative
cases) all had stenoses between 60% and 67%, with a mean
stenosis of 65%.
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Discussion |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Patients with symptomatic carotid artery occlusive disease rarely undergo arteriography as the first diagnostic test. The most widely used screening modality for noninvasive diagnosis of carotid occlusive disease is duplex Doppler ultrasound. The NASCET investigators found a beneficial effect of carotid endarterectomy for symptomatic patients with
70% carotid stenosis,
which necessitates the development of duplex criteria appropriate to
the results of the trial. Traditional duplex categories of stenosis (50% to 79%, 80% to 99%) are inadequate to properly identify patients who meet the NASCET criteria. Furthermore, the traditional duplex categories, while based on correlation of duplex and arteriogram data, used an alternative method of arteriographic stenosis measurement to that of NASCET. These former validation studies calculated stenosis using the ratio of the residual lumen of the carotid artery compared with the estimated normal bulb diameter, the method of the European Carotid Surgery Trial.4 The NASCET investigators used a comparison of the minimal residual lumen to that of the normal distal ICA measured in its cervical portion. The importance of the method of measurement of angiographic carotid stenosis has been emphasized by several authors.5 6 7 8 Clearly it is imperative that the same arteriographic method be used for validation of duplex studies as was used in the randomized trial if adequate correlation is to be achieved. We used the NASCET method of determination of arteriographic stenosis in this study.
Quantification of stenosis by duplex scanning includes a combination of the information obtained from B-mode ultrasound imaging and Doppler velocity analysis. Both components of the duplex scan are subject to the artifacts and pitfalls inherent to ultrasound technology. Calcific plaque in the carotid artery can produce acoustic shadowing of the B-mode image of the artery and can make the Doppler signal unobtainable in the region of the calcium. Often obtaining alternative views of the same portion of the artery will remedy the situation and allow the artery to be interrogated by Doppler spectral analysis. Characteristics of the Doppler signal proximal and distal to the area that is acoustically opaque because of calcifications also allow for significant information to be obtained from the artery. It is rare that no signal can be obtained as a result of calcification, and we encountered no such examinations in the course of this study. In addition, the presence of thrombus, particularly acute thrombus, within the carotid artery can often be difficult to visualize by B-mode imaging since it is weakly echogenic. This can cause difficulty with differentiation between patent and occluded arteries. The sonographer relies on not only the B-mode image but the absence of a Doppler signal and the presence of a highly obstructed waveform proximal to the suspected occlusion to confirm the diagnosis of carotid occlusion. The B-mode image rather than the Doppler velocity is relied on primarily for determination of lesser degrees of carotid stenosis that are not hemodynamically significant. For higher degrees of carotid stenosis that cause hemodynamic disturbance, the cardinal Doppler parameters described in this study are relied on chiefly to determine the degree of carotid stenosis.
Analysis of Doppler velocity criteria by ROC curves allows
the criteria chosen to be tailored to the individual patient and
institutional needs. Once the sensitivity, specificity, PPV, NPV, and
accuracy of duplex scanning parameters at an individual
institution are established, cut points can be chosen that are
appropriate for various situations. For example, if one wishes to apply
duplex to a population as a screening test for detection of a 70%
carotid stenosis, one would choose criteria with high
sensitivity and NPV so as not to miss any patients who have appropriate
lesions. On the other hand, if one desires to perform surgery based on
the results of a duplex examination alone without confirmatory
arteriography,9 10 11 12 13 high specificity and PPV would be
desirable so as not to operate on patients who do not truly meet the
criteria of the randomized trial. Once the ROC curve has been
generated, optimal criteria may be selected for either indication. It
is apparent from examination of the ROC curves that as the test becomes
more sensitive, it becomes less specific; as the PPV rises, the NPV
declines. The arbitrary criteria suggested in Table 2 were chosen to
provide the highest possible accuracy of the test while maintaining
high sensitivity and NPV and are best suited to use as a screening
test. Alternative criteria could be chosen to afford high specificity
and PPV. The duplex criteria chosen should be tailored to the specific
institutional and individual situations. This is greatly facilitated by
the use of ROC curves.
Our choice of PSVICA >210 cm/s as a criterion for
determination of 70% carotid stenosis is based on its high
accuracy, sensitivity, and NPV in the ROC analysis. This value
is intermediate between values recently suggested by other authors (130
cm/s14 and 270 cm/s6 ) and affords similar
sensitivity, specificity, predictive values, and accuracy. The wide
variability in suggested PSVICA criteria points out the
importance of establishing ROC curves for individual institutions. The
somewhat low PPV of the PSVICA criterion (68%) is due to
19 false-positive cases. However, the average stenosis in
this group of 19 false-positive cases was 66%, with a range of
60% to 68%. Thus, even though the PPV for 70% stenosis
prediction by this criterion is somewhat low, it is not due to
false-positives that stray greatly from the desired threshold value
of 70%. A higher PSV may be chosen to increase the PPV if duplex is
to be used as the sole preoperative imaging modality.
The EDV has been a useful duplex criterion for high-grade
stenoses when aliasing is problematic for
measurements of PSV. We chose EDVICA >70 cm/s because of
its high sensitivity (92%) and NPV (86%), with overall accuracy of
77%. This value is in a range comparable to previously suggested
cutoff values for determination of 70% carotid stenosis by
use of the EDVICA (100 cm/s,14 110
cm/s,6 and 130 cm/s5 ).
Velocity ratios are useful for overcoming variability in interval
measurements of PSV and EDV from examination to examination. These
ratios remain constant despite the hemodynamic effects
of ipsilateral tandem lesions or contralateral stenoses and
changes in blood pressure. We found a
PSVICA/PSVCCA ratio >3.0 to provide
high sensitivity (91%), NPV (94%), and accuracy (83%). This value is
in a range compatible with previously reported ratios
(3.3,14 >4.05 ).
The EDV ratio (EDVICA/EDVCCA >3.3) provides perfect sensitivity and NPV (100%) but an overall accuracy of 79%. This is due to eight false-positive studies with a mean stenosis of 62%.
The range of values for each suggested criterion indicates the importance of individual validation of duplex scanning results against the gold standard of arteriography. This is greatly facilitated by the use of ROC analysis. Once the ROC curve is established, criteria appropriate for use as either a broad screening test (with high sensitivity and NPV) or as a sole preoperative imaging modality before surgery (high PPV) may be selected. The actual criteria chosen for each application must be tailored to the individual situation.
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Selected Abbreviations and Acronyms |
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Footnotes |
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Reprint requests to Jeffrey P. Carpenter, MD, Hospital of the University of Pennsylvania, 4 Silverstein/3400 Spruce St, Philadelphia, PA 19104.
Received August 24, 1995; revision received November 6, 1995; accepted December 28, 1995.
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References |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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