Use of diastolic velocity ratios to predict significant carotid artery stenosis

Which Doppler velocity is best for assessing suitability for carotid endarterectomy?

OBJECTIVE

To evaluate which velocity, or combination of velocities, from carotid Doppler ultrasonography (DU), achieved the closest agreement with an assessment of suitability for carotid endarterectomy from intra-arterial angiograms (IAA).

METHODS

We prospectively collected data from 148 consecutive patients (288 carotids), who had DU and IAA (blinded assessment) before possible carotid endarterectomy. We halved our data by randomly selecting the left or right carotid artery for each patient. We used one half to calibrate our DU results to IAA (to decide which velocity corresponded with what degree of angiographic stenosis). Using this analysis, each artery in the other half of the data was defined as suitable (80-99% stenosed) or unsuitable for carotid endarterectomy. We evaluated every individual, and combination of, velocities (strategies) to see which gave the closest agreement with IAA.

RESULTS

Of all 80 strategies, six resulted in better agreement than others of the same or lower complexity. Five of these strategies gave better agreement than the internal carotid artery peak systolic velocity (ICA PSV) (kappa 0.78), but the improvement was small.

CONCLUSION

Using the ICA PSV alone is adequate for assessing carotid stenosis before endarterectomy using DU, as long as the machine is calibrated to IAA. However, the addition of the ratio of the ICA PSV to the common carotid artery PSV adds only one further measurement, slightly increases the agreement with IAA, and would be reasonable to use on a daily basis.

Influence of poststenotic collateral pressure on blood flow velocities within high-grade carotid artery stenosis: differences between morphologic and functional measurements

PURPOSE

The driving force for blood flow through a high-grade stenosis in the internal carotid artery can be expressed as the pressure gradient over the stenosis itself, which, however, might be reduced by the back pressure exerted by distal collateral vessels. Theoretically the maximum blood flow velocity as a measure of the functional grade of obstruction may therefore be lower than what is expected from morphologic gradations of the stenosis. This study was designed to test prospectively the influence of intracranial collateral vessels on blood flow velocities within high-grade internal carotid artery stenoses.

PATIENTS AND METHODS

Forty-five consecutive patients (age 66 +/- 11) with high-grade internal carotid artery stenoses were investigated before and during carotid endarterectomy. The preoperative investigations included duplex ultrasound scanning of the neck vessels, transcranial Doppler scanning for assessment of collateral flow to the middle cerebral artery and angiography. Carotid endarterectomy was performed with patients under deep general anesthesia without a shunt. Systolic and diastolic internal carotid artery blood pressures were measured before and during intraoperative cross-clamping (ie, stump pressure) of the carotid arteries.

RESULTS

Within high-grade internal carotid artery stenoses, maximum systolic and end-diastolic blood flow velocities showed a significant inverse correlation to the corresponding systolic and diastolic stump blood internal carotid artery blood pressures. All patients with spontaneous collateral flow to the ipsilateral anterior part of the circle of Willis were divided into a group with relatively high and another one with low end-diastolic blood flow velocities. The stump pressure was significantly lower in patients with high end-diastolic blood flow velocities in spite of the fact that the mean angiographic grade of stenosis did not differ significantly between the groups.

CONCLUSIONS

Flow velocities within a high-grade internal carotid artery stenosis are inversely dependent on the stump pressure, that is the poststenotic collateral perfusion pressure. This should be taken into consideration in case of discrepancies between angiography and ultrasound outcome.

Variability of Doppler US measurements along the common carotid artery: effects on estimates of internal carotid arterial stenosis in patients with angiographically proved disease

PURPOSE

To determine the effect of variability of common carotid arterial (CCA) velocities on velocity ratios used to assess internal carotid arterial (ICA) stenosis.

MATERIALS AND METHODS

Doppler ultrasonographic (US) velocity measurements were obtained at three levels in the CCA and in the carotid bulb and ICA in all patients referred for carotid US between September 1996 and October 1997. Only ICAs (n = 98, in 57 patients) without ipsilateral CCA disease at angiography were analyzed. The range of CCA peak systolic velocities (PSVs) and end diastolic velocities (EDVs) and velocity ratios were calculated for each CCA measurement. For each ICA/CCA velocity ratio, receiver operating characteristic analysis was performed.

RESULTS

CCA PSV and EDV ranges averaged 23.1 cm/sec +/- 15.7 (SD) and 5.1 cm/sec +/- 3.6, respectively. For a given side, the difference averaged 1.0 +/- 1.3 for PSV ratios and 2.7 +/- 6.9 for EDV ratios, depending on where CCA measurements were taken. By using a threshold of 60% stenosis as indication for endarterectomy, variability in CCA velocities could have altered recommendations in 16 (28%) of 57 patients. Receiver operating characteristic analysis showed that ratios made by using the three CCA velocities or their mean were not significantly different.

CONCLUSION

Variability in velocity measurements along the course of the CCA in patients with ICA disease can be substantial and can result in inaccuracies in assessment of carotid stenosis.

Assessment of stenosis: implications of variability of Doppler measurements in normal-appearing carotid arteries

PURPOSE

To examine the variability of Doppler measurements along the extra-cranial courses of the nondiseased common carotid artery (CCA) and internal carotid artery (ICA) and determine the effect of this variability on assessment of carotid arterial stenosis.

MATERIALS AND METHODS

During the study period, 580 patients were referred for carotid arterial ultrasonography (US), including Doppler measurements of flow velocities in the proximal, middle, and distal portions of the CCA, in the bulb, and in the proximal and distal portions of the ICA. Eighty-five patients (average age, 59 years) with normal ICAs and CCAs formed the cohort for this study.

RESULTS

The range of peak systolic velocity (PSV) measurement (maximum minus minimum) averaged 20 cm/sec +/- 13 in the CCA and 15 cm/sec +/- 13 in the ICA. ICA/CCA velocity ratios varied, depending on the CCA measurement location. In five arteries, PSV ratios exceeded a threshold of 1.8 (suggesting > or = 60% stenosis); in 23 arteries, end diastolic velocity ratios exceeded a threshold of 2.4 (also suggesting > or = 60% stenosis). Right-to-left CCA PSV ratios were abnormal in up to 26 patients (suggesting > 50% ICA stenosis), depending on where CCA measurements were obtained. When the CCA ratios were obtained at the same level, 16 were in the abnormal range.

CONCLUSION

Variability of Doppler measurements in the CCA and ICA in patients without visible disease is substantial and could lead to inaccuracies in carotid arterial stenosis assessment.

An approach to ascertain probands with a non-traditional risk factor for carotid atherosclerosis

In our previous studies of the determinants of carotid plaque area (CPA), we used a linear multiple regression model, which permitted us to control for the presence of known risk factors in order to reveal the contribution of putative new risk factors. We recognized that this approach could identify patients whose observed CPA was excessive when considering traditional risk factors. Subjects whose observed CPA markedly exceeded the expected CPA were easily identified because of their deviation from the regression line that was derived using all members of the study sample. We classified such subjects as having 'unexplained atherosclerosis' relative to the overall study sample when traditional risk factors were included as independent variables. We then examined the plasma homocyst(e)ine concentration in members of the subgroup with 'unexplained atherosclerosis'. We found a significantly higher mean plasma concentration of homocyst(e)ine in the subgroup with 'unexplained atherosclerosis', compared to rest of the study sample (20.4 +/- 4.3 vs. 13.2 +/- 3.2 mumol/l, P < 0.005). We also found that significantly more subjects with 'unexplained atherosclerosis' had plasma homocyst(e)ine concentrations in excess of 14 mumol/l compared to the rest of the study sample (52 vs. 33%, P < 0.002). We thus propose that systematic identification of subjects with 'unexplained atherosclerosis' relative to the rest of a well-characterized study sample might be a useful approach to identify subjects in whom there are newer, non-traditional determinants of predisposition to atherosclerosis.

Evaluation and characterization of carotid plaque

Doppler sonography, combining high-resolution imaging and Doppler spectrum analysis, has proven to be the best choice for safe, noninvasive, low-cost screening for the etiology of stroke. With high-resolution imaging, plaque can be characterized relative to the risk for intraplaque hemorrhage, thought by many to be the precursor of plaque ulceration. Using high-resolution ultrasound, heterogeneous plaque has been shown to be associated with intraplaque hemorrhage. Criteria for distinguishing heterogeneous from homogeneous plaque are discussed, as are techniques for characterizing plaque. Several published articles strongly suggest that, when heterogeneous plaques are identified, the incidence of neurological symptoms and stroke on follow-up increases, relative to patients with homogeneous plaques. Additional long-term follow-up studies of heterogeneous plaque are needed. Most recently, carotid wall thickness has been evaluated as a physiological marker for atherosclerotic disease and as a gauge of the effectiveness of medical therapies. It is likely that carotid wall thickness will become important in the diagnosis and management of atherosclerosis. Radiologist are encouraged to learn this new form of ultrasound examination, which is reviewed briefly in this article.

Duplex US of the external carotid artery

The ratio between the systolic peak velocities of the internal and common carotid arteries (vpICA/vpCCA), vpICA and grey-scale imaging measurement are generally used to evaluate internal carotid stenosis against known flow criteria in order to differentiate non-significant from significant stenosis. The same criteria are also used for evaluating the external carotid artery (ECA). Our data on 707 normal or stenotic ECA nevertheless showed that the systolic peak velocity of the normal ECA (vpECA) and its ratio to the systolic velocity of the CCA (vpECA/vpCCA) are higher than vpICA and vpICA/vpCCA. vpECA/vpCCA is about 2 in > 0-49% ECA stenosis. Only in severe stenosis are the peak velocities almost comparable. The ratio between the peak end diastolic velocities (edvECA/edvCCA) and edvECA proved to be unreliable, as did grey-scale imaging measurement of the external carotid stenosis. In addition, ipsi-lateral internal carotid stenosis greatly affects the non-stenotic external carotid flow values, and probably has the same effect on the flow values of a stenotic external artery. Thus, external carotid flow values must be considered carefully.